Unit weight or density is used to quantify the weight per unit volume of an object. When it is expressed in the basic SI Unit of mass (kg/m3), it is usually referred to as density, but when expressed in terms of weight (kN/m3), it is usually referred to as unit weight.
Density (kg/m3) = Mass (kg)/Volume (m3)
The unit weight of a material is very important in calculating the self-weight of the material, especially when evaluating the permanent actions in a structure. Without the knowledge of the unit weight of a material, it will be impossible to accurately estimate the bodyweight and the load it subsequently carries when used as a structural member. This article is dedicated to providing a handy list of the unit weight of various construction materials according to EN 1991-1-1:2002.
The density of a material can vary depending on the composition, process of manufacturing, and other factors. However, the density of man-made products is fairly constant (or controllable) when compared with the density of direct products of nature. Therefore, natural materials are expected to have slightly different properties even when they are occurring in the same place.
A footbridge (also called a pedestrian bridge) is a structure that is designed to enable pedestrians to cross over natural or man-made obstacles such as busy highways, railways, water bodies, gullies, etc, with minimal risk. Pedestrians are generally humans that are travelling on foot and also includes people in a wheelchair, or people that are pushing tramps. In the design of a footbridge, it is very important to evaluate the loads (actions) that it will encounter during its service life.
The possible loads that footbridges are subjected to are;
Human traffic
Self-weight of the bridge and its ancillaries
Wind load
Earthquake load (where applicable)
Snow load (where applicable)
Dynamic water pressure (for bridges crossing water bodies)
Thermal actions
Possible accidental action from impact
In the recent wake of the need for sustainable urban transportation, some pedestrian bridges are being designed to accommodate both pedestrians and cyclists. Furthermore, to ensure that footbridges are accessible to the disabled and mobility-impaired people, there should be provision for ramps or lifts so that they can cross over safely. The use of lifts should be a better option compared to ramps due to the high demand for space, especially in urban areas.
Pedestrian bridges can also serve as monumental structures that can be used for the decoration and beautification of cities and streets. Beautiful and well designed footbridges can adorn the skylines of a city, and form unique features that can easily attract tourists.
Figure 1: The Golden Bridge in Vietnam – A famous pedestrian bridge that opened in 2018
Actions on Footbridges
The actions on pedestrian bridges can be found in section 5 of EN 1991-2 (Eurocode 1 Part 2). The section covers explicitly actions on footways, cycle tracks, and footbridges. It is important to note that the uniformly distributed load qfk and the concentrated load Qfwk given in section 5 of EN 1991-2, where relevant can be also used for parts of road and railway bridges accessible to pedestrians. However, all other variable actions described therein apply to footbridges only.
Furthermore, it is important to note that the load models and their representative values take into account dynamic amplification effects, and can be used for all kind of serviceability and ultimate limit state static calculations, except fatigue limit states. The load models do not cover the effects of loads on construction sites and should be separately specified, where relevant.
The imposed loads defined in EN 1991-2 result from pedestrian and cycle traffic, minor common construction and maintenance loads (e.g. service vehicles), and accidental situations. These loads give rise to vertical and horizontal, static, and dynamic forces.
Static Models for Vertical Loads on Footbridges
Three mutually exclusive vertical load models can be envisaged for footbridges. They are;
a uniformly distributed load representing the static effects of a dense crowd;
one concentrated load, representing the effect of a maintenance load;
one or more, mutually exclusive, standard vehicles, to be taken into account when maintenance or emergency vehicles are expected to cross the footbridge itself.
Figure 2: Crowd on a pedestrian bridge, at Ojota Lagos, Nigeria
The characteristic values of these load models should be used for both persistent and transient design situations.
Uniformly Distributed Loads on Footbridges
The crowd effect on the bridge is represented by a uniformly distributed load. When the risk of dense crowd exists or when specified for a particular project, Load Model 4 for road bridges should be considered also for footbridges. On the contrary, where the application of the aforesaid Load Model 4 is not required, a uniformly distributed load, to be applied to the unfavourable parts of the influence surface longitudinally and transversally, qfk should be defined in the National Annex. The recommended value, depending on the loaded length L [m] is:
For road bridges supporting footways or cycle tracks, only the characteristic values (5 kN/m2) or the combination value (2.5 kN/m2) should be considered.
Figure 3: Characteristic load on a footway (or cycle track) of a road bridge
The minimum and maximum crowd load to be applied on bridges irrespective of the span length is therefore 2.5 kN/m2 and 5 kN/m2 respectively. To check the effect of span length on the value of the crowd load, equation (1) can be used. For instance, the crowd load on a pedestrian bridge of length 30 m is given by;
For local effect assessment, a 10 kN concentrated load Qfwk, representing a maintenance load, should be considered on the bridge, acting on a square surface of sides 10 cm. When the service vehicle (see paragraph below) is taken into account, Qfwk should be disregarded. The concentrated load Qfwk should not be combined with any other variable non-traffic load.
Service/Accidental vehicleon Footbridges
When service vehicles for maintenance, emergencies (e.g. ambulance, fire), or other services must be considered, they should be assigned for the particular project. If no information is available and if no permanent obstacle prevents a vehicle from being driven onto the bridge deck, the special vehicle defined in the figure below should be considered. If consideration of the service vehicle is not requested, the vehicle shown in the figure below should be considered as accidental.
Figure 4: Service or accidental vehicles on footbridges
Horizontal forces – characteristic values
A horizontal force Qflk acting along the bridge deck axis at the pavement level should be taken into account for footbridges only, whose characteristic value is equal to the greater of these two values:
– 10 per cent of the total load corresponding to the uniformly distributed load or – 60 per cent of the total weight of the service vehicle, when relevant.
This horizontal force, which is normally sufficient to ensure the horizontal longitudinal stability of the footbridge, is assumed to act simultaneously with the corresponding vertical load, and in no case with the concentrated load Qfwk.
Groups of traffic loads on footbridges
Vertical loads and horizontal forces due to traffic should be combined, when relevant, taking into account the groups of loads defined in the table below (Table 5.1, EN 1991-2). Each of these groups of loads, which are mutually exclusive, should be considered as defining a characteristic action for combination with non – traffic loads.
As a rule, except for roofed bridges, where appropriate rules are defined in EN 1991-1-3, traffic loads on footbridges are considered not to act simultaneously with significant wind or snow. Wind and thermal actions should not be taken into account as simultaneous. When a combination of traffic loads together with actions specified in other Parts of EN 1991 must be considered, any group of loads in Table 5.1 of EN 1991-2 should be considered as one action.
Application of the load models
The traffic models described above with the exception of the service vehicle model, may also be used for pedestrian and cycle traffic on the areas of the deck of road bridges limited by parapets and not included in the carriageway, or on the footpaths of railway bridges. These actions are free, so that the models of vertical loads should be applied anywhere within the relevant areas in such a way that the most adverse effect is obtained.
According to section 5 of EN 1991-1-7:2006 (Eurocode 1 Part 7), explosions shall be taken into account in the design of buildings or civil engineering structures where gas is burned or regulated. This requirement also extends to buildings where materials such as explosive gases, or liquids forming explosive vapour or gas are stored or transported (e.g. chemical facilities, vessels, bunkers, sewage constructions, dwellings with gas installations, energy ducts, road and rail tunnels).
Explosion pressures on structural members should be determined taking into account, as appropriate, reactions transmitted to the structural members by non-structural members. The basic principle for design is that the structure shall be designed to resist progressive collapse resulting from an internal explosion, in accordance with EN 1990, 2.1 (4)P. In the design, the failure of some parts of the building may be permitted, but not the key structural members such as columns, slabs, beams, shear walls, etc.
Internal Gas Explosion in a Building
A gas explosion can be defined as a process where the combustion of a premixed gas-air cloud causes a rapid increase of pressure. For the proper functioning of a building, gas is required to supply energy for heating, cooking, and electricity generation. This implies the potential for an accidental explosion. For a gas explosion to occur in a building, the following succession of circumstances is required;
A gas leakage or release occurs as a result of technical defects in gas installations (pipes, boilers, etc.), human errors during installation, repair, or maintenance of such installations, or in consequence of intentional manipulation.
The released gas forms, with the oxygen present in the air, an inflammable gas-air cloud in compliance with certain physical requirements.
A delayed ignition of the gas-air mixture through an ignition source (spark, hot surface, etc) occurs. If the ignition takes place immediately after the gas release, i.e. before an inflammable gas-oxygen cloud is being formed, a fire might occur, but the mixture will not explode.
Figure 1: Gas explosion destroys part of a building in Russia
The pressure build-up during a gas explosion in the inside of a building is a consequence of combustion in a confined environment. The combustion process results in increased temperatures due to the transformation of chemically bound energy into heat. Thereby, the expansion of the combustion products, such as CO2 and vapour, will be limited due to confinement by the building closings such as walls and floors, which will cause the pressure to increase [1].
In a completely confined compartment, such as pipes or closed vessels in industrial installations, the maximum pressure generated during a gas explosion will depend primarily on the burning velocity (velocity of the flame front relative to the unburned gas immediately ahead of the flame). This velocity depends on the composition of the inflammable gas cloud, i.e. the gas type and the proportions of gas and air [1].
The highest pressures will arise if these proportions are such that there is no excess of fuel nor oxygen after the chemical reaction has been completed, what is referred to as the stoichiometric composition. For methane gas, for instance, which is the principal constituent of natural gas, the highest explosion pressures will arise for gas concentrations of about 10%. Below a methane gas concentration of 4%, and above 17%, no explosion will occur [1].
These so-called lower and upper flammability limits depend on the type of gas involved, the initial pressure (p0), and temperature (t0). The mentioned values for methane gas correspond to ambient conditions (t0 = 20º, p0 = 1 bar).
Figure 2: Simulated explosion pressure generation in a cubical, vented compartment [1]
The ignition of a hydrocarbon gas-air cloud in a fully confined compartment might entail explosion pressures up to approximately 8 bars (800 kN/m2). Gas explosion in buildings, however, do not cause pressures of this magnitude. The main reasons are imperfect mixing of the gas-air cloud and the fact that these explosions are only partly confined. Windows, doors, light partition walls, or unrestrained brick walls act as venting elements, which, in case of failure provide explosion pressure relief. The table below culled from [1] gives typical failure pressures of these elements.
Element
Typical failure pressure (kN/m2)
Glass windows
2 – 7
Room doors
2 – 3
Light partition walls
2 – 5
Breeze block walls (50 mm)
4 – 5
Unrestrained brick walls
7 – 15
According to [2], key structural elements in a building should be designed to withstand the effects of an internal natural gas explosion, using a nominal equivalent static pressure, given by:
Where; pstat = uniformly distributed static pressure in kN/m2 at which venting components will fail, Av is the area of venting components, and V is the volume of the room.
This equation is found in Annex D of BS EN 1991-1-7:2004.
The venting components represent here the non-structural part of the enclosure (e.g. wall, floor, ceiling) with limited resistance that is intended to relieve the developing pressure from deflagration in order to reduce pressure on structural parts of the building. The explosive pressure acts effectively simultaneously on all of the bounding surfaces of the room. The expressions are valid for rooms up to a volume of 1000 m3 and venting areas over volume ratios of 0.05 m-1 ≤ Av/V ≤ 0.15 m-1.
An important issue is further raised in Clause 5.2 of EN 1991-1-7 [2]. It states that the peak pressures in the main text may be considered as having a load duration of 0.2 s. The point is that in reality, the peak will generally be larger, but the duration is shorter. So combining the loads from the above equations with a duration of 0.2 s seems to be a reasonable approximation.
For purpose of member design in the context of accidental actions, the Eurocode offers the following specific expression of the general formulation for design load combinations:
Ed = E(∑j≥1 Gk,j + P + Ad + (ψ1,1 or ψ2,1) ∙ Qk,1 + ∑i≥1 Ψ2,i ∙ Qk,i) —— (3)
Where: Gk: Characteristic value of a permanent action P: Representative value of a prestressing action Ad: Design value of an accidental action Qk: Characteristic value of a variable action ψ1,1: Factor for the frequent value of a variable action ψ2,1: Factor for the quasi-permanent value of a variable action
It should be noted that in comparison to the corresponding expression for persistent- or transient design situations no partial safety factors are to be applied to load effects within the accidental load combination. The design value of the accidental action is directly defined by means of a value Ad, which in practice often corresponds to a nominal value, such as in the case of gas explosions, where Ad is represented by a nominal, static equivalent pressure.
The reason for the use of nominal values is that a reliable statistical characterization of both occurrence and magnitude of accidental actions can only seldom be carried out because the available data is generally poor.
A Design Example of a Building for an Internal Explosion
Let us consider a kitchen on the first floor of a block of flats in Port Harcourt. The floor dimensions of the kitchen are 3 x 4 m while the storey height is 3m. The kitchen is characterised by a significant window and door opening of area 5 m2 (we are going to treat this as the venting area). Let us treat other walls in the kitchen as load-bearing.
This means that the volume V and the area of venting components Av for this case are given by:
Av = 5 m2 V = 3 × 4 × 3 = 36 m3
So the parameter Av/V can be calculated as:
Av/V = 5/36 = 0.138 m-1
As V is less than 1000 m3 and Av/V is well within the limits of 0.05 m-1 and 0.15 m-1 it is allowed to use the loads given in the code. The collapse pressure of the venting panels pstat is estimated as 4 kN/m2.
The equivalent static pressure for the internal natural gas explosion is given by:
Therefore, the design pressure should be taken as the highest, which in this case is 7.1 kN/m2
According to Eurocode EN 1990 (Basis of design), these pressures have to be combined with the self-weight of the structure and the quasi-permanent values of the variables loads. Let us consider the design consequences for the various structural elements.
Bottom floor Let us start with the bottom floor of the kitchen. Let the self-weight of the floor and finishes be 4.95 kN/m2 and the imposed load 2 kN/m2 (ψ1 = 0.5 for considered category A). This means that the design load for the explosion is given by:
The design for normal conditions is given by: pEd = γGgk + γQqk = 1.35(4.95) + 1.5(2) = 9.68 kN/m2
According to [2], we should keep in mind that for accidental actions there is no need to use a partial factor on the resistance side. So for comparison, we could increase the design load for normal conditions by a factor of 1.2. The result could be conceived as the resistance of the structure against accidental actions, if it designed for normal loads only. Hence,
pRd = 1.2 x 9.68 = 11.616 kN/m2
Dynamic Increase in Load Bearing Capacity It is now time to remember the clause in Annex B of Eurocode EN 1991-1-7. If we take into account the increase in short duration of the load we may increase the load-bearing capacity by a factor φd given by:
φd = 1 + √(gk/pRd) x √(2umax/g∆t2) —— (4)
Where; gk = 4.95 kN/m2 pRd = 11.616 kN/m2 g = acceleration due to gravity = 9.81 m/s2 umax = 0.2 m (midspan deflection at collapse) ∆t = 0.2 seconds
φd = 1 + √(4.95/11.616) x √[(2 x 0.2)/(9.81 x 0.22)] = 1.652
pREd = φd pRd = 1.652 x 11.616 = 19.189 kN/m2 > 13.05 kN/m2
Therefore under the design condition, the bottom slab can be said to be fulfilling the design requirements.
Upper floor Let us next consider the upper floor. Note that the upper floor for one explosion could be the bottom floor for the next one. The design load for the explosion, in that case, is given by (upward value positive!):
So the load is small, but will give larger problems anyway. The point is that the load is in the opposite direction of the normal dead and live load. This means that the normal resistance may simply be close to zero. What we need is top reinforcement in the field and bottom reinforcement above the supports. So it will be normal to reinforce the floor slab top and bottom.
An important additional point to consider is the reaction force at the support. Note that the floor could be lifted from its supports, especially in the upper two stories of the building where the normal forces in the walls are small. In this respect, edge walls are even more vulnerable. The uplifting may change the static system for one thing and lead to different load effects, but it may also lead to freestanding walls. If the floor to wall connection can resist the lift force, one should make sure that the also the wall itself is designed for it.
Design of the walls/columns Depending on the support condition of the walls/columns, the design pressure should be applied as a lateral load to the columns/walls, and the maximum bending moment determined. This should be combined with the axial force in the column/wall to determine the appropriate reinforcement.
References [1] Hingorani R. (2017): Acceptable life safety risks associated with the effects of gas explosions on reinforced concrete structures. PhD thesis submitted to the Department of Civil Engineering, UNIVERSIDAD POLITÉCNICA DE MADRID [2]Vrouwenvelder T., Diamantidid D. (2009): Accidental Actions (Chapter 5) in Load Effects on Buildings (Milan Hilicky et al Eds). CTU in Prague, Klokner Institute
On September 28, 2018, a large earthquake of magnitude (Mw) 7.5 struck Sulawesi Island, Indonesia. A major cause of large fatalities from this event (more than 1,300 people missing who may be still buried under the mudflows) was large-scale mass movements of soil at Petobo, Balaroa, and Jono Oge regions [1]. Generally, the capital city of Palu of Central Sulawesi Province was devastated by cascading geological hazards – strong shaking due to the mainshock, triggered tsunamis, and large scale soil movement (flow-slide).
Sulawesi Island is located within a triple junction of the Australian, Philippine, and Sunda Plates, and is heavily affected by the complex interaction of their movements. The earthquake occurred along the Palu-Koro Fault, which is known to be active and is influenced by the complex tectonic interaction of major subducting plates. The Palu-Koro fault is said to accommodate a deformation of about 40 mm/year, and has caused several devastating earthquakes in the past [1, 2].
According to researchers from Japan and Indonesia [2], the provinces of Balaroa, Petobo, Jono-Oge and Sibalaya were the worst hit in the 2018 earthquake mainly due to large-scale flow-slides and mudflows. Never before have such large-scale flow failures been triggered by an earthquake. That these failures occurred on very gentle sloping ground, sweeping away localities along with it, came as a complete surprise to one and all. The flow-slide in Jono Oge was deemed the largest in the region.
Figure 1: Overview of long distance flow-slide [2]
According to [2], the damaged area of long-distance flow-slide in Jono Oge is about 210 ha, located on the east side of Palu Valley, about 4 km south of Petobo. It is reported that a total of 500 houses were damaged. The catchment area of the affected area is estimated at 4 km2. The flow-slide occurred at the bottom of the alluvial fan created at the valley mouth where the discharged water was not observed during the survey.
The combination of many factors such as liquefaction of the sandy layer and the formation of water film due to the existence of a less-permeable cap layer, the presence of a confined aquifer, and the geology of the area are thought to have led to the flow-slide. According to an eye witness report from a person living in the area, sand ejecta was seen during and after the earthquake. Evidence of sand ejecta which suggests liquefaction was also observed in the area after the earthquake [3].
According to the article, in Jono Oge, a survivor was shooting a video while being flowed, which is currently available on Youtube, (see link below).
Researchers from Japan and Indonesia [3] used a combination of satellite imagery, aerial imagery, field surveys including collecting soil samples from the sites and conducting in-situ testing using Portable Cone Penetrometer, to investigate the mechanism of massive flow-slides. The paper was published in Elsevier – Soils and Foundations.
From the research, it was concluded that all major flow-slides in the Palu valley occurred at locations where a new alluvial fan meets an old alluvial fan based on the geological features of the area.
Furthermore, the presence of low permeable layers (silt and clay) over loosely deposited sandy and sandy gravel layers suggests the complex mechanism of the long-distance flow-slide at Jono-Oge can be explained by the ‘interlayer water film theory’.
Previous researches have revealed that liquefaction at a site with sandwiched lithology results in the liquified soil being trapped below the low permeability cap layer. This creates a thin interlayer of water, referred to as a ‘‘water film‘. The resulting inhibition of excess pore water pressure dissipation and presence of the water-film reduces the residual shear strength of the sandy soil layer to below the initial static shear stress. Consequently, even on a very gentle slope, lateral flow takes place due to the action of gravitational force until equilibrium is achieved.
It is further assumed by the researchers that damage to the underlying artesian aquifer during the earthquake may also have contributed to the development of the water film and the liquefaction induced flow-slide in the layers with very low mobilized shear resistance. The researchers however admitted the mechanism of the long-distance flow-slide is not yet fully understood.
Disclaimer Every piece of information provided in this article is from the open access research work of the authors/publishers cited. They are not owned by www.structville.com. Open access articles are under the CC BY-NC-ND license. See (http://creativecommons.org/licenses/by-nc-nd/4.0/) for information on open access articles.
References
[1] Goda K., Mori N., Yasuda T., Prasetyo A., Muhammad A. and Tsujio D. (2019): Cascading Geological Hazards and Risks of the 2018 Sulawesi Indonesia Earthquake and Sensitivity Analysis of Tsunami Inundation Simulations. Front. Earth Sci. 7:261. doi: https://doi.org/10.3389/feart.2019.0026
[2] Kiyota T., Furuichi H., Hidayat R. F., Tada N., Nawir H. (2020): Overview of long-distance flow-slide caused by the 2018 Sulawesi earthquake, Indonesia. Soils and Foundations, 60(3):722-735 https://doi.org/10.1016/j.sandf.2020.03.015
[3] Hazarika H., Rohit D., Pasha S. M. K., Maeda T., Masyhur I., Arsyad A., Nurdin S. (2020): Large distance flow-slide at Jono-Oge due to the 2018 Sulawesi Earthquake, Indonesia. Soils and Foundations, ISSN 0038-0806, https://doi.org/10.1016/j.sandf.2020.10.007
Piles are used for resisting vertical and lateral loads that are transmitted to the substructure of buildings. Structures such as tall chimneys, highrise buildings, towers, offshore structures, tall retaining structures, wharfs and jetties, etc are normally subjected to high lateral loads, which are subsequently transferred to the foundations supporting them as lateral shears. If the lateral loads are too high, inclined or batter piles can be used to aid vertical piles for lateral load resistance (Murthy, 2012). The analysis of laterally loaded piles to determine the ground deflection, bending moment, shears, load-carrying capacity, etc is therefore very important for the purpose of design.
Figure 1: Inclined concrete piles in a marine facility
Numerous researchers have worked on the experimental and theoretical solution to the problem of laterally loaded piles. Reese et al (1974) and Matlock (1970) developed the concept of p-y curves for solving laterally loaded pile problems. The method is quite popular in the United States of America. However, the p-y curve method has been reported to not predict pile response properly (Kim et al. 2004, Anderson et al. 2003).
Figure 2: Model of laterally loaded pile: (a) elevation view; (b) as elastic line; (c) p-y curves, Reese (1997)
According to Basu et al (2008), the failure of p-y curves to predict pile response properly is not surprising because the p-y curves, which describe the resistive properties of soil as a function of pile deflection, used in the p-y analysis are developed empirically by back-fitting the results of numerical analysis to match the actual field pile-load test results. Thus, p-y curves developed for a particular site are not applicable to other sites. In order to obtain an accurate prediction of lateral pile response by the p-y method, p-y curves must be developed through pile load tests for every site – an uneconomical alternative.
The problem of laterally loaded piles is closely related to the problem of beam on elastic foundation. While a beam on an elastic foundation can be loaded at any point along its length, the external loads on a pile are likely to be on or above the ground surface.
Most of the numerical solutions for laterally loaded piles involves the concept of modulus of subgrade reaction. This is based on Winkler’s assumption that a soil medium may be approximated by a series of closely spaced independent elastic springs (Murthy, 2012). It has been shown that the error inherent in Winkler’s hypothesis is not significant. Therefore in laterally loaded piles, a series of non-linear springs can be used to represent the force-deformation characteristics of the soil.
The key to the solution of laterally loaded piles problem, therefore, lies in the determination of the modulus of subgrade reaction with respect to depth along the pile. The soil reaction p at any point along the pile length x may be expressed as;
p = -Esy —— (1)
where y is the deflection at point x, and Es is the soil modulus.
The relationship between the soil resistance per unit length (p) and deflection (y) is strongly non-linear. Therefore, Es is not constant but changes with the deflection of the pile. There are many factors that influence the value of Es such as the pile width, the flexural stiffness, the magnitude of the applied load, and the soil properties.
The variation of Es with depth for any prticular load level may be expressed as;
Es = nhx —— (2)
Where nhis the coefficient of soil modulus variation. The variation of nh with relative density Dr of sand is given in the Figure below;
Figure 3: Variation of nh with relative density Dr(Reese, 1974)
Matlock and Reese (1960) provided a non-dimensional solution for the determination of deflection, slope, moment, shear, and soil reaction any point x along the pile. The equations are as follows;
Where T is the relative stiffness factor expressed as;
T = [EI/nh]0.2 —— (8)
A and B are sets of non-dimensional coefficients that can be picked from Table (see Table 16.2, Murthy, 2012). The coefficients are given as functions of the depth coefficient Z expressed as;
Z = x/T —— (9)
For free headed piles at the ground level, the deflection and slope are as given below;
The bending moment at ground level for fixed head is;
Mt = -0.93[PtT] —— (13)
Figure 4: Driving of inclined H-steel piles
Brom’s Solution for Laterally Loaded Piles
Broms’ (1964a, 1964b) developed solutions for determining lateral deflections at ground level at working loads and ultimate lateral resistance of piles under lateral load using the concept of subgrade reaction. He developed solutions for long and short piles installed in cohesive and cohesionless soils in the form of charts.
Furthermore, laterally loaded piles are usually analysed depending on whether the head is free to rotate or fixed. The head of a pile may be considered as fixed when the stiff pile cap is tied using connecting beams.
Ultimate Lateral Resistance of Piles In Saturated Clays The ultimate soil resistance of piles in cohesive soils increases with depth from 2cu (cu is the undrained shear strength) to 8 to 12cu at a depth of 3 times the pile diameter (3d) from the ground surface. However, Broms suggested a constant value of 9cu to be used as the ultimate soil resistance below a depth of 1.5d.
Figure 5:Ultimate lateral load of short pile in cohesive soil (Broms, 1964a)
Figure 6:Ultimate lateral load of long pile in cohesive soil (Broms, 1964a)
The solution for long piles involves the moment capacity of the pile section which should be calculated accordingly. A free-headed pile is considered long when βL > 2.5 and short when βL < 2.5. For a fixed-headed pile, a pile is long when βL > 1.5 and short when βL < 1.5.
β = (kd/4EI)0.25 —— (14)
Where; EI = stiffness of the pile k = coefficient of horizontal subgrade reaction d = width or diameter of pile L = Length of pile
Ultimate Lateral Resistance of Piles in Cohesionless Soil The ultimate lateral resistance of long and short piles embedded in cohesionless soils can be estimated from the chart below. For short piles, the dimensionless quantity Pu/γd3Kp is plotted against the L/d ratio while in long piles, Pu/γd3Kp is plotted against My/γd4Kp.
Where; γ = the effective unit weight of the soil Kp =Rankine’s passive earth pressure coefficient My = Ulitimate moment capacity of the section
Figure 7: Ultimate lateral load of short pile in cohesionless soil (Broms, 1964b)
Figure 8:Ultimate lateral load of long pile in cohesionless soil (Broms, 1964b)
Solved Example
A reinforced concrete square pile of dimensions 450 x 450 mm is driven to a depth of 20m in medium dense sand. The sand is in a saturated state. A lateral load of 250 kN is applied to the pile at 1 m at the ground level. Calculate the following;
(a) Lateral deflection of the pile at the ground level when the head is free (Use Matock and Reese Method) (b) Lateral deflection of the pile at the ground level when the head is restrained (Use Matock and Reese Method) (c) Ultimate lateral resistance of the pile (Using Brom’s method)
(nh = 15000 kN/m2/m; fcu = 40 N/mm2; Asprov = 12Y20 (3768 mm2); φ = 35o; Mu = 445 kNm; Submerged unit weight of soil = 8.75 kN/m3)
Solution Ignoring the reinforcements, the flexural rigidity of the pile can be calculated; fcu = 40 N/mm2 Ec = 20 + 0.2fcu = 20 + 0.2(40) = 28 kN/mm2 = 2.8 × 107 kN/m2 I = bh3/12 = (0.45 × 0.453)/12 = 3.417 × 10-3 m4
Therefore, the pile section provided is inadequate for the service lateral load coming to the substructure. The pile section will have to be changed.
References Anderson, J. B., Townsend, F. C. & Grajales, B. (2003): Case history evaluation of laterally loaded piles. J. Geotech. Geoenv. Engng., Am. Soc. Civ. Engrs. 129, No. 3, 187-196.
Basu D., Salgado R., Prezzi M. (2008): Analysis of Laterally Loaded Piles in Multilayered Soil Deposits. Publication FHWA/IN/JTRP-2007/23. Joint Transportation Research Program, Indiana Department of Transportation and Purdue University, West Lafayette, Indiana, 2008. doi: 10.5703/1288284313454
Broms B.B. (1964a): Lateral Resistance of Piles in Cohesive Soils. Journal of the Soil Mechanics and Foundations Division, 1964, Vol. 90, Issue 2, 27-6
Broms B.B. (1964b): Lateral Resistance of Piles in Cohesionless Soils. Journal of the Soil Mechanics and Foundations Division, 1964, Vol. 90, Issue 3, 123-156
Kim, T. K., Kim, N-K, Lee, W. J. & Kim, Y. S. (2004). Experimental load-transfer curves of laterally loaded piles in Nak-Dong river sand. J. Geotech. Geoenv. Engng., Am. Soc. Civ. Engrs. 130, No. 4, 416-425.
Matlock H. (1970): Correlations for Design of Laterally Loaded Piles in Soft Clay. Proceedings to the 2nd Offshore Technology Conference, Houston Texas, Vol 1
Murthy V.N.S. (2012): Soil Mechanics and Foundation Engineering. CBS Publishers and Distributors Pvt. Ltd, New Delhi, India
Reese, L.C. (1997): Analysis of laterally loaded piles in weak rock. J. Geotech. Geoenviron. Eng. 123 (11), 1010–1017
Reese, L.C., Cox, W.R., Koop, F.D.: Field testing and analysis of laterally loaded piles in stiff clay (1974): In Proceedings of the 6th Annual Offshore Technology Conference, pp. 672–690. Houston, Texas, 2(OTC 2312)
In the construction of any substructure below the ground surface, excavations must be made to the recommended depth before the construction can begin. In the circumstances where deep vertical cuts (say about 3m) are to be made in a soil, provision must be made for the support of the soil to stop it from caving in or collapsing. The design of braced cuts involves the evaluation of the stresses in the members (bracings) used for supporting the cut, and provision of adequate members sizes or sections to avoid structural failure. These are usually classified as temporary works in construction.
Most shallow foundations in cohesive-frictional soils may not need side supports to keep the cut stable. Furthermore, when there is enough space, the sides of the excavation may be sloped, thereby eliminating the need for bracings or support. However, when a vertical excavation exceeds 3m depth, the most common practice is to drive vertical timber planks called sheeting along the sides of the excavation. Horizontal beams called wales are used to hold the sheeting in place, while struts running from side to side of the excavation is used to hold the wales in place.
For deeper cuts of about 5-6m, the use of timber sheeting may become uneconomical and steel sheet pile walls may be used (see figure below). Sheet piles are convenient to use because of their high resistance to stresses. On the other hand, they are relatively lightweight and reuseable when compared with timber.
Typical construction of a braced cut using sheet pile walls
The sheet piles are driven around the boundary of the excavation. As the soil is removed from the enclosure, the wales and struts are inserted. The process continues until the excavation is complete. When the width of the excavation is too wide to permit the economical use of struts across the excavation, tiebacks may be used as shown below.
Typical schematic representation of tieback
Lateral Earth Pressure Distribution in Braced Cuts
To design braced excavations (that is to select the wales, struts, sheet piles etc), the engineer must estimate the lateral pressure that the braced cut will be subjected to. The lateral earth pressure distribution of braced cuts is different from that of conventional retaining walls. This is because the deformation of braced cut walls gradually increases with the depth of the excavation. The deformation is influenced by factors such as type of soil, type of equipment used, depth of excavation, and workmanship.
Due to very little wall yielding at the top of the excavation, the pressure will be very close to at rest at the top. At the bottom of the wall with greater yielding, the lateral earth pressure will be lower than the Rankine active earth pressure. As a result of this, the earth pressure distribution of braced cuts varies significantly with that of retaining walls.
Deformation behaviour of braced cuts and retaining walls
Peck in 1969 presented the pressure distribution diagram of braced cuts. The diagrams were developed from observations and studies obtained from the strut load of Berlin subway cut, Munich subway cut, and New York subway cut. He described the pressure as apparent pressure envelopes which represents the fictitious pressure distributions for estimating the strut loads in a braced cut.
Apparent pressure distribution in braced cuts
(a) For cuts in sand The lateral pressure for cuts in sand is given by;
σ = 0.65γHKa
Where; γ = Soil unit weight H = Height of the cut Ka = Rankine’s active earth pressure
(b) For cuts in soft-medium clay (γH/c > 4)
The lateral pressure is the larger of;
σ = γH[1 – (4c/γH)] or σ = 0.3γH
(c) For cuts in stiff clay (γH/c ≤ 4)
σ = 0.2γH to 0.4γH (average of 0.3γH)
Where c is the undrained cohesion of the soil.
When using the pressure envelops in design, it is very important to keep the following in mind;
The pressure envelope is sometimes referred to as the apparent pressure envelope. The actual pressure distribution depends on the construction sequence and the flexibility of the wall.
The water table is below the bottom of the cut
Sand is assumed to be drained with no pore water pressure
Clay is assumed to be undrained, and pore water pressure is not considered
The excavation depth is greater than about 6 m
Design of the various components of a braced cut
Struts Struts in braced cuts behave like horizontal columns. Hence, the load-carrying capacity depends on the slenderness ratio and flexural buckling capacity. During construction, it is recommended that struts should have a minimum vertical spacing of 3m. In clay soils, the depth of the first strut below the ground surface should be less than the depth of the tensile crack.
To determine the load in the strut, the following procedure can be used;
Draw the pressure envelope of the braced cut and show the strut levels. Assume that the sheet pile walls are hinged at the strut levels except for the top and bottom ones.
Determine the support reaction of the cantilever (for the top and bottom struts), and use simply supported assumption to determine the intermediate struts
Sum up the reactions to determine the load in each strut
Provide and appropriate section for the strut using a suitable design code
Sheet Piles The following steps may be taken in the design of a sheet pile wall;
For each of the section (in plan), determine the maximum bending moment
Pick the highest value of the maximum bending moments (in kNm/m)
Obtain the required section modulus of the sheet pile wall Sx = Mmax/Allowable stress of the sheet pile material
Choose a sheet pile section with a slightly higher section modulus
Wales Wales may be treated as continuous horizontal members if they are properly spliced. Conservatively, they can be assumed to be pinned at the struts.
Design of Braced Cuts – Solved Example
A long trench is excavated in a medium dense sand for the basement of a highrise building. The excavation is supported with sheet pile walls held in place by struts and wales as shown in the figure below. The struts are spaced 4m c/c in plan.
Determine;
(a) The pressure envelope on the wall (b) The loads in the struts (c) The maximum moment for determining an appropriate section for the sheet piles (d) The maximum moment for determining the section for the wales
Solution (a) The pressure envelope on the wall For a braced cut in sand, the apparent pressure envelope is given by;
(c) Bending moment on the sheet pile walls By looking at the shear force diagram, we can confirm that the maximum moment will occur at the cantilever regions and at the points of contraflexure for shear.
For section A-B, the point of contraflexure of the shear force diagram can be obtained using similar triangle;
The maximum moment for the section is 37.25 kNm/m and should be used for the design of the sheet pile wall when factored.
(d) Design moment of the wales We can use the maximum strut load to design the wale.
Mmax = PL2/8
Where L is the spacing of the strut in plan Mmax = (111.746 x 42)/8 = 223.492 kNm/m The design can be carried out accordingly.
Special considerations should be given to braced cuts in layered soils (kindly refer to standard geotechnical engineering textbooks). Furthermore, heave at the bottom of braced cuts in clay soils should also be considered where applicable.
Site managers are saddled with the responsibility of making material requisition and ensuring that construction progresses without too many glitches. One of the major causes of delay in construction is the unavailability of materials on site. Therefore a site engineer is expected to have very quick tips on how to estimate the quantity of materials required to execute a given item of work.
In this article, we are going to provide a basic quantity estimate guide for common construction materials that are used on site. The list is not exhaustive and will be updated from time to time.
(a) Quantity Estimation Guide for Concrete Works
The materials that are required for the production of normal weight concrete are cement, sand, coarse aggregates, and water. For normal strength concrete, the quantity of cement varies with the concrete grade, while the quantity of sand and coarse aggregate remains approximately constant. To estimate the quantity of materials required for concrete works, the first step is to calculate the volume of concrete required for the job.
For slabs; the volume of concrete required (m3) = Area of the floor slab (m2) × thickness of slab (m) For beams; the volume of concrete required (m3) = Depth of beam (m) × width of beam (m) × length of the beam (m) For columns; the volume of concrete required (m3) = length of column (m) × width of column (m) × height of column (m)
For instance, for a (230 x 230)mm column in a duplex, the volume of concrete required is 0.16 m3 (i.e 0.23 m × 0.23 m × 3m = 0.16 m3) assuming that the storey height is 3 m. This volume can be multiplied by the number of columns in the building to obtain the total volume of concrete required for casting all the columns in the building.
After obtaining the volume of concrete required, the quantity of materials required for the production of one cubic metre of concrete can be estimated as follows;
(i) 7 bags (350 kg) of cement is required to produce one cubic metre (1m3)of grade 25 concrete (ii) 1200 kg (1.2 tonnes) of sand is required to produce one cubic metre (1m3) of grade 25 concrete (iii) 1400 kg (1.4 tonnes) of granite is required to produce one cubic metre (1m3) of grade 25 concrete (iv) 175 – 210 litres of water is required to produce one cubic metre (1m3) of grade 25 concrete
The quantities listed above require no additional allowance for waste.
(b) Quantity Estimation Guide for Reinforcement Works
The quantity of reinforcements required can vary considerably depending on the structural design output of the structure. Therefore, it is important to refer to the reinforcement detailing and bar bending schedule for quantity estimation. It is also important to note that the bar bending schedule issued by structural engineers does not take into account offcuts and wastage. It is therefore, important to consider the propriety of bar bending schedule for construction purposes. More often than not, contractors prepare their own bar bending schedule (taking offcuts into account) before placing an order.
Reinforcements are usually quantified by weight (kg or tonnes), which can be converted to length (reinforcements are usually supplied per 12 m length). The unit weight of different sizes of reinforcements are as follows;
Diameter of bar (mm)
8
10
12
16
20
25
Unit weight (kg/m)
0.395
0.616
0.888
1.579
2.466
3.854
Reinforcement Quantity Estimation in the Floor Slab of a Duplex
In a floor slab of about 4-5 bedrooms duplex (200 – 250 m2 of floor slab area), the reinforcement requirement is about 2.5 – 4.0 tonnes of Y12 mm (main reinforcements) and 0.3 to 0.6 tonnes of Y10 mm as distribution bars. The exact quantity is determined from the design drawings.
In the absence of design drawings and an idea is needed about the probable cost of reinforcement for a floor slab, the following relationship can be used for slabs of moderate span;
12.5 kg of Y12mm bottom reinforcement is needed per square metre (1 m2) of slab
20% to 30% of the weight of Y12mm (in kg) provided at the bottom is needed at the top (hogging areas)
10% to 15% of the weight of Y12mm (in kg) provided at the bottom is needed as Y10mm (distribution bar of hogging reinforcements)
About 12 kg of binding wire is required to tie 1 tonne of reinforcement
Example Estimate the quantity of reinforcement needed to construct a 250 m2 of moderately spanned floor slab in a duplex.
Solution Quantity of bottom reinforcement required = 12.5 kg × 250 = 3125 kg = 3.125 tonnes of Y12 mm Quantity of top reinforcement required (using 25% of bottom reinforcement) = 0.25 × 3125 = 781 kg = 0.781 tonnes of Y12 mm Quantity of distribution bars required (using 12% of bottom reinforcement) = 0.12 × 3125 = 375 kg = 0.375 tonnes of Y10 mm The total quantity of reinforcement required = 4.281 tonnes Quantity of binding wire required = 12 × 4.281 = 52 kg
Note that this quantity can considerably vary based on the design result. Therefore, the quantity presented here is just a rough idea, and the reinforcements are assumed to be spaced at 150 – 175 mm c/c. This may however be an uneconomical design, but we need a rough idea of the cost, right? When the reinforcement is spaced at 200 mm c/c, 10.7 kg per square metre can be assumed for the bottom reinforcements.
Reinforcement Quantity Estimation in the Beams of a Duplex
In the absence of design results, the quantity of reinforcement required in beams of moderate spans (say equal to or less than 4m) in a duplex can be estimated as follows;
7.9 – 9.5 kg of Y16mm per linear metre of a beam
2.5 kg of Y8mm per linear metre of a beam as links/stirrups
3.55 kg of Y12 mm per linear metre of a lintel
2.5 kg of Y8mm per linear metre of a lintel as links/stirrups
Please note that the quantity stated above is not guaranteed to satisfy structural requirements. Consult a structural engineer for this purpose.
Example Estimate the quantity of floor beam reinforcement required in a moderate duplex if the total length of all the floor beams in the building is 130 m.
Solution Quantity of Y16 mm bars required = 9.5 × 130 = 1235 kg = 1.235 tonnes of Y16 mm Quantity of links reinforcement required = 2.5 × 130 = 325 kg = 0.325 tonnes of Y8 mm
Reinforcement Quantity Estimation in the Columns of a Duplex
In the absence of design results, the following can be used to estimate the quantity of reinforcement in columns of 230 x 230 mm dimensions in a duplex (one suspended floor only).
25 kg of Y16mm per 3m high column
5.33 kg of Y8mm per 3m column as links/stirrups
Note that this quantity can considerably vary based on the design result.
Example Estimate the quantity of reinforcement required to construct 25 number of 230 x 230 columns in the ground floor of a duplex. All spans in the building are moderate.
Solution Quantity of Y16mm required = 25 × 25 = 625 kg (0.625 tonnes of Y16 mm) Quantity of Y8mm required = 25 × 5.33 = 133.25 (0.133 tonnes of Y8 mm)
12 pieces of 1″ x 12″ x 12′ plank formwork is required to cover an area of ten square metres (10 m2)
4 pieces of marine board (1.2 m x 2.4m) is required to cover an area of ten square metres (10 m2) 1 piece of 2″ x 3″ x 12′ softwood is required per square meter (1 m2) of slab
(f) Miscellaneous
The weight of one bag of cement is 50 kg The volume of one bag of cement = 0.034 m3 The volume of a standard builder’s wheelbarrow = 0.065 m3 The volume of one standard head pan is = 0.0175 m3
These quantities are true when the headpan/wheelbarrow is unheaped
The demolition of a concrete structure may be done for a lot of reasons, irrespective of the cost and environmental implications. It is not always an easy easy decision to make, but sometimes, it just has to be done. It is therefore important to know the right conditions when the demolition of a building is necessary or important. The owner, consultant, or government agencies can order the demolition of a concrete structure due to the following reasons;
(1) Violation of building regulations Buildings or parts of buildings that are constructed without approval may be marked for demolition by the government or town planning agencies. These include buildings that violate regulations such as setbacks, airspace, etc.
Such demolitions may be in part or in whole. On the other hand, buildings that are built over canals, drainage, high voltage transmission lines, are also usually marked for demolition. Constructing an industrial building in a residential area without approval and vice versa can lead to a demolition order too. They are generally described as illegal structures.
(2) Court/government order The court or government can give orders for the demolition of a building due to legal reasons or otherwise. For instance, during the 2020 COVID-19 pandemic, a governor in Nigeria ordered and supervised the demolition of a hotel for violating lockdown orders. Sometimes, orders are given for the outright demolition of buildings that are used to perpetrate crimes such as criminal hideouts, kidnapper’s den, etc.
(3) Construction defects The client, consultant, or supervising agency can order the demolition of parts or the whole of a construction work due to gross defects. If the defects are deemed too serious that it will compromise the integrity of the finished structure, the demolition of the defective member(s) may be ordered.
(4) Marked as unsafe Distressed or old buildings that are deemed to pose a safety risk can be marked for demolition by regulatory agencies.
(5) Encroachment into the right of way of other developments When structures encroach into the right of way of new major developments such as highways, railways, airports, etc, they can be marked for demolition. However, for buildings that have approval, appropriate compensation is usually provided for the owners of the building. For instance, it has been reported that the proposed Fourth Mainland Bridge in Lagos, Nigeria, will affect about 800 houses.
(6) Property owner’s desire to get rid of a structure A property owner can decide to demolish an existing structure in order to erect a new one or use the space for another thing.
Generally, the demolition of a reinforced concrete structure can be a very challenging and expensive task, and oftentimes, approval may be needed from the local government before the commencement of the operation. Due to the cost and environmental implications of demolishing a building, it should always be a last resort after all other options have been explored. There are different methods of demolishing a building, but that is outside the scope of this article.
While an imminent demolition of a building is possible due to the six reasons given above, we are going to focus on the number (3), which directly affects structural engineers.
Demolition of a Building for Structural Reasons
Structural engineers are often invited to approve, evaluate, or assess existing buildings or buildings under construction. For an existing building or on-going construction to be deemed fit for human habitation, there must be a minimum safety standard for all parts and the whole of the building.
The latest design standard used all over the world for reinforced concrete design is the limit state design principle. In limit state design, a limit of performance is defined for a structure for it to be deemed acceptable. Generally, a structure is expected to satisfy the ultimate and serviceability limit state requirements.
During design, the limit state concept is achieved by factoring the loads and materials to be used in the construction. The concept also takes into account all imperfections and geometric tolerances. The generality of the concept is that the effects of the design actions should be less than the resistance of the structure. This defines the accepted limit state.
While this can easily be achieved during design, construction defects can compromise the safety of a well-designed building. Furthermore, when a building is exposed to an earthquake, fire, or accidental impact, some level of damages may be observed that will demand the assessment of the structural integrity of the building. It is also important to evaluate the capacity of an existing building when the occupancy class is to be changed from say residential to a storage house.
During construction, defects can come from poor quality of materials or poor workmanship. Some scary construction defects that require serious attention are:
(1) Extreme honeycombs (2) Very poor concrete strength (3) Inadequate yield strength of reinforcement (4) Inadequate member sizes (5) Improper placement and arrangement of reinforcements (6) Insufficient area of reinforcement (7) Poor concrete cover, etc
When a structural defect is observed in construction, something MUST be done to address it, and the onus lies on the structural engineer to provide guidance on what is to be done. This is because the safety of the occupants of the building during and after construction is not negotiable. Nothing should be left to ‘luck’ or ‘guesses’.
Due to the cost and environmental implications, a consultant should order demolition as a last resort unless nothing else can be done. Even when the member sizes and reinforcements provided are inadequate, all options should be explored before a decision is taken.
Some options to be explored are:
(1) Redesign/assessment of the building based on what is existing When the member sizes and areas of reinforcement provided are not in conformity with the design, the building can be remodelled and redesigned to check the capacity of what is existing. The design parameters such as concrete grade and yield strength should be based on the actual properties of the materials used in the construction. As a result, tests like non-destructive testing of concrete and the reinforcement should be carried out.
The engineer should also inspect the building thoroughly for cracks and evidence of structural distress. This can help him predict what is happening to the building or the nature of stresses causing the distress.
When the assessment is done, the structural engineer will then have a basis for making a decision. If the construction is still ongoing, the load path or the structural scheme can be modified accordingly. The other options listed below can be explored too. The cost of this assessment should be decided between the property owner and the contractor.
(2) Introducing additional supports/members to change the load distribution Additional supports or members can be introduced into an on-going or existing buildings in order to alter the load/stress distribution in the structure. This should be an informed decision from the structural engineer after understanding the current state of the building, or the effects of the existing structural arrangement.
(3) Introducing steel members and polymer composites Where possible, structural steel members such as stanchions, universal beams, flats, plates, or angles can be introduced to provide external support to failing structural members. This is a form of retrofitting on its own though. According to a 2016 study in Indonesia, steel plates and fasteners can be used as external reinforcements when the rebars in a structure are inadequate. The use of epoxy grouting, steel plates, and bolt fasteners have been observed to increase the flexural capacity of the strengthened members.
Materials like fibre reinforced polymers (FRP) have also found applications in the strengthening of beams and slabs. The usual strengthening method presumes the placing of the sheets bonded on the tensioned side using resins. The sheets are mounted parallel to the long edge of the slabs.
(4) Retrofitting Retrofitting is basically the process of adding or incorporating new features that were not previously included in an existing design. There are different methods of retrofitting reinforced concrete structures such as concrete jacketing, steel jacketing, use of fibre-reinforced polymer, etc.
One way of retrofitting reinforced concrete beams is by concrete jacketing. In this method, an additional layer of concrete is placed around the existing beam, together with additional longitudinal bars and stirrups, to enhance the flexural and/or shear capacities. Other options of retrofitting beams are the use of fibre-reinforced polymer (FRP) wrapping and the use of steel brackets or haunches.
Concrete jacketing of reinforced concrete columns involves adding concrete with longitudinal and transverse reinforcement around the existing columns. This type of strengthening improves the axial and shear strength of columns while the flexural strength of column and strength of the beam-column joints will likely remain the same.
Concrete jacketing involves drilling holes into the concrete members and producing a very workable but strong concrete mixture. The compatibility of deformation between the existing and new concrete, resistance against delamination, and durability should also be carefully looked into.
It is important to note that there is a price to pay for making these kinds of changes. Apart from the increased cost of construction and safety risks, the architectural concept of the building may also be altered.
If a consultant gets to a site, he can order the demolition of the structural member if the following are observed after thorough tests and investigations. This is a case where the options listed above are not feasible:
The reinforcement provided in the structural members is less than the minimum required, with respect to the expected load on the member.
The reinforcements are poorly placed. For example, placing the reinforcements of a cantilever at the bottom (the props should not be removed in the first place).
The member sizes provided are less than the design capacity
The compressive strength of concrete is less than the design strength
The yield strength of the reinforcement is less than the design strength.
Thank you very much for visiting Structville today.
Hemp and Recycled Aggregates Concrete (HRAC) is a sustainable concrete where coarse aggregates are partially replaced with industrial hemp fibers and recycled concrete aggregates (RCA) [1]. In the bid to achieve sustainable materials in construction and also reduce the carbon footprint and environmental impacts of concrete, researchers have been coming up with various ways to beneficially reuse industrial and agricultural wastes as construction materials.
According to researchers from the Department of Civil and Environmental Engineering, American University of Beirut, Lebanon, the use of HRAC in construction has two main benefits which are; savings on natural resources, and reuse/recycling of waste materials [1].
Hemp fiber is a natural material that has been used extensively in producing a lot of things such as rope, clothing, shoes, accessories, etc. It has also been used with other materials to produce plastics and composites for the automobile industry, and also in the production of bricks and mortars in building construction [2]. Hemp fiber has also been used in the production of fibreboard, thermal insulation in homes, and hempcrete. As a matter of fact, hemp fibers are regarded as one of the toughest natural fibers [3].
Fig 1: Hemp fibers (Source [3])
Recycled concrete aggregate (RCA) is obtained by breaking the lumps of old or demolished concrete works and extracting the coarse aggregates. One of the major reasons to use RCA in structural concrete is to have more environmentally friendly construction [4]. The use of RCA on a large scale may help to reduce the effects of the construction on the environment by reusing waste materials and preventing more natural resources from being harvested.
Fig 2: Recycled Concrete Aggregate (Source [4])
Some scholars have researched the use of hemp fiber in the production of concrete. A 2008 study in France showed that treating hemp fiber in alkaline improved the fiber strength and the fiber-matrix adhesion in concrete [5]. Another study by [6] showed that the strength of hemp fibers can be improved with low lignin content and good fiber separation when treated in 10% (by weight) NaOH (Sodium hydroxide) solution. The hemp fibers were observed to have an average tensile strength of 857 MPa and Young’s modulus of 58 GPa.
Li et al [7] carried out a study to determine the properties of hemp fibre reinforced concrete. The study showed that the addition of hemp fibre into the concrete matrix results in a linear reduction in the specific gravity and the water absorption ratio of the matrix. Furthermore, the compressive strength, flexural strength, toughness and toughness indices, specific gravity, and water absorption ratio of hemp fibre reinforced concrete are all correlated with aggregate size parameters, fibre factors and matrix initial mechanical properties.
A 2014 study by [8] showed that hemp fiber has no negative effect on the strength of the concrete in the long term.
Previous researches done on the strength of concrete produced with recycled concrete aggregate showed minor reductions in the various mechanical properties including compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity. According to [4], the density of RCA is lesser than the density of natural aggregates due to residual adhered mortar. A study by [9] showed that the relative density of RCA (in the saturated surface dry condition) is approximately 7–9 % lower than that of natural aggregate.
Research has shown that the natural aggregate in a concrete mix may need to be replaced with about 25-30% of RCA before significant changes can occur in the compressive strength [4]. A 2008 study attributed the reduction in compressive strength of RCA to water higher water adsorption when compared with natural aggregates [10].
In a 2020 study of HRAC by [11], the coarse aggregate content was reduced by 20% of the concrete volume, and 50% of the natural coarse aggregates were replaced by recycled concrete aggregates. The variables of the study included percentage replacement of natural aggregates with RCA (0 or 50%), maximum size aggregate (10 and 20 mm), hemp fiber length (20 and 30 mm), and fiber surface treatment (alkali, silane, and acetyl).
Fig 3: Hemp hurds (Source [12])
From the study, the replacement of 50% of natural aggregate with RCA reduced the tested mechanical properties by 1 to 10% when the maximum size of aggregate was 10 mm and by 4 to 13% when maximum size of aggregate was 20 mm. When fibers were incorporated in the mix and the coarse aggregate content was reduced by 20%, the compressive strength of the concrete reduced by 37% for maximum size aggregate of 10 mm relative to the control mix.
For maximum size aggregate of 20mm, compressive strength reduced by 31.1% relative to the control mix. The study recommended that HRAC mixes should not be used in members subjected to direct compression such as columns.
Other mechanical properties such as modulus of elasticity, modulus of rupture, and splitting tensile strength and long term/durability test results from the study can be obtained from the open access article.
References [1] Ghosn S., Hamad B. (2020): Durability Evaluation of Hemp Fibers and Recycled Aggregates Concrete. In Proceedings to the XV International Conference on Durability of Building Materials and Components DBMC 2020, Barcelona [2] https://en.wikipedia.org/wiki/Hemp#Fiber(Assessed on the 5th of December, 2020) [3] https://hempgazette.com/industrial-hemp/hemp-fiber-production/(Assessed on the 5th of December, 2020) [4] McNeil, K., Kang, T.HK (2013): Recycled Concrete Aggregates: A Review. Int J Concr Struct Mater7, 61–69. https://doi.org/10.1007/s40069-013-0032-5 [5] Sedan, D., Pagnoux, C., Smith, A., & Chotard, T. (2008). Mechanical properties of hemp fibre reinforced cement: influence of the fibre/matrix interaction. Journal of the European Ceramic Society,28(1), 183–192. [6] Pickering, K. L., Beckermann, G. W., Alam, S. N., & Foreman, N. J. (2007). Optimising industrial hemp fibre for composites. Composites Part A—Applied Science and Manufacturing,38(2), 461–468. [7] Li, Z., Wang, X., & Wang, L. (2006). Properties of hemp fibre reinforced concrete composites, Composites: Part A Applied Science and Manufacturing, 37(3), 497–505. [8] Awwad, E., Mabsout, M., Hamad, B., Farran, M.T. and Khatib, H. (2012): Studies on fiber-reinforced concrete using industrial hemp fibers. Construction and Building Materials, 35, 710-717 [9] Limbachiya, M. C., Leelawat, T., & Dhir, R. K. (2000): Use of recycled concrete aggregate in high-strength concrete. Materials and Structures, 33, 574–580 [10] Yang, K.-H., Chung, H.-S., & Ashour, A. F. (2008): Influence of type and replacement level of recycled aggregates on concrete properties. ACI Materials Journal, 105(3), 289–296. [11] Ghosn, S., Cherkawi, N. & Hamad, B. (2010): Studies on Hemp and Recycled Aggregate Concrete. Int J Concr Struct Mater14, 54 (2020). https://doi.org/10.1186/s40069-020-00429-6 [12] Novakova P., Sal J. (2019): Use of technical hemp for concrete – Hempcrete. IOP Conf. Series: Materials Science and Engineering 603 (2019) 052095 doi:10.1088/1757-899X/603/5/052095
Featured Image Credit: Novakova P., Sal J. (2019): Use of technical hemp for concrete – Hempcrete. IOP Conf. Series: Materials Science and Engineering 603 (2019) 052095 doi:10.1088/1757-899X/603/5/052095
To ensure adequate planning and delivery of a project, consultants and clients often require that the contractor submit a programme of work (POW) that shows the sequence of activities that will be carried out in order to meet the expected completion date of the project. A construction work programme is a document that shows the series of work items, their relationship, and the time allocated for their execution. They are like the road map to the proposed construction.
Clients, consultants, project managers, and construction site managers also use POW to plan milestones which is an important tool or key performance indicators (KPIs) for milestone payment or achievement.
The POW can be presented in the form of a Gantt chart showing all the tasks/activities, duration, start and finish date and milestones from site mobilisation to commissioning according to the scope of work. It also serves as a control document to monitor the progress of work with respect to the actual work done. A report could be provided weekly or monthly regarding the status of work to the project stakeholder.
The level of details required from the POW is usually defined by the consultant and it desirably kept simple. However, for POW done for in-house usage, resources may be added. POW is submitted every monthly site meeting to show project progress to project stakeholders. However, when it appears that the progress of work is critically lagging, a request for extension of time (EOT) could be initiated quickly.
Tasks/Activities in a Construction Work
The easiest way to identify tasks in construction is to break the construction work into phases. For typical building construction, these phases can be broken down as follows;
Pre-construction activities
Mobilisation and Setting Out
Substructure (foundation) construction
Concrete frame works (ground floor to first floor)
Block work
Finishes, etc
Under preconstruction activities, tasks such as site clearing, obtaining permission to commence construction, host community negotiations, environmental assessments, harmonization of construction drawings, etc can be identified. Timing and duration can also be assigned to these tasks so as to have a defined target. Under mobilisation and setting out works, all unique tasks involved should also be listed according to the peculiarities of the project.
While construction activities are usually specified in an order, it is possible for two different activities to be going on simultaneously. For instance, reinforcements works can commence while setting out operations are still on-going. The setting of blocks on the ground floor can commence during the casting of columns on the second floor, etc.
Therefore all tasks in a project are identified bearing in mind the resources that are available for their successful completion. All machinery, man-power, materials, and funds needed to execute any item of work should be given special consideration during the preparation of the programme of work. For instance, a contractor who intends to use ready mix concrete and pumping of concrete may not have the same programme of work with a contractor who wishes to use manual labour.
After the activities are defined and set up, another important aspect is to establish the relationship among the tasks. The major importance of this is to figure out the critical path and the best sequence of works. There are some activities which when they are not done can stop almost every other activity on site. For instance, during substructure works, excavation should be done quickly and completely because without it being completed, every other activity is affected.
Tasks in a construction work can be connected to each other in four ways. These are;
Finish-to-Start (FS): If the activity can’t start without finishing the predecessor activity, it is called an FS relationship.
Start-to-Start (SS): If for any reason, two or more activities must start together, it is said they have a SS relationship.
Finish-to-Finish (FF): If two or more activities finish together, they have a FF relationship.
Start-to-Finish (SF): If an activity can’t be finished until another activity starts, that means they have an SF relationship.
Once the sequence of all the activities has been identified, the project manager can issue the program of work. For simplification, programme of work can also be broken down and handed over the site manager. An example of a simplified programme of work for casting a first-floor slab is given below;
Duration of Tasks
It is important that an experienced execution team be involved in the planning or at least understand how the planning was done. Many projects have failed to achieve proper success because of an improperly thought out timeline assigned to an activity. There are some documents or magazines where someone can pick the duration of works, but the person has to pay proper attention to the conditions that warranted such duration.
In our own point of view, the duration assigned to any item of work should be based on experience. If a comparable item of work has been completed in the past, the project manager should borrow ideas from the project based on the number of workers deployed, the machinery used, the weather conditions, etc. In the image below, we present a simple POW for the finishes of a building.
Advantages of Programme of Works
It serves as part of KPIs for a project
Work plans are generated from POW
Data gathering
To boost credibility to show understanding of what the scope of the project entails from initiation to closeout.
Special Considerations for Preparation of Programme of Works
Understanding of the contract documents
Bill and drawings
Phasing, milestone, and milestone payment if any
Scope of work and work breakdown structure
Method of construction (client and contractor)
Rate and data for resources allocation to each task/activities
Weather condition, period of the year, and associated risk(s)
Cash flow (client and contractor)
Timeline and cost relationship of task and targets
Logistics.
Sample Programme of Works for a Residential Building
A simple programme for a building to be completed in 3 months has been attached in pdf format (see below for the screenshot of the attached sample of a construction programme of work). You can download the PDF file below.
PS: We can help you manage your civil engineering projects of any kind inclusive of research, design, and construction. We pride ourselves in excellence and creativity. Send an e-mail to info@structville.com
