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
Pad foundations are isolated rectangular, square, or circular slabs that are provided under reinforced concrete columns or column stubs to safely transmit the column load to the ground. They are a type of shallow foundation that is widely used all over the world especially in areas where the soil possesses good bearing capacity. They are also referred to as isolated bases or spread footings. The design of pad foundations involves sizing the base slab to satisfy geotechnical requirements and providing adequate thickness and reinforcements to satisfy structural requirements.
The dimensions of a pad foundation should not be too small so as to cause excessive settlement or bearing capacity failure of the soil. As a matter of fact, allowable bearing capacity is normally used to control settlement during the design of a pad foundation, hence it is treated as a serviceability limit state parameter. The width of a pad foundation is expected not to be less than 1000 mm, and the thickness not less than 150 mm.
Geotechnical Design of Pad Foundation
The geotechnical design of a pad foundation can be carried out according to the requirements of EN 1997-1:2004 (Eurocode 7). Eurocode 7 gives three approaches for the geotechnical design of foundations and they are as follows;
Design Approach 1 (DA1): In this approach, partial factors are applied to actions and to ground strength parameters. Design Approach 2 (DA2): In this approach, partial factors are applied to actions or to the effects of actions and to ground resistances. Design Approach 3 (DA3): In this approach, partial factors are applied to actions or to the effects of actions from the structure and to ground strength parameters.
The three approaches can give very different results when applied in design. However, the UK National Annex to Eurocode 7 permits only Design Approach 1 (DA1). For the design of pad foundation using Design Approach 1, three limit states with their appropriate load combination shall be satisfied for the structure. These limit states are;
EQU: Loss of equilibrium of the structure STR: Internal failure or excessive deformation of the structure itself GEO: Failure due to excessive deformation of the ground supporting the structure UPL: Failure due to uplift of the foundation due to water pressure HYD: Failure due to hydraulic gradient
In the design of pad foundation using DA1, there are two sets of limit state combinations for STR and GEO limit states. Combination 1 is normally used for the structural design of the foundation, while combination 2 is normally used for sizing the foundation. The partial factors for the limit states are given in the table below;
The partial factors for EQU, UPL, and HYD are given in the Table below. They can also be used in the uplift verification of all kind of buried structures.
The partial factors for soil properties is given in the table below;
It should be noted that pad foundations fall under category 2 structures which means that they are conventional structures that are founded on non-difficult grounds. They offer no exceptional geotechnical risk. As a result, routine procedures for field and laboratory testing for design and execution may be used. Geotechnical design of pad foundations can be done by geotechnical or structural engineers. However, the geotechnical design of category 3 structures with abnormal risk can be done by geotechnical engineers only.
The design of pad foundations can be done using any of the following methods;
Using the analytical (direct) method, all limit states should be verified. The ultimate bearing capacity qult of a pad foundation should be verified using the expression below;
where; c = cohesion q = overburden γ = body-weight Ni = bearing capacity factors si = shape factors di = depth factors ii = inclination factors gi = ground inclination factors bi = base inclination factors
Example on the calculation of the bearing capacity of a pad foundation using Design Approach 1 (DA1)
Calculate the bearing capacity of a 1m x 1m pad foundation founded at 0.9 m below a lateritic soil deposit. The characteristic angle of shearing resistance φk of the soil is 21° while the effective cohesion c’ is 10 kN/m2. The water table is 8m below the ground surface, and the unit weight of the soil is 18 kN/m3.
Solution
As the footing rests on a cohesive-frictional soil, the relevant material property is the angle of shearing resistance, φ and the effective cohesion c’.
Design values of angle of shearing resistance Characteristic value φk = 21°. Note that the safety factor γφ is applied to tan φk not to φk. Combination 1: γφ = 1.0, tan φd = tan φk/γφ= tan 21° = 0.383, φd = 21° Combination 2: γφ = 1.25, tan φd = tan φk/γφ = tan 21/1.25 = 0.307, φd = 17°
Step 4: Calculate the overburden pressure, q. The unit weight of soil is 18 kN/m3 and the safety factor γγ = 1 q = 18 × depth of footing = γγ × 18 × 0.9 = 16.2 kN/m2
For sizing of the foundation, Combination 2 allowable bearing capacity should be used (no other factor of safety is to be applied).
For the semi-empirical (indirect) method, a commonly recognized semi-empirical method such as bearing resistance estimation using pressuremeter test should be used. The use of experience and testing to determine SLS parameters that will also satisfy ULS requirements is generally done. An example can be found in Annex E of EN 1997-1:2004.
When using the prescriptive method, a presumed bearing resistance from BS 8004 should be used. When such a method is applied, the design result should be evaluated on the basis of comparable experience.
Example on the Structural Design of Pad Foundation
In the structural design of pad foundations, the reaction under an axially loaded column base may be assumed to be uniformly distributed if the load is concentric without any bending moment. Otherwise, the pressure distribution may be assumed to be varying linearly across the base as shown below.
(a) Design ultimate bearing pressure For a concentrically loaded pad foundation, the design earth pressure is given by;
q = P/Aprov
Where; P = Design axial force of the column = 1.35Gk + 1.5Qk (kN) Aprov = Base area provided for the footing (m2)
(b)
(b) Bending The critical section for bending is at the face of the column on a pad footing or the wall in a strip footing. The moment is taken on a section passing completely across a pad footing and is due to the ultimate loads on one side of the section. No redistribution of moments should be made.
(c) Beam Shear The vertical shear force is the sum of the loads acting outside the section considered. Shear stress is checked at a distance d from the face of the column. It is normal practice to make the base sufficiently deep so that shear reinforcement is not required. The depth of the base is often controlled by the design for shear.
(d) Punching Shear Rules for checking for punching shear resistance are given in section 6.4 of EN 1992-1-1:2004. The punching shear force is the sum of the loads outside the periphery of the critical section. Two punching shear checks should be carried out – at the column perimeter and at between d – 2d from the face of the column.
Structural Design Example of Pad Foundation
Design a square pad footing for a 250 × 250 mm column carrying a characteristic permanent load Gk of 800 kN and characteristic variable load Qk of 425 kN. The presumed allowable bearing pressure of the non-aggressive soil is 225 kN/m2. fck = 30 N/mm2; fyk = 500 N/mm2; Concrete cover = 50 mm
Pad foundation is a category 2 structure, and this design is to be done using prescriptive methods:
Let 10% of the service load account for the self-weight of the pad foundation. Base area A = 1.1(800 + 425)/225 = 5.99 m2 Minimum dimensions of footing = √5.99 = 2.447m
Adopt a square foundation of 2500 mm x 2500m x (600 mm) trial depth (Area provided Aprov = 6.25 m2)
Loading at ultimate limit state NEd = (1.35 x 800) + (1.5 x 425) = 1717.5 kN
Critical design moment at the face of the column MEd = (274.8 x 1.1252)/2 = 173.89 kNm/m
Effective depth d = 600 – 50 – 16 = 534 mm k = MEd/(bd2fck) = (173.89 x 106)/(100 x 5342 x 30) = 0.0203 ⇒ z = 0.95d = 0.95 x 534 = 507.3 mm ⇒ As = MEd/0.87fykz = (173.89 x 106)/(0.87 x 500 x 507.3) = 788 mm2/m
Provide H16 @ 225 c/c both ways (Asprov = 893 mm2/m)
Beam shear Check critical section d away from column face VEd = 274.8 x (1.125 – 0.534) = 162.4 kN/m vEd = 162.4/534 = 0.304 N/mm2
vRd, c = CRd, c × k × (100 × ρ1 × fck) 0.3333 CRd, c = 0.12 k = 1 + √ (200/d) = 1 + √ (200/534) = 1.611 ρ = 893/(534 × 1000) = 0.00167 vRd, c = 0.12 × 1.611 × (100 × 0.00167 × 30)0.333 = 0.33 N/mm2 => vEd (0.304 N/mm2) < vRd,c (0.33 N/mm2) beam shear ok
Punching Shear Punching shear: Basic control perimeter at 2d from face of column vEd = βVEd/uid < vRd,c
β = 1, ui = (250 x 4 + 534 x 2 x 2 x π) = 7710 mm
VEd = load minus net upward force within the area of the control perimeter) VEd = 1717.5 – 274.8 x (0.252 + π x 1.0682 + 1.068 x 0.25 x 4) = 422 kN vEd = (422 x 103)/(7710 x 534) = 0.102 N/mm2 Punching shear is therefore okay
Using the only data available from this picture (which is the crack pattern), what is likely caused failure of this structure?
The structure is a cantilever and we expect the maximum bending moment and shear force to occur at the support (usually taken as the centreline of the column). Predictions can be made on the cause of the failure based on the nature of the crack…
Kindly let us know your view in the comment section. Thank you and God bless you.
Gabion walls are made up of row upon row of orthogonal cages or baskets (gabions) which are filled with rock fragments/cobbles and tied together. This arrangement forms a block of gravity structure that is able to withstand lateral pressure on it. Furthermore, the permeability of the rock fragments and the flexibility of gabion cages make them particularly suitable for use at sites which are liable to become saturated and where the foundation is composed of relatively compressible materials.
Gabion walls are relatively simple to construct. Where suitable rock is readily available, the use of gabion walls is particularly attractive for reasons of economy and speed of construction. In more recent times, gabion walls are generally used as decorative walls in compounds or gardens and are aesthetically pleasing. However, gabion walls can be designed as retaining structures.
Gabions being used as fence in a home
Gabion wall decoration of a garden
Gabions acting as a retaining wall
When gabions are to be used as retaining structures, they should be considered as gravity retaining walls. There is currently no universally accepted method for designing the individual gabion units. The individual units are placed on each other to form a stable gabion wall. The basic shape of a gabion wall is trapezoidal, but the front and rear faces may be straight or stepped.
It is recommended that back batter be provided for walls higher than about 3 m to improve stability and even out ground bearing pressures. A variety of cage sizes can be produced using suitable materials to suit the terrain. Gabion units are normally in modules of 2 m x 1 m x 1 m.
However, the design method for verification and appropriate functionality of a gabion wall can be carried out at the ultimate limit state analysis (ULS), verifying the following issues;
Soil Bearing: The base pressure applied by the wall must not exceed the ultimate bearing capacity of the supporting soil.
Sliding Resistance: There must be an adequate factor of safety for sliding between the base of the wall and the underlying soil due to the lateral earth pressure. Active earth pressure conditions should be assumed.
Overturning stability: The overturning of the wall due to horizontal earth pressure forces when the retained soil mass become unstable (active failure) should satisfy the required conditions.
Internal stability verification: For each layer of gabion a bearing capacity and sliding resistance verification should be made
Global Stability
Design Considerations of Gabion Walls
The maximum pressure transferred from the gabion wall to the ground should be compared with the safe bearing capacity of the soil. This issue depends on the type of gabion foundation also, which for this case it is considered a non-stiff footing (the gabion is not placed on top of concrete).
Since gabion retaining walls consist of several layers, the internal stability verification should verify that these layers are not collapsing, neither sliding nor the bearing capacity of lower layers lesser than required. These conditions must be satisfied for each layer starting from the top layer to the bottom layer.
The overturning stability of the gabion retaining wall ensures that the soil retained behind the gabion wall is not causing lateral collapse of the wall. For this analysis, it is assumed the gabion wall is rigid body, with no sliding occurrence or partial breakdown between layers. The stabilising force is the weight of the gabion wall, while the destabilising force is the lateral pressure from the retained material or surcharge.
Limit state checks should be carried out at selected planes through the gabion wall, ignoring the resistance contributed by the cage material and the connections between the cages. For stepped walls, stability checks should be carried out at each major change in section shape.
In order to limit deformation, gabion walls should be proportioned in such a way that the resultant force acts within the middle third of the wall’s cross-section. The mobilised angle of wall friction, δ, used in the design should not exceed ϕ’/2, where ϕ’ is the angle of shearing resistance of the compacted backfill. In order for the assumption of δ = ϕ’/2 to hold, the gabion infill must be placed in such manner to achieve a dense mass which will not settle relative to the backfill after construction. Otherwise δ should be assumed to be zero. For a wall to be founded on relatively compressible materials, δ should also be assumed to be zero.
Materials for construction of gabion retaining walls
(1) Gabion Baskets Gabion baskets can be made from a range of materials. Nylon, polypropylene and polyethylene grids have been used. They have the advantage of being lightweight and corrosion-resistant. However, these materials are susceptible to attack by fire and ultraviolet light. A material widely used in the commercial production of gabions is steel wire-mesh, of which there are two types, hexagonal woven and square welded.
The wires used for the wire-mesh in gabion baskets should be mild steel wire with a minimum tensile strength of 350 N/mm2. For permanent applications, the wires should be at least 2.7 mm in diameter and galvanized. Where it is suspected that the infill or retained materials or groundwater are aggressive, the wire mesh should be protected with PVC coating.
(2) Infill Material Rock used for filling gabions should be sound, clean and well-graded. The maximum size of the rock should not exceed two-thirds the depth of the gabion to be filled or 300 mm, whichever is less. The preferred size is 150 mm to 300 mm. The smallest dimension of the rock should at least be twice the largest dimension of the mesh aperture.
(3) Backfill, Filter, and Drainage Materials For a partially-submerged gabion wall, a free-draining granular backfill should be provided so that water pressure will not build up behind the wall when the water level in front of the wall is lowered.
Drainage Considerations for Gabion Walls
A geotextile filter should be provided behind the rear face of the gabion wall to prevent the migration of fines from the backfill into the coarse rock infill. Drainage layers at the rear face are normally not warranted. However, a drainage layer of adequate permeability should be provided at the base of the wall to guard against erosion of the foundation material.
The high permeability of the gabion units will permit direct infiltration through the body of the wall at times of heavy rainfall. In order to minimize the possibility of saturation and erosion of the foundation material under a non-submerged gabion wall, it is good practice to provide a blinding layer with adequate drainage provisions at the level of the foundation. For submerged gabion walls, appropriate measures should be incorporated to prevent scouring and erosion of the foundation.
Design Example of Gabion Walls
In the example below, a 3 m high gabion retaining wall is to be designed according to the requirements of Eurocode 7 (EN 1997-1).
The design parameters and loadings are as follows;
Wall geometry Width of gabion 1; w1 = 2000 mm Height of gabion 1; h1 = 1000 mm Width of gabion 2; w2 = 1700 mm Height of gabion 2; h2 = 1000 mm Step to front face between courses 1 and 2; s2 = 150 mm Width of gabion 3; w3 = 1400 mm Height of gabion 3; h3 = 1000 mm Step to front face between courses 2 and 3; s3 = 150 mm Wall inclination; ε = 0 deg
Gabion properties Unit weight of fill; γd = 16.0 kN/m3 Friction between gabions; δbg.k = 35.0 deg
ASDIP Structural Engineering Software company has announced the release of STEEL-5, the latest version of ASDIP STEEL software module. The Florida, USA, based structural engineering software company has been operational since the year 1992 and offers different modules for civil engineering designs such as reinforced concrete, steel, foundation, and retaining walls. The STEEL-5 is the latest version of the steel package which offers advances and improvements from the previous versions.
ASDIP STEEL software is utilised by professional engineers for the design of steel members and connections. It works with any operating system, and assists structural engineers to transparently design, analyze, check and optimize structural design work. According to the company’s official website, the following are some of the most important features and benefits included in version 5 of ASDIP STEEL;
The new version includes the following five modules:
Base Plate / Anchorage Design – Any combination of axial vertical and horizontal loads, and biaxial moments. It includes uplift and partial bearing analysis. Design of anchor rods and shear lugs. Graphical generation of the breakout areas.
Steel Column Design– Either sway or non-sway columns. Second-order moments analysis to account for slenderness. Multiple load types and load cases. Results sorted by load combination. Graphical generation of the interaction diagram.
Steel / Composite Beam Design – Up to five spans and two overhangs. Multiple load types and load cases. Separate Construction and Final loads analysis. Graphical generation of the shear, and moment diagrams.
Shear Connection Design – Single angle, double angle, Shear Plate, and Tee connections. Multiple options to specify different conditions of the connection elements. Check of the limit states. Graphical generation of the connection in different views.
Moment Connection Design – Flange-Plated, and Welded Flange moment connections. Any combination of vertical loads and moments. Check of the limit states for moment and shear. Graphical generation of the connection in different views.
A Linkedin Post by the Founder of ASDIP, Javier Encinas PE, invited structural engineers across the world to download the 15-day trial of the new release in order to check the power and capabilities of the STEEL-5. According to him,
“For some weeks we have announced in advance the coming release of ASDIP STEEL 5, our software for structural steel design. As promised, ASDIP STEEL 5 has been released. In addition to an upgraded layout design and literally dozens of improvements, it includes great new features, such as:
Moment connections (NEW module) AISC 360-16 and ACI 318-19 compliance Custom load combinations Biaxial base plates and anchorage Continuous steel / composite beam
You are invited to download the 15-day free trial and check the software by yourself, hands-on. I have prepared a blog post and a short video with a brief overview of this new version of ASDIP STEEL. Your comments and suggestions are always welcome.”
Reinforced concrete (R.C.) slabs are plate elements used to form the floors of buildings. In a typical reinforced concrete building, reinforcement bars arranged as mats are incorporated into a concrete plate of minimum thickness 125 mm to form a reinforced concrete solid slab. The provision of adequate reinforcement, slab thickness, and proper detailing to satisfy ultimate and serviceability limit state requirements forms the basis of the design of reinforced concrete (R.C.) slab. Satisfying other requirements such as durability, fire resistance, etc are also necessary.
Floor slabs are usually subjected to uniformly distributed loads, partially distributed loads, line loads, or concentrated loads in the transverse direction. A beam is similar to a slab in so many ways but there are fundamental differences in behaviour and stress distribution of the two elements. While a beam is generically a one-dimensional element, a plate is a two-dimensional element. Due to the two-dimensional nature of a slab, it is subjected not only to bending moments Mxx and Myy and shear forces Vxand Vy but also to twisting moments Mxy on all the four faces.
Fig 1: Beam element and slab element
Types of Reinforced Concrete Slabs
A monolithic reinforced concrete slab is essentially a statically indeterminate structure. For a slab of a given shape and support conditions, the distribution of shear forces, bending, and twisting moments in the slab due to externally applied loads cannot be determined easily. The column layout in a monolithic reinforced concrete structure often forms a rectangular grid. Continuous beams may be provided in one direction or two orthogonal directions, to support slabs that may be solid or ribbed in cross-section. Alternatively, slabs can be supported directly on columns to form flat slabs.
Slabs may be simply supported or continuous over one or more supports and are classified according to the method of support as follows:
One-way spanning slab between beams or walls
Two-way spanning slab between the support beams or walls
Flat slabs carried on columns and edge beams or walls with no interior beams
Ribbed slab transferring slab load to beams and walls in one direction
Waffle slab transferring slab load to beams and slabs in two direction
One-way Spanning Solid Slabs
Clause 5.3.1(5) of EN 1992-1-1:2004 suggests that a slab subjected dominantly to uniformly distributed loads may be considered as one-way spanning if either:
It possesses two free (unsupported) and sensibly parallel edges.
It is the central part of a sensibly rectangular slab supported on four edges with a ratio of longer (Ly) to shorter span (Lx) greater than 2.
Fig 2: Typical general arrangement of a one-way slab system
In other words, when beams are provided in one parallel direction only, the slab is a one-way slab. Furthermore, if the longer side of a slab panel exceeds twice the shorter side, the slab is generally designed as a one-way slab, but that does not mean that the slab is transmitting load in one direction only.
One-way slabs may be simply supported or continuous. For one-way slabs supported on two opposite sides, the bending moments are calculated in the same way as for beams. Continuity across a beam is treated as fixed support. In detailing, if a slab is assumed to be simply supported at an end support, it is advisable to provide reinforcement for a probable negative bending moment due to the monolithic construction of beams and slabs (Reynolds and Steedman, 2005).
Fig 3: Typical reinforcement detailing of a simply supported one-way slab
The effective span for one-way slabs is the same as that of beams. If ln is the clear span (distance between faces of supports), the effective span leffis given by;
leff = ln + a1 + a2
Fig 4: Effective span of a simply supported one-way slab
One-way slabs should be designed to resist the most unfavourable arrangement of loads. In clause 5.1.3 of Eurocode 2, the following two loading arrangements are recommended for buildings.
Alternate spans carrying (γGGk + γQQk) other spans carrying only γGGk.
Any two adjacent spans carrying (γGGk + γQQk). All other spans carrying only γGGk
Steps in the designof a one-way slab
The steps in the design of a slab are as follows;
Determine the design life of the structure
Choose a slab thickness determined using deflection requirements, experience, or otherwise
Establish the durability requirements, fire resistance, and adequate concrete cover
Calculate and apply the loads on the slab comprising of the dead and imposed loads
Apply the appropriate load combination
Idealise each slab element and analyse to determine the critical design moments MEd and shear forces VEd
Carry out the flexural design
Check the deflection
Check the shear capacity
Check bar spacing and cracking
For the flexural design of slabs, determine k from;
k = MEd/(fckbd2) If k < 0.167, no compression reinforcement is required, and you can calculate the lever arm; z = d[0.5 + √(0.25 – 0.882k)]
The area of reinforcement required is given by; As1 = MEd/(0.87fykz)
(a) Concrete cover The nominal concrete cover in slabs is expected to satisfy the requirement;
Cnom = Cmin + ∆c,dev
Cmin is expected to satisfy the requirement for durability, fire resistance, and bond, while ∆c,dev is the allowance made for construction deviation (usually 10 mm). The minimum cover for bond should not be less than the bar diameter. For a one-way slab to have a fire rating of one hour (REI 60), the minimum thickness should be 80 mm and the concrete cover (from the surface to the centre of bar) should be minimum of 20 mm. A minimum cover of 15mm + ∆c,dev is adequate for the durability of slabs under exposure class XC1. Therefore under normal circumstances, a concrete cover of 25 mm is usually adequate for floor slabs.
(b) Minimum tension steel The main moment steel spans between supports and over the interior supports of a continuous slab. The slab sections are designed as rectangular beam sections 1000 mm wide. The minimum area of main reinforcement has to satisfy clause 9.2.1.1(1) of Eurocode 2;
As,min = (0.26fctm/fyk)btd but not less than 0.0013bd
where bt = width (for slab design 1000 mm), d = effective depth
The maximum area of steel provided should not exceed 0.04Ac.
(c) Distribution steel The distribution, transverse, or secondary steel runs at right angles to the main moment steel and serves the purpose of tying the slab together and distributing non-uniform loads through the slab. Clause 9.3.1.1(2) states that in the case of one-way slabs, secondary reinforcement of not less than 20 percent of principal reinforcement should be provided. Note that distribution steel is required at the top parallel to the supports of continuous slabs. The main steel is placed nearest to the surface to give the greatest effective depth.
Fig 5: Typical reinforcement detailing of a continuous one-way slab
(d) Slab main reinforcement Slab reinforcement is a mesh and may be formed from two sets of bars placed at right angles. The table below gives bar spacing data in the form of areas of steel per metre width for various bar diameters and spacings. Reinforcement in slabs consists of a large number bars both ways which need to be tied together to form a mat. This is actually an expensive operation (see cost of fixing reinforcement in Nigeria).
It is necessary to point out that the critical span in the analysis of solid slabs is the short span. Therefore, the main reinforcements will lie parallel to the short span, and that is where you check your deflections (see why short spans are critical in floor slabs).
Table 1: Area of reinforcement per metre width of spacing
Spacing
ϕ8 mm
ϕ10 mm
ϕ12 mm
ϕ16 mm
ϕ20 mm
100
502
785
1130
2010
3141
125
402
628
904
1608
2513
150
335
523
753
1340
2094
175
287
448
646
1148
1795
200
251
392
565
1005
1570
225
223
349
502
893
1396
250
201
314
452
804
1256
275
182
285
411
731
1142
300
167
261
376
670
1047
For instance, if the area of reinforcement required (As,req) from calculation is 523 mm2/m, you can provide H12@200 c/c (Area of steel provided As,prov = 565 mm2/m).
(e) Crack Control According to Clause 9.3.1.1(3) of Eurocode 2, if h is the total depth of the slab, then the maximum spacing of reinforcements is normally restricted to;
3h ≤ 400 mm for principal reinforcement
3.5 h ≤ 450 for secondary reinforcement
However, in areas of maximum moment, maximum spacing is restricted to;
2h ≤ 250 mm for principal reinforcement
3 h ≤ 400 mm for secondary reinforcement
For slabs 200 mm thick or greater the bar size and spacing should be limited to control the crack width and reference should be made to section 7.3.3 of the Eurocode 2.
(f) Curtailment of bars in slabs Curtailment of bars is done according to the moment envelope. However, clause 9.3.1.2(1) requires that half the calculated span reinforcement must continue up to support. It is further stated that in monolithic construction, where partial fixity occurs along an edge of a slab but is not taken into account, the top reinforcement should be capable of resisting at least 25 percent of the maximum moment in the adjacent span and this reinforcement should extend at least 0.2 times the length of the adjacent span measured from the face of the support.
Fig 6: Typical curtailment rules of solid slabs
The above situation occurs in the case of simply supported slabs or the end support of a continuous slab cast integral with an L-beam which has been taken as a simple support for analysis but the end of the slab might not be permitted to rotate freely as assumed. Hence negative moments may arise and cause cracking.
(g) Shear capacity of slabs Under normal loads, shear stresses are not critical and shear reinforcement is not required in floor slabs. Shear reinforcement is provided in heavily loaded thick slabs but should not be used in slabs less than 200 mm thick (clause 9.3.2 (1)). To check the shear capacity of slabs, the shear stress (vEd = VEd/bd) must be checked against the shear capacity of an unreinforced section (VRc,d). This is given by;
Where; CRd,c = 0.18/γc k = 1 + √(200/d) < 0.02 (d in mm); ρ1 = As1/bd < 0.02 (In which As1 is the area of tensile reinforcement which extends ≥ (lbd + d) beyond the section considered) Vmin = 0.035k(3/2)fck0.5 K1 = 0.15; σcp = NEd/Ac < 0.2fcd (Where NEd is the axial force at the section, Ac = cross sectional area of the concrete), fcd = design compressive strength of the concrete).
In a one-way slab, the design shear force is calculated from the support reactions or end-shears at the support, while in a two-way slab, they can be obtained from the coefficients in Table 3.15 of BS 8110-1:1997.
(h) Check for deflection The check for deflection is a very important consideration in slab design and usually controls the slab depth. In normal cases, a strip of slab 1 m wide is checked against span-to-effective depth ratios. A slab should not deflect excessively under service load. Excessive deflection of slabs can cause cracking to partitions and finishes.
For deemed to satisfy basic span/effective depth (limiting to depth/250);
Actual L/d of the slab must be ≤ Limiting L/d × βs
The limiting basic span/ effective depth ratio is given by;
L/d = K [11 + 1.5√(fck)ρ0/ρ + 3.2√(fck) (ρ0/ρ – 1)1.5] if ρ ≤ ρ0
L/d = K [11 + 1.5√(fck) ρ0/(ρ – ρ’) + 1/12 √(fck) (ρ0/ρ)0.5 ] if ρ > ρ0
Where; L/d is the limiting span/depth ratio K = Factor to take into account different structural systems ρ0 = reference reinforcement ratio = 10-3 √(fck) ρ = Tension reinforcement ratio to resist moment due to design load ρ’ = Compression reinforcement ratio
The value of K depends on the structural configuration of the member and relates the basic span/depth ratio of reinforced concrete members. This is given in the table 2;
Table 2: Basic span/effective depth ratio of different structural systems
Structural System
K
Highly stressed ρ = 1.5%
Lightly stressed ρ = 0.5%
Simply supported slabs
1.0
14
20
End span of interior slabs
1.3
18
26
Interior span of continuous slabs
1.5
20
30
Flat slab
1.2
17
24
Cantilever slabs
0.4
6
8
βs = (500As,prov)/(fykAs,req)
Two-way Spanning Slabs
Two-way action occurs when a slab is supported on all four sides. If the two dimensions and support conditions are the same, then the load is distributed to all supporting beams equally. In design, a slab is considered to be two-way if the ratio of the longer side to the shorter side is less than two.
For two way slabs, the precise amount and distribution of the load taken by each support, and consequently the magnitude of the bending moments on the slab, are not easily calculated if assumptions resembling real conditions are made. Therefore, approximate analyses are generally used. The method applicable in any particular case depends on the shape of the slab panel, the conditions of restraint at the supports and the type of load.
Two basic methods are commonly used to analyse slabs that span in two directions. They are;
The theory of plates, which is based on elastic analysis, is particularly appropriate to the behaviour under service loads
Yield-line theory, which considers the behaviour of the slab as a collapse condition approaches.
Generally, for rectangular slabs with standard edge conditions and subject to uniformly distributed loads, normally the bending moments are obtained using tabulated coefficients. These coefficients are based on elastic analysis from thin plate theory. The loads used in the analysis are factored to represent the ultimate limit state condition. This is the approach used in BS 8110 for slabs with corners that are not held down (no consideration for torsion). The analysis must take into account the support conditions which can be idealised as fixed, hinged, or free.
For slabs with irregular plan shapes and slabs subject to a combination of point loads and distributed loads, Johansen’s yield line analysis and the Hillerborg strip method provide powerful methods for strength calculations.
Simply supported two-way slabs Where the corners of slabs are free to lift and no provision is made to resist forces at the corners, the maximum moments per unit width are given by the following expressions:
Msx = bending moment in strips with span lx = Msx = αsxqlx2 Msy = bending moment in strips with span ly = Msy = αsyqlx2
where lx is the shorter span of the panel, ly is the longer span of the panel and q is the design ultimate load per unit area. Values of αsx and αsy are given in Table 3.13 of BS 8110-1:1997 for different ratios of ly and lx, where ly is the longer span.
Rectangular panels with restrained edges Where corners of a two-way slab are prevented from lifting and reinforced to resist torsion, the maximum bending moments per unit width are given by the following expressions:
Msx = βsxqlx2 Msy = βsyqlx2
where; Msx is the maximum design moment either over supports or at midspan on strips with span lx Msy is the maximum design moment either over supports or at midspan on strips with span ly q is the design ultimate load per unit area, lx is the shorter span, and ly is the longer span
The coefficients can be obtained from Table 3.14 of BS 8110-1:1997.
Design Exampleof a two-way slab
The general arrangement of the floor plan of a building is shown below. Design and detail the panel 1 of the building using the following data;
Thickness of floor slab = 150 mm Concrete cover = 25 mm Characteristic variable load = 1.5 kN/m2 fck = 25 N/mm2 fyk = 460 N/mm2
Fig 7: General arrangement of a floor slab
Load Analysisof Panel 1
Permanent Loads Self weight of slab = 25 kN/m3 × 0.15m = 3.75 kN/m2 Weight of finishes = 1.2 kN/m2 Partition allowance = 1.5 kN/m2 Total dead load (gk) = 6.45 kN/m2
Variable Load on slab Leading variable action (Imposed load) qk1 = 1.5 kN/m2
Total load on slab (ULS) = 1.35gk + 1.5qk = 1.35(6.45) + 1.5(1.5) = 10.9575 kN/m2
The floor slab (PANEL 1) is spanning in two directions, since the ratio (k) of the longer side (ly) to the shorter side (lx) is less than 2. Hence, k = Ly/Lx = 3.825/3.625 = 1.055 (say 1.1, however it is more appropriate to interpolate from the table).
Moment coefficient for two adjacent edges discontinuous
Short Span Mid-span = 0.042 Continuous edge = 0.056
Long Span Mid-span = 0.034 Continuous edge = 0.045
Design of short span Mid span MEd = αnlx2 = 0.042 × 10.9575 × 3.6252 = 6.0475 kNm/m d = h – Cc – ϕ/2
Assuming ϕ12mm bars will be employed for the construction d = 150 – 25 – 6 = 119mm; b = 1000mm (designing per unit width)
Taking the distance between supports as the effective span, L = 3625mm The allowable span/depth ratio = βs × 30.838 = 2.0 × 190.327 = 280.645 Actual deflection L/d = 3625/119 = 30.462 Since 280.645< 30.462 Therefore, deflection is ok.
Shear Verification Maximum shear force coefficient for slab(β) = 0.44 VEd = βnlx
Ultimate shear force VEd = 0.44 × 10.9575 × 3.625 = 17.477 kN/m
VRd,c = [CRd,c.k. (100ρ1 fck)1/3
CRd,c = 0.18/γc = 0.18/1.5 = 0.12
k = 1 + √(200/d) = 1 + √(200/119) = 2.296 > 2.0, therefore, k = 2
Installation of reinforcements is an important item of work in the construction of reinforced concrete structures. The cost of reinforcement works therefore significantly impacts the total contract sum of any construction project. In the preparation of a bill of quantity (BOQ) or bill of engineering measurement and evaluation (BEME), concrete and reinforcement works are listed as separate items of work.
Therefore, a contractor bidding for such a job is expected to provide his rate of buying, cutting, bending, and installing the reinforcements according to the construction drawing in one sum. While concrete is priced per cubic meters (m3), reinforcement is usually priced per kilogram (kg).
The price to be quoted per kg of reinforcement is expected to cover the cost of purchase and supply of the materials to the site, cost of cutting, bending, and installation according to the design specification. It is also expected to cover the contractor’s profit and overhead. The use of bar bending schedule (BBS) can enhance the process of reinforcement handling and evaluation.
Therefore, a contractor bidding for a job in reinforced concrete construction is expected to know the basic cost of reinforcement in the market, the cost of transportation, the cost of labour, and the cost of machinery that will be needed to execute the job. Initial market research may be required before the tender is computed and submitted to the client.
In Nigeria, there are different categories of reinforcements that are used for construction works, and each of them is priced differently in the market. As found all over the world, the use of mild steel (fy = 250 N/mm2) is no longer common, and it is rarely utilised in construction in Nigeria. However, some people still use R6mm (commonly called the ‘quarter rod’) and R8mm as links for columns and beams, but it is rarely specified in construction drawings these days. The categories of reinforcements in the Nigerian markets are;
Local reinforcements (manufactured locally)
TMT reinforcements (manufactured locally)
Foreign reinforcements (imported into the country)
Local reinforcements are manufactured locally by the steel industries in Nigeria and are the cheapest class of reinforcements in the market. They have a reputation of not always meeting the minimum yield strength requirements, and are not usually employed in large-scale or serious projects.
TMT reinforcements are high-quality reinforcements produced from recycled steel through the process of thermomechanical treatment. They usually satisfy the yield strength requirements and are widely used for all kinds of projects in the country, including bridges. They are more expensive than local reinforcements. As at July 2022, while one tonne of reinforcement is sold at about ₦415,000.
Foreign reinforcements are usually imported from Germany, Ukraine, or Russia, and are slightly more expensive than TMT reinforcements. However, the presence of TMT reinforcements is making foreign reinforcements less attractive.
Therefore, careful attention to design details and specifications is needed during the pricing of reinforcements. We are going to show below the process of pricing for reinforcement works in Nigeria. The information that will be needed for the evaluation of the cost of reinforcement works in Nigeria are as follows;
Cost of reinforcement in the market (based on the size and type of reinforcement)
Cost of binding wire
Cost of delivery to site
Cost of labour for cutting, bending, and installation
Cost of machinery (cutting machine and bending machine)
Calculation of the Cost of Reinforcement Works in Nigeria
For example, let us calculate the cost of installing 3650 kg of Y12 reinforcements in the floor slab of a building.
As a rule of thumb, about 12 kg of binding wire is required to tie 1 tonne of reinforcement (this also means that about 0.012 kg of binding wire is required to tie 1 kg of reinforcement). Therefore about 44 kg of binding wire is required for this project.
Cost calculation Basic cost of reinforcement (including delivery to site) = say ₦430,000 per tonne = ₦430 per kg Basic cost of binding wire (including delivery to site) = ₦17,000 per 20 kg = ₦ 850 per kg (= ₦10.2 per kg of reinforcement) Basic cost of labour (cutting, bending, and installation) = ₦25,000 per tonne = ₦25 per kg Assuming that the use of machinery is not applicable in the project or factored into the cost of labour, total basic cost = ₦430 + ₦10.2 + ₦25 = ₦465.2 per kg
Make 10% allowance for waste and laps = 1.1 x 465.2 = ₦512 Allow 20% for contractor’s profit and overhead = 1.2 x ₦465.2 = ₦ 559 per kg
Therefore the current cost of reinforcement works in Nigeria is ₦559 per kg of reinforcement.
Therefore, the cost of fixing the floor slab reinforcement = ₦559 x 3650 = ₦2,040,350:00 (Two million, forty-six thousand and three hundred and fifty Naira)
For your construction and building design works, contact;
Shear walls are structural members that provide additional lateral stiffness to a building by resisting shear, moment, and axial forces, which are produced due to gravity and lateral loads. In some design cases, the entire lateral loads coming to a building assumed to be resisted by shear walls alone, especially when it is the major stabilising/bracing component of the building. The design of shear walls involves providing adequate cross-section and reinforcements to resist bending, shear, axial, and twisting forces due to gravity and lateral loads.
For an element to be described as a reinforced concrete wall, the length to thickness ratio should be equal to or greater than 4 (Clause 9.6.1, Eurocode 2). A shear wall is said to be short when the height-to-width ratio is less than or equal to one, while it is said to be slender when the height-to-width ratio is greater than or equal to four. For slender walls, bending deformation is dominant, while in short walls, shear deformation is dominant. When the height-to-width ratio is in-between one and four, then the shear wall undergoes both shear and bending deformations.
Shear walls and columns in a building
Forces Acting on Shear Walls
Shear walls are designed to resist bending moment, shear, axial, and uplift forces, especially when they are subjected to lateral actions. The lateral forces acting in the plane of a shear wall attempts to lift up one end of the wall and push the other end down. Uplift forces are greater on tall walls and less on low walls. Shear walls resist the shear force parallel to the plane of the wall by cantilever action.
Axial forces in a Shear wall The axial load in a wall may be calculated assuming the beams and slabs transmitting the loads to it are simply supported.
Transverse moments For continuous construction, transverse moments can be calculated using elastic analysis. The eccentricity is not to be less than h/30 or 20 mm where h is the wall thickness.
In-plane moments Moments in the plane of a single shear wall can be calculated from statics. When several walls resist forces, the proportion allocated to each wall should be in proportion to its stiffness.
Location/Placement of Shear of Shear Walls
In a tall building where the shear wall is used for lateral stability, it should be located on each level of the structure. Preferably, shear walls of equal length should be placed symmetrically on all four exterior walls of the building to form an effective box structure. When the shear walls in the exterior frame could not provide sufficient strength and stiffness, the shear walls should be added to the interior frame as well.
If the shear wall is placed at the interior frame of a building, it attracts and resists higher internal forces but may not be too effective in reducing the maximum lateral deflection of the building. However, if a shear wall is placed at the ends of a building, the lateral deflection is reduced considerably when compared to the later.
Architectural disposition in a building may not always give the room for optimum placement of shear walls for good structural performance. In some areas such as lift areas or window/door openings, coupled shear walls may have to be used. However, the best position or arrangement of shear walls in a building is a symmetrical arrangement whereby the shear centre (centre of rotation) will coincide with the centre of gravity of the building in order to reduce or eliminate torsion (twisting) due to lateral loads.
Coupled shear wall in a building
Functions of Shear Walls
Shear walls must provide the necessary lateral strength to resist horizontal wind and earthquake forces.
They provide resistance against sliding through connections.
They also provide lateral stiffness to prevent the roof or floor above from excessive side sway.
When the shear walls are stiff enough, they prevent floor and roof framing members from moving off their supports.
Buildings that are sufficiently stiff usually suffer less structural damage due to the presence of shear walls.
Structural Action of a Shear Wall
In a shear wall, the primary mode of deformation is mainly due to flexure but not shear. The mode of deformation of shear walls is such that it has maximum slope at the top and least at the bottom which is the flexural mode shape. The structural action of a shear wall resembles that of a cantilever beam.
Typical deflected shape of a shear wall
Structural Design of Shear Walls
The amount of reinforcement needed for proper detailing of a reinforced concrete wall may be derived using strut-and-tie model. If a wall is however subjected predominantly to out-of-plane bending, the design rules and guidelines for slabs apply. In Eurocode 2, the requirements for the design of columns and shear walls are not so different except in the following areas;
■ The requirements for fire resistance ■ Bending will be critical about the weak axis ■ Rules for spacing and quantity of reinforcement
Reinforcement Detailing of Shear Walls
(a) Minimum and maximum area of vertical reinforcement According to clause 9.6.2 of Eurocode 2, the minimum and maximum amounts of reinforcement required for a reinforced concrete wall are 0.002Ac and 0.04Ac outside lap locations respectively. It is further stated that where minimum reinforcement controls design, half of this area should be located on each face. The distance between two adjacent vertical bars should not exceed three times the wall thickness or 400 mm, whichever is lesser.
(b) Area of horizontal reinforcement According to clause 9.6.3 of Eurocode 2, horizontal reinforcement should be provided at each face and should have a minimum area of 25% of the vertical reinforcement or 0.001Ac, whichever is greater. The spacing between two adjacent horizontal bars should not be greater than 400 mm.
(c) Provision of links If the compression reinforcement in the wall exceeds 0.02Ac, links must be provided through the wall thickness in accordance with the rules for columns in clause 9.5.3 which are:
The diameter of the transverse reinforcement should not be less than 6 mm or one-quarter of the diameter of the largest longitudinal bar whichever is greater. The maximum spacing is to be Scl, max
Scl, max is the minimum of;
20 times the diameter of the smallest longitudinal bar
The lesser dimension of the wall i.e. the thickness
400 mm
The maximum spacing should be reduced by a factor of 0.6 in the following cases;
In sections within a distance equal to 4 × thickness of wall above or below a beam or slab.
Near lapped joints, if the diameter of the longitudinal bar is greater than 14 mm. A minimum of three bars evenly placed in the lap length is required.
Where the main reinforcement (i.e., vertical bars) is placed nearest to the wall faces, transverse reinforcement should be provided in the form of links with 4 per m2 of the wall area.
Design Example of Shear Walls
Design a 225 mm thick shear wall of 3.6 m height at the ground floor of the building. The shear wall is carrying a 200 mm thick slab on the first floor of the building. The action effects on the shear wall are as follows;
Vertical loads Dead Load Gk = 300 kN/m Live load Qk = 55 kN/m
Vertical load due to in-plane bending and wind Wk = ±650 kN/m Vertical load due to in-plane bending and imperfections GkH = ±60 kN/m
Maximum moment out-of-plane, floor imposed load as leading action M = 40 kN/m @ ULS Maximum moment out-of-plane, floor imposed load as accompanying action M = 35 kN/m @ ULS
Design load cases Consolidate (c) into (a) and (b) to consider two load cases:
NEd = 1518.75 kN/m MEd = 46.732 kNm/m (out of plane)
and
NEd = –756 kN/m (tension) MEd = 46.732 kNm/m (out of plane)
Design: cover above ground cnom = cmin + ∆cdev
where; cmin = max[cmin,b; cmin,dur] (Exp. (4.1))
where cmin,b = diameter of bar = 20 mm vertical or 10 mm lacers cmin,dur = for XC1 = 15 mm ∆cdev = 10 mm cnom = 15 + 10 = 25 mm to lacers (35 mm to vertical bars)
Design using charts For compressive load: d2/h = (25 + 10 + 16/2)/225 = 0.19
Interpolate between charts for d2/h = 0.15 and d2/h = 0.2 as shown below
Rectangular column interaction chart for d2/h = 0.15
Rectangular column interaction chart for d2/h = 0.2
By inspection all concrete is in tension zone and may be ignored. Use 7H16 @ 175 c/c on both sides for at least 1 m each end of wall (Asprov = 2814 mm2).
Horizontal reinforcement As,hmin = 0.001Ac or 25% As,vert (Cl. 9.6.3(1) & NA) = 225 mm2 or (0.25 × 2814) = 704 mm2/m This therefore requires 352 mm2/m each side Use H10 @ 200 (393 mm2/m) both sides
Beams are horizontal structural elements used for supporting lateral loads. In conventional reinforced concrete structures, beams usually receive load from the floor slab, but may also be subjected to other loads such as wall load, finishes, services installation, etc. The design and detailing of reinforced concrete beams involves the selection of the proper beam size and the quantity of longitudinal and shear reinforcement that will satisfy ultimate and serviceability limit state requirements. Afterward, it is very important that the beam reinforcements are placed and arranged properly on site, to avoid construction error.
This article presents guides and short notes on how to properly place and arrange beam reinforcements on site. This is very important for students (on industrial training) or fresh graduates that are new to construction site practices.
Typical detailing of a reinforced concrete beam
The technical guides to the detailing and arrangement of beam reinforcements are as follows;
(1) Confirm the formwork dimensions and stability Beam reinforcement placement commences immediately after the carpenters complete the soffit formwork of the floor. At this point, it is important to verify that the formwork dimensions have been done according to the design specification. The checks should include measuring the beam width and the drop of the beam relative to the proposed surface of the slab. The sides of the beam drop should be checked for verticality to avoid having slanted beam sides, and the soffit should be checked for perfect horizontality. It is usually very difficult and depressing to make corrections after the reinforcement has been placed.
Poorly sized beam dimensions can also lead to compromised concrete cover, which can impact the bonding of concrete and reinforcement, the fire rating, and the durability of the building. It is typical to take a bent link (stirrup) and insert it in the beam drop to check if the desired concrete cover has been achieved. It can also be done to confirm that the beam size has been done according to specification. Furthermore, it is important to check the stability and bracing of the formwork to avoid collapse, bulging, or bursting.
(2) Confirm cut length and bending dimensions Before the placement of the rebars commences, the site engineer should confirm that reinforcements have been cut and bent according to the design requirements. Bar bending schedule (BBS) can be used as a check document, provided it is very accurate and synchronised with what has been done on-site. All changes in dimensions or arrangements should be communicated ahead of time.
Cut and bent beam reinforcements at a construction site awaiting tying and placement
(3) Tying of Beam Reinforcements Links and stirrups are used for resisting shear stresses and torsion in a reinforced concrete beam. They are also used for holding the top and bottom reinforcement of beams in place. The correct quantity required should be installed and spaced as recommended in the structural drawing.
Tying and arrangement of beam reinforcement on site
(4) Arrangement of Primary and Secondary Beams For an internal beam (secondary beam), which is supported by another beam on either or both sides, the top bars should rest on the top bars of the primary beam. Also, the bottom bars should rest on the bottom bars of the primary beam as shown in the image below.
(5) Arrangement of Beam-Column Junction at Corners For beams occurring at a column junction at the corner of a building, either beam reinforcements can rest on each other.
Corner beam-column junction
(6) Arrangement of Continuous Beam-Column Junction At a continuous beam-column junction as shown below, the continuous beam with heavier reinforcement will have to be placed on the beam with lighter reinforcement or the beam terminating at that point.
Internal beam-column Junction
(7) Reinforcement Arrangement of Overhang (Cantilever) Beams For cantilever beams, the main bars are the top reinforcements. If a side beam is resting on it (when the cantilever beam is acting as a primary beam) as shown in the image below, the top bars of the secondary beams beam must rest on the top bars of the cantilever beams. In the same vein, the bottoms bars of the side beam (main rebar) must rest on the bottom bars of the cantilever beam.
Well designed and constructed cantilever beam
The arrangement of beams at a site should be checked strictly to conform to the engineer’s design drawings. Failure to check construction details properly may lead to the failure of the member or lack of robustness. We believe that beams are arguably the most common structural members in a building. Hence, attention should be given to its arrangements for robustness and efficient load transfer.
