Effects of Trees on Wind Comfort of Pedestrians

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July 12, 2020

Pedestrian wind comfort can be improved when horizontally incoming airflow passes through trees in the urban areas. This is according to recent research carried out in the Department of Environmental Atmospheric Sciences, Pukyong National University, Busan, Republic of Korea, and published in Elsevier – Sustainable Cities and Societies. In the study, the authors applied computational fluid dynamics incorporating tree drag parameters to evaluate how trees improved the wind comfort of pedestrians.

The way wind is perceived at the ground level depends on a lot of factors such as wind direction, wind speed, obstacles, and many other parameters. This experience affects pedestrians’ comfort and safety, and can impact the financial returns or economic viability of an area. According to Lawson-based wind comfort criterion, wind speed exceeding 10 m/s can be uncomfortable for pedestrians at the ground level, while wind speed above 15 m/s is outrightly dangerous. Other wind comfort criteria exist such as that proposed by Davenport.

Trees are known to function as porous obstacles to airflow, and they eventually affect wind speed and direction. The presence of high rise buildings and urban densification has been observed to reduce airflow in city centers and may lead to increased urban heat and poor dispersal of pollutants. However, the effect of wind on tall buildings can amplify wind pressure in the surroundings due to issues like vortex shedding, reverse flow, channeling effects, etc. This can lead to discomforting wind effects on pedestrians. Therefore, by applying computational fluid dynamics model (CFD) with tree drag parameterization scheme, the researchers were able to evaluate the effect of trees on pedestrian wind comfort in Pukyong National University campus. The CFD model used in the study was verified with field observations.

The CFD model used in the study was based on Reynolds-averaged Navier–Stokes (RANS) equations and assumes a three-dimensional, non-rotating, non-hydrostatic, incompressible airflow system. Turbulence was parameterized using the renormalization group (RNG) k-ε turbulence closure scheme. Tree drag terms were introduced into the momentum, turbulent kinetic energy (TKE), and TKE dissipation rates equations to account for the loss of airflow pressure due to winds.

The target area for the research was the Pukyong National University (PKNU) campus (See Fig. 1a) which is located in the downtown area and is surrounded by commercial and residential areas. PKNU boasts relatively high vegetation density for student recreational spaces and ecologically friendly landscaping within the campus. Regions A and B (Fig. 1b) contains a dense forest of trees taller than 10–15 m. Several tree species are planted along the PKNU boundary (region C in Fig. 1b).

The authors adopted the wind comfort criteria proposed by Isyumov and Davenport (1975), which distinguish four sensory levels (good, tolerable, unpleasant, and dangerous) for four categories of human activity or activity location (Table 1). These sensory levels are determined by the Beaufort wind force scale (BWS), represented by wind speeds at 10 m above the ground level. To evaluate wind comfort at the pedestrian level (z =1.75 m), they used the BWS values converted into pedestrian height.

Table 1: Sensory levels in terms of suitability for outdoor activities, represented by the Beaufort wind force scale (BWS) (Isyumov & Davenport, 1975).

From the study, poor wind comfort for outdoor activities (BWSs ≥ 4) was observed in the areas without trees, mainly around the edges of buildings, in the windward regions of buildings, in the spaces between buildings, and in wide, unobstructed areas. This was attributed to venturi effects between the spaces in buildings. In the case of where trees were present, the BWS values declined by one to three levels, improving the overall level of wind comfort within the PKNU campus.

The highest ABWS and TBWS values (≥ 4) were observed near the southeast perimeter of the PKNU campus, where a 10-lane road is located. By contrast, the lowest ABWS and TBWS values (≤ 3) were observed in the southwest and northwest of the campus. Where trees were present, the overall wind speeds inside the campus were reduced due to drag.

The authors concluded that tree arrangement can reduce wind speeds in the lee of the trees by more than half and proposed that trees should be planted at 90° to the dominant wind direction. The presence of trees decrease wind speeds. However, because wind speeds can increase in surrounding areas without trees, the effects of trees on strong winds in such areas should be assessed.

Reference
Kang G., Kim J., Choi W. (2020): Computational fluid dynamics simulation of tree effects on pedestrian wind comfort in an urban area. Sustainable Cities and Societies 56 (2020)102086. https://doi.org/10.1016/j.scs.2020.102086

Disclaimer:
Contents of this research article have been shown on www.structville.com because it is an open access article under creative commons licence (http://creativecommons.org/licenses/BY/4.0). All other rights belong to the authors and Elsevier.

Difference Between Pile Load Test and Pile Integrity Test

By

Ubani Obinna

-

July 10, 2020

Pile foundations are slender structures used to transmit superstructure load to firmer sub-soil stratum beneath the natural ground surface. They can also be used for other purposes such as resisting heavy lateral forces, compaction of soils (compaction piles), avoiding excessive settlement, etc. Due to their importance in civil engineering structures, piles are usually subjected to tests such as pile load tests and pile integrity tests before they are loaded. These two tests are completely different and are sometimes confused for one another, even though they do not serve the same purpose. This article aims to highlight the difference between these two tests.

Pile Load Test

Generically, pile load test can be described as a reliable method of pile foundation design which involves loading constructed piles on-site to determine their load-carrying capacity. A pile load test involves applying increments of static loads to a test pile and measuring the settlement. The load is usually jacked onto the pile using either a large deadweight or a beam connected to two uplift anchor piles to supply reaction for the jack. Generally, an installed pile, weights, deflection gauge, hydraulic jack, and load indicator are required for a pile load test.

Loading of test piles is usually applied in increments of 25% of the total test load which should be 200% of the proposed design load. After the load test, the load-settlement curve is plotted and the failure load determined. Eurocode 7 permits three different methods for the design of pile foundations which are;

By testing (static load test, ground testing result, dynamic ground testing)
By calculation (empirical or analytical)
By observation

It is important that the validity of static load test be checked using calculations.

Pile Integrity Test

Pile integrity test (PIT) is a non-destructive method of testing of piles that is used for qualitative evaluation of the physical dimensions, continuity, and consistency of materials in a bored (cast in-situ) pile. This test is very important for quality control and quality assurance of piles at great depth.

The three most common methods of carrying out pile integrity tests are;

Low-strain pile integrity test
Crosshole sonic logging
Thermal integrity test

In the low-strain impact integrity testing, the head of the pile shaft is subjected to impact using a tool like a simple hammer and the response is determined using a high precision transducer. The transducer can either be an accelerator, or a velocity sensor. Low-strain pile integrity tests can provide information such as embedment length, changes in cross-section (such as bulging), discontinuity (such as voids), and consistency of pile materials (such as soil inclusion and segregation). However, it cannot provide information such as bearing capacity and cannot be applied to pile caps.

Differences and similarities between pile load test and pile integrity test at a glance

Pile Load Test	Pile Integrity Test
Used for determining bearing capacity of piles	Used for determining physical properties of constructed piles
Can evaluate pile settlement under load	Cannot evaluate pile settlement
Expensive to set up	Cost effective
Takes time to complete	Very quick test
Cannot provide the embedment length of the pile	Provides embedment length of the pile
Cannot give information on the quality of the piling job	Provides information on the quality of the piling job

Therefore, pile load test and pile integrity tests should be carried out as soon as piling jobs are concluded on site before the next stage of the construction commences.

Design of Pile Cap Using Staad Foundation Advanced

By

Peace Ikpotokin

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July 9, 2020

Pile caps are rigid plate structures that are used to transfer superstructure load from columns to a group of piles. They are usually subjected to bending and shear forces, and shear considerations usually govern the thickness design of pile caps. The three main approaches that are used in the analysis of pile caps are;

Truss Analogy
Bending analogy, and
Finite element analysis

While truss analogy and bending theory can be easily carried using quick manual calculations, finite element analysis usually require the use of computer models. In this article, we are going to explore the potentials of Staad Foundation Advanced Software in the analysis and design of pile caps.

A quick design of pile caps can be done on Staad Foundation Advanced using the Foundation Toolkit option. This approach does not require importing models and can be used for quick stand-alone design when the column load and geotechnical parameters of the soil are available. To use this option, launch the ‘Staad Foundation Advanced‘, click on ‘New Project‘, and select ‘Foundation Toolkit‘ labelled as shown below.

Step 1: Launch the foundation toolkit

Step 2: Create Pile Cap Job

When the Foundation Toolkit opens, go to ‘Main Navigator‘, and from ‘Project Info‘ drawdown list, select ‘Create Pile Cap Job‘ as shown below.

Step 3: Select design code, units, and pile layout

When the ‘Pile Cap Job’ is launched, select the desired code of practice, unit, and click ‘Next’. The pile layout can be left as predefined.

Step 4: Define the load

On clicking ‘Next’, the dialog box for load comes up. Make sure that the unit is consistent as desired, and for this exercise, I am applying a factored column load of 3500 kN. If there are other forces such as moment and shear coming from the column, you can define them also.

Step 5: Define Load Combination

Since we are dealing with an already factored load, select ‘User Defined‘ from the drawdown list of load combination. If you have defined dead load, live load, wind load etc in Step 4, you can select the desired code of practice for the combination of the loads. Since I defined my factored load as dead load, I assigned a factor of 1.0 to dead load at ULS and SLS (actually I am not interested in SLS in this design). Then click ‘Next‘

Step 6: Define the design parameters

In this case, the column dimensions are taken as 450 mm x 450 mm, and the thickness of the pile cap was taken as 1300 mm. Other design parameters specified are as shown below.

Step 7: Select pile arrangement

The diameter of the pile was selected as 750 mm, with a spacing of 2250 mm. The safe working load of the pile was taken as 900 kN. You can also input the uplift and lateral load capacity of the pile. The edge distance is taken as (diameter of pile/2 + 150 mm) – where 150 mm is the overhang from the edge of the pile to the edge of the pile cap. Then click on ‘Calculate‘.

This brings the possible pile arrangements based on the safe working load and the superstructure load. For this tutorial, the arrangement below was adopted. The simple idea behind it is simply (Column load/pile safe working load). Note that for practical purposes, serviceability limit state load should be used when selecting the number of piles. Then click ‘Ok‘ and ‘Next‘.

Step 8: Finish the model

Step 9: Carry out the Design

Clicking ‘Finish‘ returns you to the ‘Main Navigator‘ page, where you can click on ‘Design‘ to carry out the design of the pile cap.

Step 10: View the output

The output page is where you can view the geometry drawing, details and schedule drawing, calculation sheet, and graphs.

The design approved the 1300 mm thick pile cap provided, and provided Y16@100 mm c/c reinforcement. You can go ahead and print the calculation sheet which you can download below.

FOUR-PILE-CAPS-STRUCTVILLE Download

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Modeling of Soils Using Isogeometric Analysis

By

Ivy Ugorji

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July 8, 2020

Researchers from the Department of Construction Sciences, Lund University, Sweden have presented isogeometric analysis as an alternative to finite element analysis for modelling of soil plasticity. In a study published in the year 2017 in Geomaterials Journal, the researchers were able to show that isogeometric analysis showed good agreement with finite element method for drained soils in two- and three-dimensions. The research, therefore, suggested that isogeometric analysis is a good alternative to conventional finite element analysis for simulations of soil behavior.

Isogeometric analysis is a numerical method that uses non-uniform rational B-splines (NURBS) as basis functions instead of the Lagrangian polynomials often used in the finite element method. These functions have a higher-order of continuity and therefore makes it possible to represent complex geometries exactly. The basic idea behind isogeometric analysis is to use splines (NURBS) as basis functions for computational analysis by applying them directly. This allows the same basis function to be used for discretzation and for analysis.

A1 1 — Representation of quadratic B-spline basis function with knot vector Ξ = {0, 0, 0, 1, 2, 3, 4, 5, 5, 5} *(Spetz et al, 2017)*

A2 2 — Representation of quadratic Lagrangian basis function (Spetz et al, 2017)

Since being introduced by Thomas J.R. Hughes at the University of Austin, Texas in the year 2005, isogeometric analysis has found numerous applications in engineering such as analysis of thin plates and shells, soil-structure interaction, fluid-structure interaction, flow through porous media, etc. However, finite element analysis has been used extensively for constitutive modelling of soils for the design of foundations, retaining walls, slope stability problems etc.

For the purpose of the research, Drucker-Prager criterion and the theory of plasticity was used by the researchers to evaluate the influence of the NURBS-basis functions on the plastic strains for granular materials against the conventional finite element analysis approach.

To compare the results of the findings, a 2D model of a strip footing on sandy silt was analyzed. The problem was solved for plane strain conditions using quadratic NURBS based IGA and conventional FEA with 5 different meshes. In order to compare the two methods, the element meshes were constructed using quadratic isoparametric elements for both IGA and conventional FEA.

k11 — Load/displacement response at point A and B for the finest mesh (4800 elements) *(Spetz et al, 2017)*

The first point, A, denotes the center of the footing and the second point, B denotes the edge of the footing. It was observed that the displacements in the model are in good agreement at center of the footing but a minor difference was observed between the displacements from the isogeometric and conventional finite element analysis at the edge of the footing.

References
Spetz, A. , Tudisco, E. , Denzer, R. and Dahlblom, O. (2017): Isogeometric Analysis of Soil Plasticity. Geomaterials, 7, 96-116. doi: 10.4236/gm.2017.73008.

Featured Image: Institute for Structural Mechanics, University of Stuttgart. https://www.ibb.uni-stuttgart.de/en/research/shells-fem-iga/

Disclaimer:
This research article has been featured on www.structville.com because it is an open access article that permits unrestricted use and distribution provided the original source of the article has been cited. See the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/

Free Vibration of Masts

By

Ubani Obinna

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July 8, 2020

Tall masts are used in a variety of applications such as telecommunication, radio/television broadcasting, supporting lighting fixtures on the highway, raising flags, etc. Due to their height above the ground, these structures are usually subjected to wind action, and this makes the knowledge of their behavior under the effect of wind very important.

The dynamic behavior of telecommunication masts under the effect of wind is studied under ‘flow-induced vibration of structures’. This term is used to denote the phenomena associated with the response of a structure immersed in or conveying fluid flow. It covers those cases in which an interaction develops between fluid dynamic forces and the inertia, damping, or elastic forces in the structure. The study of these phenomena draws on three disciplines which are, structural mechanics, mechanical vibration, and fluid dynamics (Ahmad, 2009).

This article investigates the natural frequency of vibration of X-braced and V-braced (Chevron bracing) telecommunication masts. To advance the study, the wind-induced vibration can be investigated using Staad Pro software (finite element analysis) in order to determine the critical wind speed that will potentially lead to resonance in the structure under the effect of wind. Resonance occurs when one of the natural frequencies of the structure coincides with the frequency of the vortices shedding around the mast due to the effect of wind.

Methodology

To carry out this study, two models of telecommunication masts were considered;

MODEL 1

Type: X-Bracing
Height: 30 m
Width at the base: 3m x 3m
Width at the top: 1m x 1m
Section of the legs: UA 100 x 100 x 8
Section of the horizontal bracings and diagonals: UA 50 x 50 x 6
Solidity ratio: 20.6%

The first four fundamental natural frequencies of the structure were obtained as follows;

Mode	Frequency (Hz)	Period (Sec)	MPF-X (%)	MPF-Y (%)	MPF-Z (%)
1	2.637	0.379	32.303	0.000	24.153
2	2.637	0.379	24.153	0.000	32.303
3	11.568	0.086	13.073	0.000	9.760
4	11.568	0.086	9.760	0.000	13.073

Table 1: Natural frequency of x-braced telecom masts

N/B: MPF – Mass participation factor

*3rd mode of vibration of x-braced mast*

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MODEL 2

Type: V-Bracing (Chevron Bracing)
Height: 30 m
Width at the base: 3m x 3m
Width at the top: 1m x 1m
Section of the legs: UA 100 x 100 x 8
Section of the horizontal bracings and diagonals: UA 50 x 50 x 6
Solidity ratio: 19.51 %

The first four fundamental natural frequency of the structure were obtained as follows;

Mode	Frequency (Hz)	Period (Sec)	MPF-X (%)	MPF-Y (%)	MPF-Z (%)
1	2.775	0.360	26.153	0.000	30.132
2	2.775	0.360	30.132	0.000	26.153
3	11.390	0.088	9.021	0.000	14.375
4	11.390	0.088	14.375	0.000	9.021

Table 2: Natural frequency of v-braced telecom masts

N/B: MPF – Mass participation factor

mode 2a — *2nd mode of vibration of v-braced (chevron) mast*

mode 3a — *3rd mode of vibration of v-braced (chevron) mast*

Time history analysis can be used to evaluate the response of the mast to periodic vortex shedding excitations. It is a step-by-step analysis that involves solving the dynamic equilibrium equations given by;

Kx(t) + Cẋ(t) + Ṁẍ(t) = r(t) (1)

Where,

K is the stiffness matrix
C is the proportional damping matrix
Ṁ is the diagonal mass matrix
x is the relative displacements with respect to the ground
ẋ is the relative velocities with respect to the ground
ẍ is the relative accelerations with respect to the ground
r is the vector of the applied load.
t is time

The load r(t) applied in a given Time-History analysis may be an arbitrary function of space and time. It can be written as a finite sum of spatial load vectors multiplied by time function. The frequency of vortex shedding (f_v) is related to the Strouhal number (S) and the relationship is given by;

S = f_vW/U —— (2)

Where W is the average outside width of the mast (m). The value of ݂f_v is calculated for different values of velocities. The velocity values chosen should result in Reynolds number (Re) values that still fall in ranges of constant Strouhal number S = 0.21 for low speeds and 300 < ܴ݁Re < 2 x 10⁶ or ܵS = 0.27 for high speeds and ܴ݁Re > 3.5 x 10⁶.

From equation (2) we can deduce that;

f_v = SU/W ——- (2a)

For resonance to occur, note that f_v = f_s (where f_s is the natural frequency of the structure)

Note that we can calculate the range of velocities based on Reynolds’s number. The Reynolds numbers that can be used for this purpose are 300, 2 x 10⁶ and 3.5 x 10⁶.

Re = UW/v_air (3)

Where v_air is the kinematic viscosity of flowing air (ν = 1.51 x 10^-5 m²/s at standard atmospheric pressure and 20^oC).

However, it should be noted that a latticed tower has a complex aerodynamic shape to the wind such that consistent vortex shedding to cause complete oscillation of the structure over a prolonged period is almost impossible. This is because the vortex shed from the different trussed members can rarely have a uniform frequency, thereby reducing the possibility of vortex-induced vibration. However, for towers with circular or tubular cross-section over the whole or part of its body, vortex-induced vibration is a possibility.

Effects of Wind on Telecom Masts

By

Ubani Obinna

-

July 2, 2020

Telecommunication masts or towers are tall structures that are designed for the transmission of telecommunication signals or for radio/television broadcasting. Telecom masts are usually made of steel structures of which the members can be made of hot rolled angle sections or circular hollow tube sections. Due to their height above the ground, the effect of wind is quite significant in the design of telecom masts and may govern the structural design.

Telecom masts can collapse for a variety of reasons of which wind can be one of them. According to Balc z o et al (2006),

There are very few cases where masts collapsed, and the reasons for them are in most cases not the wind forces which were taken according to the standard into consideration at stress analyses, but coincidence of other fatal circumstances. … Out of 225 mast failures in Europe only 7 were caused by wind overload. It is also thought-provoking that the wind-storm in 1999 in Denmark caused no collapse of any masts although a number of masts should have been crashed according to the stress analyses carried out by using Eurocode.

However, in Nigeria, a telecom mast was reported to have collapsed in Jalingo after a heavy storm in May 2018, which led to loss of lives. A similar incident was also reported in Edo State in the year 2018, after a heavy storm. However, no technical analysis was done to evaluate the cause of the collapse of the telecommunication masts.

failed telecom masts — Collapsed Telecom mast in Edo State, Nigeria (May, 2018)

telecom mast collapse 3 — Collapsed telecom mast in Jalingo, Nigeria (May, 2018)

Different structural configurations can be adopted for telecom masts but it appears that the structural efficiency of all configurations is not the same. Therefore, this article aims to explore the effect of wind on different configurations of steel telecom masts.

In Europe, wind action on telecom mast can be evaluated according to the procedure described in EN 1991-1-4:2005 (Eurocode 1 Part 4) with emphasis on clause 7.11 of the code. In America, this can be evaluated according to the procedure described in ASCE-7 (2010).

In this article, the response of cross-bracing configuration (X-braced) and V-bracing structural configuration to wind load will be investigated using Staad Pro software and ASCE-7-2002.

The models used in the study are shown below.

Model 1: X-braced telecom masts

Type: X-Bracing
Height: 30 m
Width at the base: 3m x 3m
Width at the top: 1m x 1m
Section of the legs: UA 100 x 100 x 8
Section of the horizontal bracings and diagonals: UA 50 x 50 x 6
Design wind speed: 50 m/s
Building classification category: II
Exposure category: Category B
Solidity ratio: 20.6 %

The following results were obtained when analysed for self-weight and the effect of wind load.

Maximum deflection at the top = 52.63 mm

The maximum internal forces are shown below;

Model 2: V-braced telecom masts

Type: V-Bracing
Height: 30 m
Width at the base: 3m x 3m
Width at the top: 1m x 1m
Section of the legs: UA 100 x 100 x 8
Section of the horizontal bracings and diagonals: UA 50 x 50 x 6
Design wind speed: 50 m/s
Building classification category: II
Exposure category: Category B
Solidity ratio: 19.51 %

When analysed for the effect of wind, the result below was obtained;

Maximum deflection at the top = 42.802 mm

The maximum internal forces are shown below;

The table of comparison is shown below;

Action Effect	X-Bracing	V-Bracing	% Difference
Deflection (mm)	52.63	42.80	18.67%
Maximum axial tension (kN)	82.615	65.883	20.25 %
Maximum axial compression (kN)	91.478	74.095	19.00%

From the table above, it can be seen that under the same conditions, V-braced masts perform better than X-braced masts under the effect of the wind.

When a lateral load is applied to an X-braced frame, the diagonal braces are subjected to compression while the horizontal web acts as the axial tension member in order to maintain the frame structure in equilibrium under lateral load. For V-braced frames (chevron bracing), one of the braces resists the tension while the other brace resists compression at each storey.

Both the tension and compression brace distribute the lateral load equally in the elastic range before the buckling point. However, the tension brace will remain in tension while the compression brace will lose all the axial load capacity after buckling. This contributes to the unbalanced distributed lateral load and can cause a large bending moment in the intersection of the beam and braces.

However, V-braced frames possess high elastic stiffness and strength when compared with X-braced frames.

Lapping of Reinforcements and Use of Rebar Couplers – Peculiarities and Differences

By

Ubani Obinna

-

July 1, 2020

Table of Contents

In reinforced concrete buildings, it is almost impossible to carry out construction without lapping reinforcements at one point or another. This is due to obvious reasons such as ease of transportation and handling. Also at very long lengths, reinforcements will become unstable under their own weight when placed in a vertical position. Lapping (reinforcement splicing) is the traditional way that has been used to join two different reinforcements during construction. However, in recent times, mechanical splices (such as rebar couplers) have been introduced to make two different pieces of reinforcements joined together to behave as ‘one continuous unit’.

The three general methods identified for reinforcement splicing are;

Lap splices
Welded splices, and
Mechanical splices

Read Also…
Detailing of columns to Eurocode 2

It is generally known that when reinforcements are lapped traditionally (lap splices), the two rebars will depend on the bond of the concrete for load transfer. However, with the use of mechanical splices concrete is not needed for load transfer since the two bars tend to behave as ‘continuous unit’. Furthermore, the idea of lapping is inherently wasteful and may lead to heavy congestion of reinforcement in a section of a concrete member. On the other hand, welding of bars is considered a more expensive alternative which may depend on the chemical properties of the reinforcing bar for adequate weldability.

detailing at laps — Lap region of a concrete column

The advantages of using mechanical splices over lap splices in reinforcements are;

(1) Enhanced Structural Performance

The structural integrity of joints connected with the use of mechanical splices is enhanced since the connection does not need to rely entirely on concrete bond for load transfer. In seismic applications, mechanical splices tend to maintain structural integrity when bars are stressed into the inelastic range. On the other hand, lap splices can infringe into the plastic hinge region, which is in violation of code limitations.

(2) Saves design effort

In reinforced concrete design, the engineer is expected to carry out calculation of lap length, which can depend on the size and type of the reinforcement, the grade of the concrete, the bond condition, and concrete cover. But with the use of mechanical splices, such computational effort is eliminated.

rbar — Rebars joined using mechanical couplers

(3) Savings in Materials

With the use of mechanical splices, reinforcement bars do not overlap, thereby leading to savings in materials.

(4) Reduction in bar congestion

Lapping of reinforcement effectively doubles the steel-to-concrete ratio, and the resulting congestion can make the placement and consolidation of concrete difficult. Furthermore, the code limits the area of reinforcement in lap regions of columns to 0.08A_c. Using rebar couplers completely eliminates this challenge.

A peculiar disadvantage of mechanical splices is the extra step of preparation of bars required before couplers are installed. A special care is needed in cutting the threads correctly and to protect the threads from corrosion before installing the couplers as these reasons could lead to improper fixing of couplers.

REBAR COUPLER — Typical threaded end of a rebar for coupling

However in a research carried out in Sri Lanka in the year 2018, the additional cost incurred in preparation of bars which includes the cost of two machineries (forging machine and thread cutting machine) and additional workmanship (one electrician and an unskilled labourer), seems to produce attractive cost benefits compared to lap splicing for larger diameter bars such as 32 mm and 40 mm.

In a study to check the failure pattern of reinforcements joined using couplers, two failure modes were identified which are;

Failure of the rebar, and
Failure of connection

However, it was found that the failure stress of the bars which failed due to failure of connection is greater than the yield stress of steel and closer to the fracture point of steel. Due to the fact that the design stress of rebars is taken as the yield stress of steel and also due to the reduced stress in steel when steel are in contact with concrete, the risk of failure due to improper fixing of couplers may not be critical.

Water Bars (Water Stops) – Uses, Types, and Applications

By

Ubani Obinna

-

June 27, 2020

Table of Contents

In reinforced concrete water retaining/excluding structures, a potential source of leakage or water ingress is the construction joint, and that is why water bars (also called water stops) must be provided whether there is risk of ground water or not. Water bars are barriers that are provided in construction joint of concrete structures to prevent the egress or ingress of water into the structure.

Typical water stop details 1 — Water bar arrangement in a water retaining structure

Constructions joints are usually created due to the difficulty in casting multiple members of a water retaining structure on the same day (such as casting tank base and walls on the same day). Apart providing reinforcements to limit crack width to 0.2mm and providing water stops, good workmanship is important because inadequacies such as honeycombs, poor compaction, foreign materials in the concrete, poor installation of water bars etc can compromise the entire effort.

In the construction of tanks and swimming pools, the construction joint between the walls and the base is facilitated with the use of kickers, which are best cast monolithically with the base slab. Kickers should be at least 150 mm thick, and should incorporate the water bars. Kickerless construction can be carried out by some contractors, but that will increase the risk of water penetration. However, hydrophilic (water swellable) water bars can be used when there is no kicker, and this has performed well in construction from experience. Accurate positioning of PVC water bar is important in this case.

kicker details and water bar installation — Typical water bar and kicker details

Types of Water Stops

The most popular types of water stops are;

Preformed strips water stops
Hydrophilic or swellable water stops
Cementitious crystalline water stops
Injectable water stops

(a) Preformed Strips Water Stops

Preformed strips are made of impermeable but durable materials which are embedded in the concrete during concreting. Typically they are installed at the construction joints to provide watertight seal during movements within the joint. These are popularly called water bars. Water bars are usually made of flexible PVC material or steel, with the former being more popular. However, care must taken during placement to ensure that the flexible PVC water bar does not collapse during placement of concrete. This is the advantage that steel water bar has over PVC water bar . Steel water bar is usually made of 1.5 mm thick black steel or stainless steel, which by virtue of its stiffness remains permanently in place during concreting.

pvc water bar — PVC water bar arrangement

Preformed water stops are better used with kickers as described above.

(b) Hydrophilic/Swellable Water Stops

These water stops are produced with hydrophilic material which swells and develops a sealing pressure when it comes in contact with water. They are usually bonded or nailed to the old concrete before the first pour of the new concrete and their use is limited to joints where the movement is low. Swellable water bars are popular in kickerless construction.

hydrophilic water bar 1 — Hydrophilic or swellable water bar

For swellable water bars to be effective, the strip must lie flush with the substrate (hardened concrete). Appropriate care must also be taken at the joints to ensure that there will be no room for water penetration.

(c) Cementitious Crystalline Water Stops

The water stopping action of this material is from salt crystallisation in the presence of water within the pores and capillaries of the concrete. They are prepared by mixing cement, fillers, and chemicals on site as slurry, which is then applied on the surface of the old concrete before pouring the new concrete. They are not suitable for use in expansion joints but construction joints only.

KWS application — *Cementitious crystalline water stops*

(d) Injectable Water Stops

With this method, resins or other proprietary fluid in injected under pressure through perforated or permeable hoses/tubes that have been preinstalled on the old concrete before the pour of the new concrete. This is done after temperature and shrinkage movements have stabilised sufficiently, and the resin flows out of the hose into cracks, fissures or holes in the joint, thereby sealing the water paths in the joint. This is suitable for construction joints only.

injectable water bar — *Injectable water stops*

Structville Announces Webinar on Design of Roof Trusses (July, 2020)

By

Ubani Obinna

-

June 26, 2020

In our core commitment to provide a flexible platform for learning, improvement, and disseminating civil engineering knowledge, we are delighted to announce that we will be holding our webinar for the month of July, 2020. Details are as follows;

Theme: Structural Design of Roof Trusses
Date: Saturday, 4th July, 2020
Time: 7:00 pm (WAT)
Platform: Zoom
Fee: NGN 1,500 only ($5.00 USD)

Features:

Introduction to structural behaviour of trusses
Types of trusses
Analysis of trusses
Design of Steel trusses
Interactive question and answer sessions
Detailing of trusses and connections

To book a space for this webinar, click HERE.

For more information, contact:
WhatsApp: +2347053638996
E-mail: info@structville.com

Over a time of about 4 years, www.structville.com has published over 200 free unique articles on different topics in civil engineering, and continues to get better. We sincerely appreciate the patronage and support we have received over the years, as more and more people all over the world continue to benefit from the services we offer. We look forward to an exciting future together in the civil engineering community. God bless us all.

WhatsApp Image 2020 06 26 at 2.54.37 PM 1

Massive Diaphragm Wall Reinforcement Installation (Video and Discussion)

By

Ubani Obinna

-

June 20, 2020

A video was recently shared on Linkedin showing a huge and long reinforcement cage for a diaphragm wall being put in place. Please watch the video below.

Some professionals who viewed the video suggested that the reinforcement cage would have be done in segments, in order to guarantee the quality of the concreting. To understand the perspectives properly, let us look at what a diaphragm wall is.

Diaphragm wall is a continuous reinforced concrete wall constructed under the ground. The depth of a diaphragm walls can get up to 25m-50m, and they are constructed in-situ, panel by panel. As a result, the excavation works begin with the mechanical grep being lowered to the ground by a crawler crane and taking excavation. Some of the functions of diaphragm walls are;

As a retaining wall
As a cut-off provision to support deep excavation
As the final wall for basement or other underground structure (e.g. tunnel and shaft)
As a separating structure between major underground facilities
As a form of foundation (barrette pile – rectangular pile)

Knowing that such construction and concreting must be done at great depth which may not be easily accessible for human inspection, it would be impractical to prepare the reinforcement cage in segments. The option for quality work to ensure properly consolidated concrete will have to come from the contractor through professional concreting and carefulness using pumps. Let us look at some of the comments from the post.

Why? are they trying to save on lap joints? this structure can be cast cheaply on hop up instead of using hugely expensive cranes. And how will they keep it safe while they are casting it not to mention tensile forces from all that rebar flapping around..

It’s diaphragm wall & tremie will be used to pour concrete to avoid segregation of concrete due to higher level. On time casting Ensures integrity and no weak point in structure as after casting excavation will be done on one side, other side would add large earth pressure.

Wow that is a deep one, the concrete pour must be exceptionally well controlled to ensure no voiding

Someone asked,

Unbelievable… How all this height of reinforcing steel cage stable? …. we need explanation

The response given to the question is shown below;

Standard procedure for a diaphragm wall completed in sections. There will be spacers to keep the cage in position. If you go onto any installer web page you will see the procedure in detail or watch on youtube

Other important comments are as follows;

This is for Diaphragm wall. Placing concrete is not the problem . The main problem if any defects happens that’s the challenge.

Appears to be for a diaphragm wall or the like, hence the trench. Tremie pipes would likely be used for the placing of the concrete.

There is no vibrators used in such castings. Pouring concrete using tremie pipes in. Especially the pit is filled with slurry or fluids to support unstable soil strata until concreting.

Really a daring try.. Assuming the concreting by ultra high performance SSC without vibrators. But what it’s going to shore for what depth. Also I assume the above comment on segments are for rebar cages not the concreting or construction as the diaphragm walls shall go in single pore (please correct me if wrong). So the question is what for this not how!

In 1970 Diaphram wall of length 25+ meter were cast in Kolkata Metro Project using tremie for concrete placement. The entire pit was dug using grab cutter and used to be filled with bentonite slurry for stability of the pit.

I understand that Self Compacting concrete with allowable aggregate with allowable degree of strength gain during pour without cold joints will be a good option.

Very interesting video. Diaphragm walls have been used in UK reaching depths about 40m in circular shape. From the design point of view, it has been a challenge in all aspects as numerical modelling, groundwater strategy, tolerances, etc

Thank you so much for reading. I hope you have learnt something from the discussion. If you have some time to spare, you can take some free online quiz to refresh your mind on some important topics in Civil Engineering.

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Lapping of Reinforcements and Use of Rebar Couplers – Peculiarities and Differences

(1) Enhanced Structural Performance

(2) Saves design effort

(3) Savings in Materials

(4) Reduction in bar congestion

Water Bars (Water Stops) – Uses, Types, and Applications

Types of Water Stops

(a) Preformed Strips Water Stops

(b) Hydrophilic/Swellable Water Stops

(c) Cementitious Crystalline Water Stops

(d) Injectable Water Stops

Structville Announces Webinar on Design of Roof Trusses (July, 2020)

Massive Diaphragm Wall Reinforcement Installation (Video and Discussion)

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