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A review on the use of shear walls in The Proposed 4 th Ngong Avenue Towers CONTENTS Chapter Page no 1. Introduction 1.1 Introduction to structural concrete shear walls ………………..…...……. 2 1.2 Introduction to the case study project …………………………....……… 2 1.3 Purpose of this case study ………………………………………...……… 2 2. Basic concept of how shear walls resist lateral loads 3. Basic seismic design process for shear walls design and detailing in relation to earthquake resistance for the proposed 4th Ngong Avenue Towers. 3.1 Design brief …………………………...……………………..…..…... 6 3.2 Consequences of earthquake damage ...……. ………………..….…... 6 3.3 Cost of earthquake resistance…………...…….……………….. …….... 6 3.4 Seismic activity of the region ..…...……. ……………………..……... 7 3.5 Site response to earthquake geotechnical data ......………..…….... 8 3.6 Determination of structural form R.c,shear walls, frames etc ……….. 9 3.7 Determination of structural ductility ………………………………... 11 3.8 Non seismic stresses acting on the structure ………………………… 15 3.9 Detailing for seismic resistance ie damping using structural and architectural methods that enhance damping properties of the structure - 15 4. Conclusions of the case study …………….…. ............................................. 16 5. Appendixes 1
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Page 1: Shear Walls in the Building 2

A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

CONTENTS

Chapter Page no

1. Introduction 1.1 Introduction to structural concrete shear walls ………………..…...……. 21.2 Introduction to the case study project …………………………....……… 21.3 Purpose of this case study ………………………………………...……… 2

2. Basic concept of how shear walls resist lateral loads 3. Basic seismic design process for shear walls design and detailing in relation to

earthquake resistance for the proposed 4th Ngong Avenue Towers.3.1 Design brief …………………………...……………………..…..…... 63.2 Consequences of earthquake damage ...…….………………..….…... 63.3 Cost of earthquake resistance…………...…….………………..…….... 63.4 Seismic activity of the region ..…...…….……………………..……... 73.5 Site response to earthquake geotechnical data …......………..…….... 8 3.6 Determination of structural form R.c,shear walls, frames etc ……….. 93.7 Determination of structural ductility ………………………………... 113.8 Non seismic stresses acting on the structure ………………………… 153.9 Detailing for seismic resistance ie damping using structural and

architectural methods that enhance damping properties of the structure - 15 4. Conclusions of the case study …………….….…............................................. 165. Appendixes

A. APPENDIX A: Sample architectural drawings of the proposed 4th Ngong avenue towers …...…...……………………..…..…... 17

B. APPENDIX B: Sample structural drawings of the proposed 4th Ngong avenue towers …………………….……………...…... 18

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

C hapter 1

1.1 Introduction to structural concrete shear walls

Shear walls are reinforced concrete walls designed to provide high in-plane stiffness and strength for both lateral and gravity loads. Shear walls are ideally suitable for tall buildings of over 15 stories. In the preliminary stages of design, the designer in liaison with the architect will always try to ensure that the position and general arrangement of the shear walls is such that it enhances the stiffness of the building in both flexure and torsion. In rectangular buildings, external shear walls in the short direction can be used to resist lateral loads acting on the wide faces, with rigid frames or infill panels in the long direction. In buildings of a square plan form a strong central service core, surrounded by flexible external frames can be used. If strong points are placed at both ends of a long building, the resistance provided to the subsequent shrinkage and thermal movements of floors and roof should be carefully considered. In all cases the floors and the roof are considered to act as stiff plates so that at each level the horizontal displacement of all walls and columns are taken to be the same provided the total lateral load acts through the shear centre of the system. If the total load acts eccentrically then the additional effects of the resulting torsion moment needs to be considered. Depending on the architectural and client demands, size and use/nature of structure and adversity of lateral loads, the structure can end up with shear walls without openings or shear walls with openings and in either case the shear walls could be combined with columns. Shear wall systems can be categorised into cross wall system, long wall system and two way system.

1.2 Introduction to the case study project

The Proposed 4th Ngong Avenue Towers is a 23 storey (including the 2 basement floors) office and parking building in plan for construction by the client Upward scale Investments Co. Ltd. The structural engineer consultant is Gath consulting engineers. At the time of preparing this document only the architectural drawings and final structural layouts and sections were completed. The reinforcement detailed drawings are yet to be produced therefore not attached to this case study. However since detailing is a critical element in the success of a shear wall a proposal shall be made as to what is expected in the detailed drawings.

1.3 Purpose of this case study

In this case study we are going to use The Proposed 4 th Ngong Avenue Towers architectural and proposed structural layouts as a case study to review the shear wall design criteria and steps. The following critical aspects of shear wall design shall be considered:

The location and general arrangement of the shear walls Basic concept of how shear walls resist lateral loads. Check the basic seismic design process for shear walls design and detailing in

relation to earthquake resistance for the proposed 4th Ngong Avenue Towers.

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C hapter 2

2.0 Basic concept of how shear walls resist lateral loads

The shear wall loading is idealised in design as follows; The loading of the lateral load (wind or earthquake) is concentrated at the floor levels due to the fact that the floor slab has a high lateral stiffness.

At each floor level the lateral load is distributed to all lateral load resisting elements including the frames, shear walls and cores in the ration of their stiffness.

When the lateral load is to be resisted by a shear wall as for the case in this case study then the shear wall acts best as a lateral load resisting element if the load acts in the direction parallel to the thickness of the wall. Basically the wall should be well anchored and detailed in a manner that will ensure the wall acts as a single unit in as it transfers the resisted lateral forces to the foundation. When the design lateral load strikes the structure a tension force develops on the front face (where the load strikes the structure) of the the wall and a compression force on the back face of the wall and the shear forces as shown in the sketch bellow,

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

Diagram showing force concentrarionts in a shear wall during lateral load resisting action of the wall.

The shears at the centre of the panel cancel out, the tension and compression create a couple which is equal to a moment.

For both very tall and small shear walls the basic load resisting concept as described above will apply.

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

In the design of shear walls to resist seismic forces it is important to consider the characteristics of seismic forces that lead to structural damage or collapse.The following illustrations summarise the seismic characteristic

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C hapter 3

3.1 Design brief

The client, Upward scale Investments Co. Ltd intends to develop a 23 storey (including the 2 basement floors) office and parking tower. The proposed buildings name has been set by the client as the 4th Ngong Avenue Towers. The structural engineer consultant is Gath consulting engineers. The project architect in liason with the client have proposed a tower characterised by large spans of 8.25 meters and split hexagonal shaped building with limited number of internal columns and a central core of comprising of lifts and staircases servicing the tower.The first 8 floors are dedicated for parking and hence the need for versatile vehicular movements. The detailed description of the architectural proposed requirements is as indicated in appendix A Sample architectural drawings of the proposed 4th Ngong Avenue towers. The tower is well packaged as an architectural masterpiece with regard to the architectural shape which is anticipated to be one of a kind in its location – 4th Ngong Avenue.

3.2 Consequences of earthquake damage

The main consequences of earthquake damage to high rise structures as in the context of the case study include but not limited to the following:-

Loss of human life Damage and loss of property

For the case study building the consequence of earthquake damage can be estimated based on estimated occupancy of the structure which roughly estimates the number of human lives that are endangered during an earthquake. Three main methods used in estimation of occupancy of office structures include estimation based on office floor space area, estimation based on parking space provided and estimation based on survey of similar use buildings.

- For the case study tower Total office space area is calculated from the sum of the dedicated office space areas per floor for the whole tower and is equal to 10,269M2

assuming 2meter square per person the building can be estimated to host human life in the range of 10269/2 = 5,134 persons. The consequence of earthquake damage could mean the loss of nearly 5,000 people.

- On the other hand construction cost of a tower like the proposed 4 th Ngong avenue is estimated to be close to half a billion shilling. This will be lost if the structure is not well designed to resist earthquakes. In addition to structural losses the property of the businesses and offices that will be in operation will be destroyed.

3.3 Cost of earthquake resistance

There are two broad ways of dealing with earthquake stresses:1. By use of conventional building design of most common box shaped structural form

and reduce the stresses acting upon it by use of shear walls, frames, tubes etc in very tall structures this could be incorporated with other systems such as pendulum dumpers and sometimes base isolation can be done by placing dampers between foundation and the superficial structural walls

2. We can also change the design to relieve stresses more effectively. This involves use of shapes such as one shown in figure 3.1 bellow which are by nature efficient stress relievers.

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figure 3.1A comparison of the efficiency versus the cost of these two basic ways of dealing with earthquake forces. Domes (and their planar cousin, the pyramid) distribute lateral forces very well for three reasons. First, and foremost, even movements strong enough to get a dome to sway will not produce areas of the structure that have no support against gravity, because the base is much wider than the top. Second, domes distribute forces in all directions naturally, and thus the design is much better at dissipating energy. And third, most of the mass of a dome is low, and this lower centre of gravity greatly reduces the chance of collapse. But why wasn’t a pyramid or a dome shape not considered for the case study tower? A solution to this question lies partly in the associated cost. Until recently; domes were very hard to build. Basically every piece that goes into a dome has to be custom made, and until the keystone is in place the structure is actually rather fragile. To build such a tower would take very many years and thus making it very expensive. Though domes result in more office space than its equivalent box shape, the form of the space is more difficult to utilise. Consequently the use of shear walls which are faster to erect as well as well understood turned out to the choice of lateral load and earthquake load resistance for the proposed 4th Ngong avenue Towers. The cost of earthquake resistance can be attributed to the tower tall reinforced concrete shear walls that have to be provided in four independent directions of the building in addition to the reinforced central core that also hosts the lift and the staircases. See the attached structural layouts. It should be noted that though we may desire to have a more earthquake proof structure, we are limited with required funds hence the engineers norm is to strive for something slightly less ambitious - earthquake-resistant building - which is designed to prevent total collapse and preserve life, as well as construction budgets.

3.4 Seismic activity of the region

The proposed 4th Ngong avenue towers is located in within Nairobi in a zone category VI according to the the Kenyan Code of Practice for the Design of Buildings and other Structures in relation to earthquakes(1973). According to this code the Tower can further be categorized as a class A. The earthquake design coefficients corresponding to zone VI will therefore be used for establishing the earthquake design forces. These coefficients are derived based on the design perceived earthquake strength and ground motions expected under such conditions.

3.5 Site response to Earthquake geotechnical dataThe rock formation of upper hill area consists of hard rock strata located within approximately 2 meter depth underneath. If this proves to be the case for the project site it

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will come as good news to the foundation design engineer. This is due to the fact that to be able to anchor the structure safely against earthquake it is highly recommended to put foundations onto a hard rock stratum. This is due to the fact that soft rocks, sand and loose soils tend to compact during earthquakes producing large-scale displacements and movement that could highly interfere with the structural integrity and even lead to total collapse. However a proper geological study shall be carried out to establish the extent of the rock, the strength of the rock and whether the rock has faults, cracks or is susceptible to rapid weathering. Based on the finding of the geological report for the site an appropriate foundation sizing and design shall be carried out.

Soil Liquefaction — loose granular soil compact under shaking, settling, and putting ground water under pressure. The pressure is relieved when the water migrates to the surface, where it produces sand boils. This produces mega settlements that will lead to structural failure

Generally shaking in soft sediments is larger and longer than when compared with the shaking experienced at a "hard rock" site

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

Building tilted by ground failure caused by liquefaction, Kobe earthquake

3.6 Determination of structural form

In this project the decision to use shear walls as the seismic and lateral load resisting forms are based on the following key advantages of shear walls:

Shear wall structures have shown very good performance in resisting earthquakes all over the world and form the popular choice in many earthquake prone areas like chile New Zealand USA etc.

Shear walls are easy to construct, because reinforcement detailing of walls isrelatively straight-forward and therefore easily implemented at site.

Shear walls are efficient, both in terms of construction cost and effectiveness inminimizing earthquake damage in structural and non-structural elements (like glass windows and building contents).

Shear walls provide adequate lateral strength to resist horizontal earthquake or wind forces placed upon the structure Provide lateral Stiffness preventing excessive movement in the roof or floors above Resist two types of forces: Shear forces and uplift forces some vertical loads could be

loaded on to the shear walls. Provide efficient transfer horizontal forces to the next element in the load path below

themIt should be noted the positioning and general shape of the shear walls and structure as a whole plays a vital role in the efficiency in resisting the seismic and lateral load effects on the structure. Irregular shapes, L- shapes even though they are very visually appealing, they perform poorly with regard to resistance of seismic and lateral load effects. The proposed 4th Ngong Avenue towers shall have a hexagonal shape with shear walls as shown in figure. 3.6.1. A close examination of the arrangement of the shear walls and the central core clearly satisfies the fact that all directions are well catered for to resist the seismic forces.

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

Figure 3.6.1 structural layout of the proposed 4th Ngong avenue towers showing the general arrangement of shear walls

Since shear walls carry large horizontal earthquake forces, the overturning effects on them are large. Thus, design of their foundations requires special attention.Shear walls should be provided along preferably both length and width. However, if they are provided alongonly one direction, a proper grid of beams and columns in the vertical plane (called a moment-resistant frame) must be provided along the other direction to resist strong earthquake effects. Door or window openings can be provided in shear walls, but their size must be small to ensure least interruption to force flow through walls. Moreover,openings should be symmetrically located. Special design checks are required to ensure that the net cross-sectional area of a wall at an opening is sufficient to carry the horizontal earthquake force. Shear walls in buildings must be symmetrically located in plan to reduce ill-effects of twist in buildings. They could be placed symmetrically along one or both directions in plan. It should be noted that shear walls are more effective when located along exterior perimeter of the building – such a layout increases resistance of the building to twisting. These shear wall design considerations have been catered for in the proposed 4 th Ngong Avenue towers

3.7 Determination of structural ductility

Shear walls perform much better if designed to be ductile. Mainly due to the fact that a ductile structure will absorb more energy thereby delaying the structural collapse as well as will

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

display large warning signs/cracks before collapse. Studies have shown that the structural ductility of shear walls is influenced by various factors such as overall geometrical proportions (section length), reinforcement type, reinforcement ratio, reinforcement distribution (uniform and concentrated), axial force level, connection with remaining elements in the building and material properties. For example increasing reinforcement ratio will reduce ductility in case of uniform reinforced shear walls, but it stays stable in case of concentrated reinforcement. As for axial force level, curvature ductility becomes lower as axial force increases and almost diminishes when it becomes more than half ultimate axial force bearing capacity. Most design codes demand a reinforcement area ratio of at least 0.25% for a shear wall for it to be categorised as a ductile shear wall.The steel reinforcing bars are to be provided in walls in regularly spaced vertical and horizontal grids as shown in figure 3.7.1. The vertical and horizontal reinforcement in the wall be placed in two parallel layers. Horizontal reinforcement needs to be anchored at the ends of walls. Also under the large overturning effects caused by horizontal earthquake forces, edgesof shear walls experience high compressive and tensile stresses. To ensure that shear walls behave in a ductile way, concrete in the wall end regions must be reinforced in a special manner to sustain these load reversals without loosing strength as shown in figure 3.7.2. End regions of a wall with increased confinement called boundary elements must be used. This special confining transverse reinforcement in boundary elements is similar to that provided in columns of RC frames. Sometimes, the thickness of the shear wall in these boundary elements is also increased see figure 3.7.3 (though this will not be used in the proposed 4th Ngong Avenue towers). RC walls with boundary elements have substantially higher bending strength and horizontal shear force carrying capacity, and are therefore less susceptible to earthquake damage than walls without boundary elements.

Figure 3.7.1. For enhanced structural ductility steel reinforcing bars are to be provided in walls in regularly spaced vertical and horizontal grids

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

Confining reinforcement inboundary elements:135° hooks, closely spaced ties

Figure 3.7.2. For enhanced ductility End regions of a wall with increased confinement called boundary elements must be used. In the proposed 4th Ngong avenue towers boundary elements without increased thickness are to be used

.Figure 3.7.3. alternative boundary element with increase thickness.

For better performance the ductility of joints is also very important. This is achieved through proper detailing techniques as shown in the following diagrams and sketches,

Beam /wall or column detail to enhance structural ductility. The circled in this beam the circled zone shows the extra rebar length that is to be provided beyond the wall/column face

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Bent sometimes depending on the design result additional independentBars bars are to be provided

Beam joint details to enhance ductility

Also the detailing of other structural members such as Columns is vital for better performance under seismic and lateral loads as shown in the following sketches,

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

Steel reinforcement in columns – closed ties at close spacing improve the performance of columns under strong earthquake shaking.

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It should be noted that depending on the design the walls and columns could be enhance for ductility by use of extra sausage shaped bars as shown bellow.

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3.8 Non seismic stresses acting on the structure

The non seismic forces acting on the proposed 4 th Ngong Avenue towers is primarily wind forces and gravity loads based on floor to floor usage. The vibration caused by movement of cars in the lower floors though minor will need also to be considered in the design and checks made to ensure that the vibrations will not be so huge/amplified to a level that will cause fear to the building users. In the detailed design the wind induced stresses shall be determined based on the Bs 6399 and the higher between wind and seismic stresses used for design. Gravity loads shall be computed based on floor to floor usage and each element designed accordingly using either the British BS8110 code or euro codes guidelines.

3.9 Detailing for seismic resistance i.e. damping using structural and

architectural methods that enhance damping properties of the structure

Detailing shall be as discussed above under ductility. The basic shape of the proposed 4th Ngong avenue towers (split hexagonal) is good enough architectural and structural shape that will enhance structural stability during seismic events. It should be noted that by virtue of the structural size - 49.5 meters longest dimension at ground level this reduces to 33m after the mezzanine floor levels -, it may not be necessary to introduce expansion joints. Other architectural considerations that will boost structural stability to seismic loading include the fact that rather than using a soft first storey structural type as is the habit with many architects, the architect proposed a structural form with continuous/uninterrupted shear walls.

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C hapter 4

Conclusions of the case study

The main objectives of the case study were achieved. The key elements to design criterion guide in the critical assessment of the use of shear walls for particularly seismic loading design for the proposed 4th Ngong Avenue Towers were discussed. The more detailed aspects are however to be investigated by the design team.

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

APPENDIX: A

Sample architectural drawings of the proposed 4th Ngong avenue towers

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A review on the use of shear walls in The Proposed 4th Ngong Avenue Towers

APPENDIX:B

Sample structural drawings of the proposed 4th Ngong avenue towers

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