Question

A ) Briefly explain the main characteristic of shear wall structures in tall building.
Sketch as necessary. Provide examples as well.
Describe the difference of proportionate and non-proportionate twisting structure.
B) Briefly explain the main characteristic of wall-frame structures in tall building.
Sketch as necessary. Provide examples as well.
C) Describe the factors affecting the stability of tall building. State the modes
involved in buckling of tall building

Answer 1

a)

Shear wall is a structural member used to resist lateral forces i.e. parallel to the plane of the wall. For slender walls where the bending deformation is more, Shear wall resists the loads due to Cantilever Action. In other words, Shear walls are vertical elements of the horizontal force resisting system.

In building construction, a rigid vertical diaphragm capable of transferring lateral forces from exterior walls, floors, and roofs to the ground foundation in a direction parallel to their planes. Examples are the reinforced-concrete wall. Lateral forces caused by wind, earthquake, and uneven settlement loads, in addition to the weight of structure and occupants, create powerful twisting (torsional) forces. This leads to the failure of the structures by shear.

Shear walls are especially important in high-rise buildings subject to lateral wind and seismic forces. Generally, shear walls are either plane or flanged in section, while core walls consist of channel sections. They also provide adequate strength and stiffness to control lateral displacements.

The shape and plan position of the shear wall influences the behavior of the structure considerably. Structurally, the best position for the shear walls is in the center of each half of the building. This is rarely practical, since it also utilizes the space a lot, so they are positioned at the ends. It is better to use walls with no openings in them. So, usually, the walls around lift shafts and stairwells are used. Also, walls on the sides of buildings that have no windows can be used.

When a structural member experiences failure by shear, two parts of it are pushed in different directions, for example, when a piece of paper is cut by scissors.

Proportionate twisting structure- A structure that is not symmetrical on plan about the axis of loading will generally twist as well as translate and twists under the action of horizontal load the resulting horizontal displacement of any floor is combination of translation and rotation of floor about the a centre of twist.

Non Proportionate twisting structure -A structure that is symmetrical on plan about the axis of loading.

b)

Main characteristics of wall frame structure in tall building are.

It consists of wall and frame that interact horizontally to provide stronger and stiffer system.

The walls are usually solid (not perforated by openings) and they can be found around the stairwells, elevator shafts, and/or at the perimeter of the building.

The walls may have a positive effect on the performance of the frames such as by preventing a soft storey collapse.

Wall-frame system suitable for buildings with storey number ranges from 40-60 storey which is greater than that of shear or rigid frame separately.

Braced frames and steel rigid frames provide similar advantages of horizontal interaction.For Example, Burj Al Khalifa which is the tallest structure in the world is constructed using rigid frame system.

c) Factor affecting the stability of High rise building as a whole are the lateral load which include.

1) Seismic Load 2) Wind Load

Modes of buckling are

• Plate Buckling
• Flexural-torsional buckling
• Lateral-torsional buckling
• Plastic buckling
• Crippling
• Diagonal tension
• Dynamic buckling