Vertical Building Research Paper

CHAPTER-5

LATERAL LOAD RESISTING SYSTEMS

5.1 GENERAL Seismic loads are actually generated by the dead weight of the building construction. In visualizing the application of seismic forces, each part of the building has to be considered and its weight as a horizontal force. The major lateral loads on building structure are wind and earthquake loads. In buildings, lateral load is transferred to the foundation in 3 stages..

1. Primary collection (load which transfers from wall to diaphragms)

2. Horizontal distribution (load which transfer from diaphragms to vertical members such as columns etc.,)

3. Vertical transfer (load which transfer from vertical members to the foundation).

FIG 5.1 – LOAD TRANSFER TO FOUNDATION

5.2 TYPES OF LATERAL

In an earthquake (seismic waves), a frame with suitable properties and data can develop plastic hinges that will absorb energy and allow the frame to withstand actual displacements that are larger than calculated in an elastic-based design concepts. In modern moment frames, the ends of beams and columns, being the locations of max seismic moment, are designed to sustain inelastic behavior associated with plastic hinging over many cycles and load reversals. Frames that are designed and detailed for the ductile behavior are called “special” moment frames. Frames without special seismic detailing depend on the reserve strength inherent in the design of the members. The basis of this reserve strength is the load factors in the safety under working-stress design methods. Such frames are called “ordinary” moment frames. For ordinary moment frames, failure occurs due to instant (sudden) brittle mechanism, such as shear failure in concrete members or

Shear walls are generally made of concrete or masonry. They are usually provided between columns, in staircases, lift wells, toilets, and utility shafts. Tall buildings with flat slabs must have shear walls. Such systems as compared to slabs with beams have very little resistance even to average lateral loads. At first shear walls were used in reinforced concrete buildings to resist wind forces. These came into practice only as late as 1940s. With the introduction of shear walls, concrete construction can be used for tall super structure also. Earlier, tall buildings were made only of steel, as bracings to take lateral wind loads could be easily provided in steel construction technique. However, since recent observations have consistently shown the excellent performance of buildings with shear walls even under seismic forces, such walls are now largely used for all earthquake resistant designs. Surveys of buildings after earthquakes have consistently shown that the loss of life due to complete collapse was minimal in buildings with shear walls. However, the most significant property of shear walls for seismic design is different from design for wind, such that it should have good ductility under reversible and repeated overloads or vibrations. In planning shear walls, it is necessary to reduce the bending tensile stresses due to lateral loads as much as