Simplified Design for Structural Walls - Simplified Design of Reinforced Concrete Buildings - page 192

Chapter 6
Simplified Design for Structural Walls

6.1 INTRODUCTION

For buildings in the low to moderate height range, frame action alone is usually sufficient to provide adequate
resistance to lateral loads. Whether directly considered or not, nonstructural walls and partitions can also add
to the total rigidity of a building and provide reserve capacity against lateral loads.

Structural walls or shearwalls are extremely important members in high-rise buildings. If unaided by walls,
high-rise frames often could not be efficiently designed to satisfy strength requirements or to be within
acceptable lateral drift limits. Since frame buildings depend primarily on the rigidity of member connections
(slab-column or beam-column) for their resistance to lateral loads, they tend to be uneconomical beyond
a certain height range (11-14 stories in regions of high to moderate seismicity, 15-20 stories elsewhere).
To improve overall economy, structural walls are usually required in taller buildings.

If structural walls are to be incorporated into the framing system, a tentative decision needs to be made at the
conceptual design stage concerning their location in plan. Most multi-story buildings are constructed with a
central core area. The core usually contains, among other things, elevator hoistways, plumbing and HVAC
shafts, and possibly exit stairs. In addition, there may be other exit stairs at one or more locations remote from
the core area. All of these involve openings in floors, which are generally required by building codes to be
enclosed with walls having a fire resistance rating of one hour or two hours, depending on the number of
stories connected. In general, it is possible to use such walls for structural purposes.

If at all possible, the structural walls should be located within the plan of the building so that the center of
rigidity of the walls coincides with the line of action of the resultant wind loads or center of mass for seismic
design (see Chapter 11). This will prevent torsional effects on the structure. Since concrete floor systems act
as rigid horizontal diaphragms, they distribute the lateral loads to the vertical framing elements in proportion
to their rigidities. The structural walls significantly stiffen the structure and reduce the amount of lateral drift.
This is especially true when shearwalls are used with a flat plate floor system.

6.2 FRAME-WALL INTERACTION

The analysis and design of the structural system for a building frame of moderate height can be simplified if
the structural walls are sized to carry the entire lateral load. Members of the frame (columns and beams or
slabs) can then be proportioned to resist the gravity loads only. Neglecting frame-wall interaction for buildings
of moderate size and height will result in reasonable member sizes and overall costs. When the walls stiffness
is much higher than the stiffness of the columns in a given direction within a story, the frame takes only a small
portion of the lateral loads. Thus, for low-rise buildings, neglecting the contribution of frame action in resisting
lateral loads and assigning the total lateral load resistance to walls is an entirely reasonable assumption. In