Civil Engineering Reference
In-Depth Information
contrast, frame-wall interaction must be considered for high-rise structures where the walls have a significant
effect on the frame: in the upper stories, the frame must resist more than 100% of the story shears caused by
the wind loads. Thus, neglecting frame-wall interaction would not be conservative at these levels. Clearly, a
more economical high-rise structure will be obtained when frame-wall interaction is considered.
With adequate wall bracing, the frame can be considered non-sway for column design. Slenderness effects
can usually be neglected, except for very slender columns. Consideration of slenderness effects for sway and
non-sway columns is discussed in Chapter 5, Section 5.6.
6.3
WALL SIZING FOR LATERAL BRACING
The size of openings required for stairwells and elevators will usually dictate minimum wall plan layouts.
From a practical standpoint, a minimum thickness of 6 in. will be required for a wall with a single layer of
reinforcement, and 10 in. for a wall with a double layer (ACI 14.3.4). While fire resistance requirements will
seldom govern wall thickness, the building code requirements should not be overlooked. See Chapter 10 for
design considerations for fire resistance. The above requirements will, in most cases, provide stiff enough walls
so that the frame can be considered non-sway.
The designer has to distinguish between sway and non-sway frames. This can be done by comparing the total
lateral stiffness of the columns in a story to that of the bracing elements. A compression member may be
considered non-sway if it is located in a story in which the bracing elements (shearwalls) have such substantial
lateral stiffness to resist the lateral deflection of the story that any resulting deflection is not large enough to affect
the column strength substantially. A simple criterion is given in ACI 10.10.1 to establish whether structural walls
provide sufficient lateral bracing to qualify the frame as braced: The structural walls must have a total stiffness at
least equal to twelve times the sum of the stiffnesses of all the columns in a given direction within a story:
12EI
(columns)
The above criterion can be used to size the structural walls within the range of structures covered in this
publication so that the frame can be considered non-sway.
EI
(walls)
≥
6.3.1
Example: Wall Sizing for Non-Sway Condition
Using the approximate criteria given in ACI 10.10.1, size the structural walls for Alternate (2) of Building #2
(5 story flat plate)*. In general, both the N-S and E-W directions must be considered. The E-W direction will
be considered in this example since the moment of inertia of the walls will be less in this direction. The plan
of Building #2 is shown in Fig. 6-1.
Required fire resistance rating of exit stair enclosure walls = 2 hours
For interior columns: I = (
1
/
12
)(16
4
) = 5461 in.
4
For edge columns: I = (
1
/
12
)(12
4
) 1728 in.
4
I
(columns)
= 8(5461) + 12(1728) = 64,424 in.
4
12I
(columns)
= 773,088 in.
4
* The 5-story flat plate frame of Building #2 is certainly within the lower height range for structural wall consideration.
Both architectural and economic considerations need to be evaluated to effectively conclude if structural walls need to be
included in low-to-moderate height buildings.
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