Shearwalls & Boundary Elements
By Javed B. Malik
First published in Concrete International Magazine, December 2007
Critical intersections require critical review
Boundary elements are the heavily-reinforced, critical
zones of shearwalls normally located close to the
edges of the wall or next to large wall openings (Fig. 1).
In the depth of the floor system, where floor beams and link
beams must frame into the wall, boundary elements can
become very congested. As Fig. 2 shows, the horizontal
reinforcement for two floor beams as well as the horizontal
and diagonal reinforcement for the link beam must be
threaded between:
- Vertical reinforcement for the boundary element and the wall;
- Horizontal reinforcement for the wall; and
- Ties needed to confine the boundary element vertical reinforcement.
Some of this reinforcement may be located in several
layers and some of the bars may be hooked, making the
congestion even more severe.
If the walls are sized without proper consideration of
the potential conflicts, two problems typically occur.
First, it becomes very difficult to fit all the bars in the
allotted space—bars may interfere with each other and
may not fit. Even if there is enough room for all of the
bars to fit in their final positions, however, congestion
may make it difficult, if not impossible, to assemble the
reinforcing bar cages. Hooked bars can be particularly
challenging to place in their final position when having to
thread them through a congested area of reinforcement.
The second problem is that concrete placement is
very difficult around congested areas. Not only is it
difficult to get the concrete into these areas, it is also
difficult to insert the vibrators properly. If not properly
vibrated, voids can be created in the wall at the most
critical locations. If undetected, these voids may jeopardize
the structural performance of the wall.
Suggestions for constructibility
A short time spent during the schematic design and
construction document phases can save a lot of time and
trouble during construction. The following suggestions
will help ensure a constructible design. When considering
these suggestions, designers should keep in mind that
they are made strictly from a constructibility point of
view. Their impact on structural performance should be
carefully evaluated by the designer. Reference 1 deals
with some of these issues in detail.
The starting point for the designer should be to draw a
sketch of the critical areas to scale, study the clearances,
and make sure that all of the reinforcement can fit. With
modern software, critical joints can be drawn and studied
in three dimensions.
An obvious way to reduce congestion is to increase the
thickness of the wall, creating more room for concrete and
reinforcing bars. This may not always be possible however,
due to architectural constraints and loss of precious lease
space. An alternative is to thicken only the boundary
elements. Making the boundary elements only a few inches
thicker than the wall can greatly reduce congestion in the
joint by moving the boundary element vertical reinforcement
outside of the link beam horizontal reinforcement, as shown
in Fig. 3. If the boundary element is thickened, the link beam
can also be easily widened to provide additional room for
link beam reinforcement. Because it
may pose problems with space
planning, the option of thickened
boundary elements should be carefully
studied, particularly where they
encroach into elevator shafts.
Similar to increasing the thickness
of a boundary element, increasing the
length of boundary elements can help
spread the vertical bars apart, leaving
more room for floor beam reinforcement
perpendicular to the wall. This,
however, may also increase the
quantity of the vertical reinforcing
steel required for the design.
Splices for both vertical and
horizontal bars should be moved
outside of the joint, if possible.
As shown in Fig. 4, this reduces the
number of bars taking up space in the
highly congested area of the joint.
Similarly, terminating the longitudinal
link beam reinforcement with a
straight development length instead of
a hook can reduce congestion but also
makes the bars much easier to install.
When hooks are required at each end
of a bar in a floor beam perpendicular
to a wall, splicing the bar in the middle
of the beam allows much easier
installation than placing a single bar
with hooks on both ends in a congested
joint (Fig. 5). The savings in placement time can easily offset
the increased steel quantity. Another option may be to use
headed bars instead of hooks.[2]
Diagonal bars for link beams can be especially difficult
to properly coordinate with other reinforcement. It's
important to keep in mind that each bar is located in a
separate layer and to be certain that the wall or the
boundary element is wide enough to accommodate all
these bar layers. Another location that can be difficult for
installing diagonal bars is where they extend into the wall
or boundary element. Often, this location is also where
longitudinal bars from floor beams perpendicular to the
wall enter the joint. This may require moving the floor
beam bars to clear the diagonal bars from the link beam.
Another issue to look for is when the floor beams and link
beams are of the same depth. The floor beam bars will have
to be raised or lowered to clear the longitudinal bars from the
link beam. This will change the height of the beam stirrups.
Similarly, if the link beams and the shearwall are the
same width, the link beam horizontal bars will be located
inside of the wall vertical bars. This will increase the
clear cover for the link beams and make the stirrups
narrower. This needs to be brought to the attention of
the steel detailer by a section cut through the floor
beams. If not addressed properly, the detailer would
probably deduct 3 in. (75 mm) from the overall width and
depth of the beam to get the stirrup dimension.
Some other suggestions for keeping these joints
constructible include placing the horizontal wall bars and
boundary element ties in the same plane and using
mechanical splices. Placing the horizontal bars and the ties
in the same plane reduces the number of reinforcement
planes and increases clearances. Vertical wall bars will
thus be located inside the horizontal bars. Mechanical
splices can be especially helpful in alleviating congestion
at splices located in joints, but relocating the splice to
another location is often an even better choice.
As a final note, remember that the actual bar diameter
for calculating clearances is larger than the nominal
diameter due to the deformations. Similarly, the curvature
of column ties, beam stirrups, and hooks should be taken
into account because these also reduce clearances.
Acknowledgments
The author is thankful to the members of ACI Committee 315-B, Details of Concrete Reinforcement—Constructibility, for their valuable suggestions and contributions.
References
1. Wyllie, L.A. Jr., and La Plante, R.W., "The Designer's Responsibility
for Rebar Design," Structural Bulletin Series 1, Concrete
Reinforcing Steel Institute, Schaumburg, IL, Aug. 2003, 16 pp.
2. Mobeen, S.S.; Elwi, A.E.; and Ghali, A., "Double-Headed Studs in
Shearwalls," Concrete International, V. 27, No. 3, Mar. 2005, pp. 59-63.
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