shearwalls & boundary elements

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Shearwalls & Boundary Elements

By Javed B. MalikFirst 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 f loor system, where floor beams and link beams must frame into the wall, boundary elements canbecome very congested. As Fig. 2 shows, the horizontal reinforcement for two f loor 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; andTies needed to confine the boundary element vertical reinforcement.

-A Paradigm Shift

- Constraints on Reinforcing Bar Modeling

- Reinforcing Bars Exceeding Stock Lengths

- Rebar and Waterstops

- Design to Minimum Dimensions

- Shearwalls & Boundary Elements

- Sloped vs Stepped Footings

- Calculating the Length of Bent Bars

- Beam-Column Joints

-Avoiding the Dead Zone

- Placing Drawings are not Shop Drawings

- The Tolerance Cloud

- Placement Tolerance Clouds

- Forming Tolerance Cloud

- Detailing & Fabrication Tolerance Cloud

arwalls & Boundary Elements http://www.condor-rebar.com/news-articles/2011-03-shearwalls-bound...

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Some of this reinforcement may be located in several layers and some of the bars may be hooked, making thecongestion 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 spacebars may interfere with each other and may not fit. Evenif 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 diff icult to getthe 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 performanceof the wall.

Suggestions for constructibility

A short time spent during the schematic design and construction document phases can save a lot of time and troubleduring 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. Theirimpact 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, andmake 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 andreinforcing 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 linkbeam 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, particularlywhere they encroach into elevator shafts.


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Similar to increasing the thickness of a boundary element, increasing the length of boundary elements can help spreadthe 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, thisreduces 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 hookcan 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]


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