Avoiding the Dead Zone

Constructibility concerns at intersections of curved and tangential members

Constructibility issues aren't always the result of members that are heavily reinforced—geometry can also be a culprit. The connection of a beam with a circular column (Fig. 1) and the tangential intersection of a straight beam (or wall) with a curved wall (Fig. 2) are common examples. The curvilinear layout of the vertical bars in the column or wall results in slight offsets that can create "dead zones" through which horizontal bars cannot pass. This problem is most serious in large heavily reinforced columns such as those found on many highway structures. Bridge bents, for instance, often have very heavily reinforced caps sitting on round columns with the same diameter as the cap width. The problem is compounded when the cap has multiple sets of stirrups that allow little, if any, lateral adjustment of the longitudinal beam bars. Double or triple layers of vertical bars can make the dead zone even worse.


Possible solutions

The following are some of the best methods for dealing with this problem. For each individual case, however, the best solution will depend on the specific layout of the reinforcement as well as structural design issues:

• Often, the best solution is to design the beam to be wider than the supporting member so the outer continuous bars of the beam pass outside of the dead zone. Additionally, the inner continuous beam bars can then be arranged so they are clear of the dead zone. If the beam has extra bars shorter than the span between supports, they can be located at the dead zones;





• Prior to placing the wall or column concrete, it's sometimes possible to deflect the problem vertical bars so they don't interfere with the beam bars. Placing a spacer such as a wooden 2x4 on edge along the top of the forms exactly where the beam bars will be located works well. Unfortunately, large-diameter vertical bars may make it impractical to achieve the needed amount of deflection;





• Stop the problem vertical bars at the soffit of the beam and install appropriate and properly located dowels to maintain member integrity; or



• Stop the beam bottom bars at the faces of the supporting member and install appropriate splice bars. This is practical only if the column or wall stops at the beam and does not rise above it because, in such a case, the vertical bars will terminate below the top bars. The top bars cannot be cut at the faces of the supporting member, so if the verticals must pass through the beam to the member above, another solution must be found.



Advantages and Disadvantages

Advantages and disadvantages for the solutions presented herein depend on the specifics of each case. Bar conflicts should immediately come to mind in all curvilinear-tangential situations and should be thoroughly thought out at the design stage—the best time to look for a solution.

The cost differences among the possible solutions are usually not significant, but any additional up-front cost—for example, for extra splices—will be minor compared to the cost of trying to correct the situation after concrete in the supporting member is cast. Structural issues such as selecting which reinforcement to splice and splice locations will often be the determining factors when choosing a design solution.


Design Considerations

Any congestion issue is clearly one that is best solved prior to construction. Unfortunately, people tend to overlook the potential difficulties that can arise at curvilinear-tangential connections. If a problem isn't solved before it's discovered in the field, it's often perceived as a typical situation of too many bars in too small of a space, rather than as a problem caused by the arrangement of the bars along a curve.

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