Posted on December 5, 2007 - by Elliott
Brace Yourself for the 2006 IBC
The 2006 International Building Code (IBC) is coming and for some jurisdictions it is already here. Las Vegas has already seen its adoption; Northern Nevada and California are close behind. BJG has studied up and is providing you with the following story of the 2006 IBC and your building’s braced frame particularly with respect to industrial buildings.
Not so long ago, the concentric braced frame was a thing of simplicity and reliability. Inexpensive to implement and relatively simple to design, it was an important system of choice for economical structures where the architectural floor plan could accommodate the brace layout. Several typical braced frame configurations are shown immediately below and will be discussed later where appropriate.
A typical braced frame “works” by transferring horizontal forces to foundations by pushing or pulling on the diagonal steel member. See Figure 8 for an illustration. Engineers have tried many varieties of braced frames over the centuries. Some have performed well, some have performed poorly and some have been “just ok”. Much of what we have learned has been by seeing what does and does not work in earthquakes.
Some configurations, such as the “K” brace, where the braces intersect the side of a column, failed in an undesirable way (the column buckled) and these configurations were prohibited. This made sense because a failed column can lead to a roof collapse.
The most popular configuration of concentric brace frames are the “V” and inverted “V”. These configurations work well because they provide areas for large openings that allow access through the frame.
As codes and research have evolved, the ability of buildings to absorb energy (ductility) during earthquakes has become the dominant theme in earthquake design. Concentric braced frames are normally very rigid and not very ductile - they tend to take load until they either break or buckle.
Another issue with braced frames is that test results have shown that failure often occurs at the connection. This observation has led the code writing bodies to add requirements to ensure the attachments of the brace are stronger than the other components, including the brace. Good idea but tough in practice. The compression or “pushing” strength of a brace is much lower than the tension or “pulling” strength. (Think of pushing or pulling on drinking straw.) The end result is that we are required to make the connections stronger than the pulling strength which is already much more than the “design” strength of the brace in compression and, subsequently, the connections quickly increase in size.
Even with larger, stronger connections, concentric braced frames were still economical and practical. However, since about 2000, there has been a “loss of confidence” in the concentric braced frame based primarily on research in structural laboratories. This has been reflected in higher design force requirements in the building code and more complicated design requirements.
The old concentric braced frame has become the “ordinary” concentric braced frame and its use in higher seismic areas is either prohibited or severely limited by the IBC and its adopted references. “Special” concentric braced frames, which are designed to enhance ductility, are required in higher seismic areas.
The latest and most recent change in the IBC from the 2003 to the 2006 is effectively an end to the old “V” and chevron bracing system in large warehouse applications. An exception that previously exempted one story buildings from having to meet post buckled strength requirements has been removed. This requirement affects the “V” and inverted “V” brace. The beam at the intersection of the two braces is required to resist the force of only one of the braces pulling on the beam assuming the other brace has buckled and has no residual strength. In a typical warehouse, this force results in the biggest wide flange (300 lb/ft) failing.
Luckily, the team at BJG is here to help. We have begun implementing “zipper” columns and/or “X” bracing in our braced frame designs which solve many of the concerns with the old braced frames for a relatively small cost. It is important to note that these solutions reduce the size of openings available through the frame, which for industrial buildings effects truck and forklift traffic at the frame location.
Lastly we would like to introduce you to the “protected zone”. The 2006 IBC adopted references have specified that items cannot be connected to a brace at either end or in the middle, this includes pins or welds. See figure 9. The reasoning is that these attachments could be crack initiators inducing a non-ductile fracture failure mode. This includes limiting demising wall framing attachments. This requirement makes framing a wall in the plane of a brace virtually impossible. BJG has solved this issue by sliding demising walls to one side or the other of the brace and running them full height. Installing the sheetrock, taping, texturing, calculating rentable square footage, etc is harder, but not impossible.
In summation, the code changes governing braced frame design makes braced frames far more ductile, somewhat more expensive and much less user friendly for industrial buildings. They are still the logical choice for many applications but we do not think they are the automatic solution that they were a few years back. We recommend that all lateral bracing solutions including interior shear walls, moment frames and, yes, braced frames be investigated early in the design process and the appropriate solution for each facility be determined by your BJG design team.
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