I was recently requested to review a detail for a federally funded design-build project currently under construction. The building is a two-story structure with a sloped metal roof. The supporting structure for the roof is a typical bar joist system. The bar joists bear on wide flange steel beams. The wide flange beams are at different elevations to create the slope for the roof. The slope for the roof results in interior non-loading bearing partitions with a maximum height at the top of the slope of 28 feet.
The detail in question was for kickers for the steel studs. The designer had selected a 362S162-43. For those of you not up to speed on stud identification, it is a 3 5/8” stud with a 1 5/8” flange width, 18 gauge. The problem with this stud is that the height limit is 25’ 9” using a deflection of L/120. Clearly the wrong stud was selected. The designer became concerned and changed the deflection to L/240 after calculating the horizontal deflection to be 2 1/2”. The problem here is that the calculation is for the stud only; it does not take into consideration the stability added to the wall when drywall is applied to both sides of the stud. By the way, a calculation to determine the deflection of the composite construction does not exist. By changing the deflection criteria to L/240, the height limit was lowered to 19’ 1”. To correct for this shortcoming the designer added kickers at the 19’ 1” height, a seemingly simple solution to the problem.
The first step in this situation is to work with the architect to convince him that L/120 will likely be adequate for interior partitions clad with gypsum sheathing. There are tables for L/240 and L/360, and most specifiers gravitate toward L/240 because they “want the wall to feel stiffer.” However, L/120 is adequate for most gypsum board applications, and cracked joints and screw or nail pops have other root causes. In addition, although the 5 PSF load is a code requirement, interior partitions typically never experience a 5 PSF uniform pressure over their entire area and thus never reach the larger deflections calculated.
Several other factors come into play that were not addressed in developing this particular solution. First and foremost, we all know that a bar joist system introduces deflection into the roof system. Not a problem; just install a deflection track at the head of wall. Second, this particular building falls under the seismic requirements of the building code. This means there will be splayed suspension wires and compression posts holding up the ceiling grid. It would be opening a real can of worms to not mention plenum pollution with the electrical and HVAC components that will be located there as well.
The major problem with the kickers is that they will have a detrimental effect on the deflection track at the head of wall. Either securing them to the roof deck or the wide flange beams will introduce forces and loads on the partition resulting in damage to the drywall. While the installation of kickers seems to be an effective solution, their introduction will cause problems that will not become apparent until sometime after the building is completed and occupied—definitely a callback with a lot of correction work required.
Since this was a design-build project, it occurs to me that an in-depth review of the project would have turned up the need for a stronger stud, and the problem would have been solved prior to purchasing and installing the 3 5/8” studs in the wall system. In fact, the appropriate stud selection probably would have been a 6” stud. Even though the 6” stud would have been more expensive than the 3 5/8” stud, the additional cost incurred by the after-the-fact installation of kickers would most likely have balanced it out.
So what do you do in a situation like this? As a contractor you follow the contract documents presented by the designer—after all, they are the design professionals. That said, the fact of the matter is you, as a knowledgeable contractor, are expected to speak up when you see a situation that does not work—sort of the old catch-22. However, this particular project is design build. In the design-build process all parties to the contract should be working together to provide information to move the project along and save time. Saving time is the goal of design build because, after all, “time is money.”
A special thanks to Don Allen of the Steel Framing Alliance for his help in preparing this month’s column.
Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to [email protected] or call him directly at (703) 538.1611.
