Readers Respond - Bearing Area
Readers Respond - Bearing Area
The November 2011 Technical Q&A discussed options to consider when dealing with insufficient bearing area. We received some great feedback, which warranted revisiting this topic. Below are letters from two engineers involved in the design of structural building components who give their opinion and outline their preferred methods for resolving insufficient bearing area. Their suggestions offer an engineer’s perspective on how component manufacturers can provide customer service on projects, but for any option, manufacturers must make their own decision based on their business, the project, and the component manufacturer’s and truss designer’s responsibilities per TPI-1 and the IBC.
The article was very informative and presented some excellent insight. Although wood-bearing failure is rarely catastrophic, it is a design consideration that must be investigated because it can result in an undesirable finish appearance. This should receive more attention by truss technicians, especially in “flagging” minimal bearing lengths or providing a suggested solution.
Photo 1 shows the dramatic result that may occur when there is insufficient bearing area. Photo 2 shows a subtler sheet rock or joint tape crack that is localized to each truss location along a bearing wall. These are real performance issues that truss technicians should be proactive in helping to solve when the software alerts them to this issue.
I applaud you for pointing out that both truss and wood plates must be checked. My experience has shown that many truss technicians focus specifically on the truss to eliminate insufficient bearing without taking into consideration the support structure. It could be construed as very bad form, and it certainly is a customer disservice to solve the truss side of the equation while “hiding” a potential wood plate bearing problem.
Although the article presents solutions, it’s prudent to point out the challenges associated with some. Bearing blocks may not be installed when required, and when present, may be mismatched to the truss chord—either installed slightly below or above the bottom chord. Consequently, a 1-½ in member bears on the plate until deformation, which only then engages the other member(s). In addition, fastener quantity, location and size often do not meet what is specified.
Another option is compression parallel to grain, which extends a vertical member through the truss, cutting the wall plates, and extending a stud up to the truss. The design professional of record should be involved when proposing this solution, since diaphragm design using the wall plates may be adversely affected.
Regarding TPI 1 Section 188.8.131.52, it is my understanding the metal truss plates must extend across the full bearing plate width to permit a bearing capacity increase for truss lumber. Truss technicians need to know this alternative requires metal plate placement within ¼ in of the lumber edge contacting the bearing surface, which will likely require larger butt cuts to accommodate the metal plate width.
Which option works best? It is my experience a bearing enhancer is the best solution. Enhancers are typically listed in a code acceptance report, and published design values make them easy to size using the catalog or design software. Field inspections are simplified—enhancers and their fasteners are either present or missing. Inquiries posed by building code officials, contractors, framers or the building designer can easily be addressed using the manufacturer’s published literature and the truss design drawing. Questions about installation, cut top plates and top plate lumber grades are eliminated, which allows the truss manufacturer to focus on what they do best—selling, designing and fabricating trusses!
Scott D. Coffman, P.E.
I read your article on bearing area in the November 2011 issue. While I am not in particular agreement of the 18 percent increase you mention, this is only because it doesn’t go to the 23 percent I need to get the maximum improvement for the 1-½ in bearing length (along the top plate). Instead, I have always specified that the plates be flush with the bottom, and this will (I assume) strengthen the allowable bearing stress by at least 23 percent.
I am of the opinion that the truss industry (fabricators and designers) are not helpful enough to the rest of the building team when it comes to bearings. I say this not only for bearing, but also for reaction uplift numbers that the software renders, which is many times unrealistic and erroneous due to the “infinitely rigid” assumptions the software must make to analyze the trusses.
The bearing improvement scheme (see Figure 1 attached below) I have used for 30 years increases the allowable reaction for a top plate as allowed by the code referenced National Design Specification (NDS®) for Wood Construction published by the American Wood Council, using the formula for increasing the bearing stress as allowed for bearing lengths less than 6 in [see NDS Section 3.10.4 (see Figure 2 attached below)]. The bearing surface of the truss is reinforced with flush plates, and this is assumed to strengthen the bottom surface of the truss beyond the Fc(perp) of the top plate by more than 23 percent.
The example in the Technical Q&A showed the bearing allowable reaction would be increased (for Douglas Fir) from 3,280 lbs to 4,258 lbs. I feel that many times trusses are unnecessarily doubled only to account for the reactions. Builders are not friendly toward making walls bigger for the truss fabricators. They are okay with adding a post, but not changing from 2x4 to 2x6 walls or 2x6 to 2x8 walls.
Don’t consider this as a critique of your work—just some information to put in with all the other comments you will get on this.
Norman Scheel, S.E.