Restrictions of knots in wood trusses

[mfstructural]

I've come across a case where the bottom chord of a truss was fractured while someone was walking on it. The bottom chord of the truss reportedly fractured when the person stepped on a knot. My question is are there any documents that restrict the knot size in the members of a engineered wood truss? I understand wood trusses fall under conventional wood construction, and that knots are allowed and are part of the wood. From what I've read, as long as you can show that the allowable wood stress at the enveloping reduced section at the location of a knot exceeds the maximum LC applied stresses, you are ok. Anyone have any insights into this?
Thanks

 

[KootK]

To my knowledge there are no rules specific to trusses. However, there are definitely rules pertaining to the individual wood members comprising trusses. They would be the grading rules that applied to the original grading of the lumber.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

KootK is correct. This is a grading rules compliance issue; however, in most cases, the bottom chord can be No. 2 KD, which allows under the rules in my area (Southern Pine Inspection Bureau), a knot of up to 2" in diameter on the interior face of a 2x4 or 1-1/4" if at the edge.

 

[dhengr]

Mfstructural :
The bottom chords of many (most) roof trusses are the most critical and highly stresses members in the truss. Almost invariably they will be the best lumber in the truss, highest allowable stresses and modulus of elasticity, the best grade lumber, whatever the species. I wouldn’t expect to see many large knots or edge knots in these tension members, and unless specially spec’ed and designed they are not designed for 250lb.+ concentrated loads, although many times they may carry that loading. They may be designed for a 5, 10 or 15lb./sq.ft. ceiling loading and maybe some mechanical loads. But, again not some big hulk bouncing around on them. The grading rules do apply, and the truss manuf’er. will usually use the appropriate grade for the situation and stresses at hand. I would want to know much more about what actually happened in this case. I would want to see a bunch of photos of the trusses, of the knot and the failure area. I would look for any lumber grading info stamped on the lumber pieces. I would look for shop drawings, erection plans, truss manuf’er., for the original bldg. plans, etc. All of this type of info is needed to start to making an assessment of what happened here. It may have broken at a knot, but it might not really (totally?) be the fault of the knot, if the material met the grading rules in all respects.

 

[XR250]

I see wood trusses that have failed at knots frequently in my inspections of homes. Appears to usually happen during handling and erection.

 

[KootK]

I see wood trusses that have failed at knots frequently in my inspections of homes. Appears to usually happen during handling and erection.

My experience too. And often in the context of weak axis bending causing the initial fracture. For most situations, just screw on a scab on either side, per an engineer's direction, and carry on.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.