Built up beams ssytem factor

[ajk1]

Why is it that the for built-up beams, the Woodworks Program, as well as the "Built-up Beam Selection Table" in the Canadian Wood Design Manual, both seem to use a system factor of 1.1 for resisting moment, while the "Span Book" says in "Design Assumptions" that it uses a 1.2 system factor for bending in built-up floor beams?

 

[racookpe1978]

Well, consider that a "floor beam" is - "by definition and logical assumption" going to be loaded from above with a wide secondary plate (the floor members) securely attached to the top of the wooden beam, that single wooden beam being matched between many partners of equal capacity.

No other construction loading or installation is likely, right?

Therefore, given that there is additional solid structure tieing together several uniform beams in a uniform fashion at a known location at the top of every beam, why should not that same beam be assigned a greater load than an unknown beam placed in unknown directions getting unknown forces from unknown directions while attached in who knows what manner at what location?

 

[Canuck67]

Woodworks, as the O86 standard considers this a Case 1 scenario with 10% extra strength over that of a single joist.

 

[ajk1]

Hi racookpe1978 (Nuclear)- Thanks for the response. I am not clear on what you mean. The diagram in The Span Book shows a single built-up beam in a floor. It is not matched by "many partners of equal capacity", although I am not sure what you mean by "partners" (do you mean other adjacent built-up beams)? In any event, it says in this same book that it has a greater load sharing factor than given in the Code and the Canadian Wood Design Manual for built up beams.

 

[ajk1]

Hi Canuck67 (Structural)- yes I know that is what Woodworks does. But I cannot understand why the Span Book uses a different factor than the Code and Woodworks. Also, when I try to check out the spans in the Span Book, some check out right on using the 1.2 factor they say they use, and some are about 5% overstress. Maybe the best thing is for me to ask the Canadian Wood Council.

 

[Triangled]

could be repetitive member increase? if i am understanding your system correctly?