IBC 2009 Chapter 34

[SteelPE]

I don't know why I am always having questions with Chapter 34 of the IBC but I do.

I was asked to look at an existing wood structure that was built in 1951. The existing structure has a hip roof formed out of plywood plate trusses with conventional framing at each hip end. There is some concern about an addition that is going onto the building and the additional loading the additional framing will be placing on the existing structure. We have been asked to submit a proposal to investigate the structure to make sure the roof is acceptable, if not, propose a fix for the roof to support the new loads.

The problems comes up because the GC has all of the original drawings from when the structure was built (amazing). At the end of each hip, the structure was suppose to have a double truss. A field visit reveals that there was only 1 truss installed. It has been this way for the past 60 years and there is no sign of distress in the roof.

We planned on trying to use section 3403.3 of the IBC (the 5% allowable increase) but now we are not sure if we can. What do others think?

 

[hawkaz]

Are these prefab wood trusses (with shop installed press plates) or were the trusses hand built?

If these are pre-fab, the shop may have value engineered the design and got 1 to work.

 

[SteelPE]

These trusses have plywood plates. They were hand built. The building is 1 of 50 in the complex. All the trusses are exactly the same so I imagine they may have been built in a shop....but given the age, I'm not sure.

 

[OHIOMatt]

The question is not whether you can or can not (basically if you will be called to task by the building official), but whether you should. In my opinion, you should not, at least until you have investigated the potential discrepancy. I use the work "potential" as this single truss may have been designed differently or altered in RFI or shop drawing process.

My suggestion, analyze the truss. If underdesigned, do not add load unless you take care of it by strenthening. If the truss is %50 percent underdesigned, I would not feel comfortable making the situation worse, the "straw that broke the camels back".

 

[woodman88]

Pick a high stress joint. Count the number of nails/staples in each member for the hip and standard trusses. If the hip trusses has twice (plus/minus) the nails/staples of the standard truss or has larger (longer and clinched?) nails, hand calculate the forces in the joint. Divide the forces by the number of nails.
Are the force per nail reasonable?
If you need to, you can download the 1944 NDS from

http://www.awc.org/standards/nds.html.

At the lower right of the page. Click on the “NDS Archives + Historical Design Values” button.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.