Strengthening Wood Roof Trusses 1

[frv]

I have a situation where we are going to remove a portion of a loadbearing wall supporting roof wood trusses. We are going to support the trusses with a new steel beam.

The complication comes from the fact that the new steel beam will be located about two feet from where the wall was supporting the truss (i.e., we have moved the truss support "in" about two feet from where it was originally located). Obviously, this changes the load distribution within the truss.

Does anyone have any literature on strengthening existing wood trusses? I know typically structural panels are nailed to each side of the truss to "sheath" the truss, but I'd like some further information. Thanks!

 

[MikeHalloran]

You need to rent a structural engineer to look at exactly what you are proposing; money well spent.



Mike Halloran
Pembroke Pines, FL, USA

 

[MiketheEngineer]

You might be able to find the original truss supplier who could help out. If the new beam is not at a panel point - you may/will have a real problem!

 

[frv]

I am a structural engineer.

Wood trusses are typically proprietary and thus difficult to modify without consulting the original manufacturer. My question was whether there was some generally accepted, documented method for strengthening the trusses. For example, with OWSJ documentation is readily available detailing analysis/strengthening options.

 

[ToadJones]

if you're only moving the support center in two feet, I'd fill between the top and bottom chord completely with blocking and sheath.

 

[woodman88]

Calculate the forces in each member for the gravity case. This should give you the worst case forces unless you have a high lift condition. If so, calculate the uplift forces too. Do the same for the new bearing condition. Any member force that increases will need a repair or carefully checked for allowable stresses (one way to do this is to compared the same grade members and size plates at other locations for greater forces) in the member and the metal plate. If you know who manufactured the plates you can get the ICC report for the plate values. Attached is a fairly good article on truss repairs you can read.

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.

 

[frv]

Thanks, woodman; that should be helpful.

Toad.. I was considering something similar, but I was looking for some verification for what I intended to do.

 

[ToadJones]

yea...mine was the nail-bender version

 

[woodman88]

Oh, I forgot to mention that some truss plate designs assume that compression web members only need to be plated for half the force as the other half is resisted by wood to wood contact. So any members that change from compression to tension will need the metal plates to be carefully checked.

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.

 

[Forensic74]

Proprietary wood trusses? It's pieces of graded wood with nailer plates. You're making a bigger deal of this than it is. Get the grade stamps if you must, analyze the frame however you choose (you are an SE and should know all the different unbalanced combinations right?), sheath or sister as required. It's not like a proprietary metal joist with special chord and web sections.

 

[woodman88]

MainMan10, It is proprietary in that the truss programs (or at least the ones I dealt with) had certain ways of modeling the joints to decrease the truss cost. To hand match this type of computer calculation without full knowledge of the joint modeling would be very difficult. If you do not think so, I suggest you try to hand match exactly the member stresses on the next truss drawing you see.

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.

 

[Brad805]

I am with Toad and Mainman. Use a common sense approach and all will be well. Have you ever looked at a truss repair from a truss manufacturer? They all give quite conservative repairs based upon the engineers experience.

Brad

 

[woodman88]

Oh, and another thing about compression members to tension members, (Sorry it has been awhile since I have done a truss repair) If the tension member becomes 0.70 stressed your truss plate needs to cover 70% of the member cross section or you will need to repair the joint for the tension force. But this is just common sense so I should not have to mention it.

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.