Knee brace force for truss design

[BadgerPE]

On pole type structures, we typically note on plan the maximum compressive/tension forces in the knee braces. This note is for the truss designer and usually states that the maximum forces are a combination of gravity and lateral forces. Also, our drawings show the connection detail so that the truss designer can reduce the section if necessary.

However, we get a fair amount of pushback from the truss designers stating "we have never accounted for knee brace forces in our truss designs for anyone else and if we did, they would only include lateral forces". I think I have the issue resolved by simply having them include a note on their submittal drawings stating "this truss has been designed for a concentrated force of x kips at x location due to knee bracing". I have told them how they analyze it is up to them and I feel as though them putting the note on their truss plans puts the liability on them if anything happens at that location.

How does everyone else handle this situation? Is it over-analyzing the gravity forces in the knee brace because 3D modeling is used on these structures? I can't simply be on board with neglecting what is often a relatively large force (5k +) because they have never done it before. Seems kinda like the typical contractor excuse and not that of a licensed engineer.

 

[msquared48]

"we have never accounted for knee brace forces in our truss designs for anyone else and if we did, they would only include lateral forces".

Structurally, this statement makes no sense whatsoever unless they expect you to install vertical slip holes for the gravity forces but not for the lateral, which is not going to happen, let alone function properly and as intended. The knee brace will just not work that way.

Personally, although it may not be possible, I would really try to look for another truss supplier who has a better knowledge of structure.

Mike McCann
MMC Engineering

 

[BadgerPE]

Mike,

In your experience, do you typically get truss drawings that show that knee braces have been accounted for?

 

[msquared48]

Not yet...

I think over the years, that I have only designed two with knee braces. Most clients need do not want to raise the eave line to get the extra clearance they need under the knee brace.

Mike McCann
MMC Engineering

 

[Splitrings]

I always include the location and magnitude of my knee brace load on my drawings. We have not had an issues with truss manufacturers. Somewhere I believe I have a truss design showing the knee brace load. I'll see if I can find it.

 

[Splitrings]

Here is a photo of a truss with a BC that has alarger member in the area of the knee brace