Pre-Engineered Wood Truss Design Question

[MJB315]

How do pre-engineered wood truss engineers design the bottom chord? If the drawings specify that the bottom chord should carry an uniform load... does the design engineer assume that the hung load will be screwed into it? How does the industry approach this?

"We shape our buildings, thereafter they shape us." -WSC

 

[ztengguy]

It generally is assumed screwed. Drywall can be glued, but hard to do in this application.

 

[dhengr]

MJB315:
You could always tell them on you plans what you intend to do to the bot. chord, so they can design accordingly; or so they can tell you how to accomplish what you want. They do not like to see the bot. chord riddled with nails and screws, etc. They don’t want your activities splitting or otherwise reducing the strength of the bot. chord. It is generally the best and strongest piece of lumber in the truss and is fairly highly stressed, in tension.

 

[a2mfk]

Most of us know a technician and a computer program design pre-engineered trusses, and then an engineer spends a half minute having a look before signing and sealing. With that said, and to add to Dhengr, the program assumes a uniform load along the bottom chord due to attic live load (if any) and dead load, and I believe there is a point LL requirement. Then the bottom chord is usually in tension (gravity loads) as part of the truss, plus the bending stress from the uniform load applied directly to the bottom chord. I haven't thought about it much but the tension from the truss gravity loads probably helps as much as it hurts, especially in lateral-torsional buckling, sort of pre-stresses it in tension.

But if you plan on any penetrations other than fasteners at 3-4" o.c., that would be something to coordinate with the truss designer, or rather avoid.

 

[focuseng]

They design the chord by whatever the program or someone else tells them. No more than that. I know a lot of truss guys and have some very good friends in the job description. Most just plug and chug and check for various issues but those are the good guys. God help you if the trusses get sent to a Menards or other box store for supply. If you have specific requirements -loading, material bearing, fastening- you need to spell it out on the drawings very explicitly. Otherwise assume the designer is not taking anything special into account -ie. small diameter dowel fastening only and typical uniform loads. They will account for girder truss loading reactions which they design but 8/10 times they won't check their own hanger specs for the load reactions or applicability for fastener penetrations...

______________
MAP

 

[AELLC]

In many circumstances, the truss calculations are finished before the structural drawings are even started, so it is necessary to require a shop drawing submittal that you can redline.

 

[Triangled]

Agreed, don't assume anything good. Specify precisely what is required.

 

[charliealphabravo]

I would say it depends on the type of load.

If it is a typical attic space then the bottom chord loads are applied conventionally without much direction from the EOR or truss design engineer. The drywall is either nailed to the bottom chord or to hat channel. The load might also includes lighting, ductwork, insulation and perhaps light storage on plywood planks.

If the loads include fire suppression, ceiling mounted fixtures or anything more than 10 or 20 psf then the bottom chord will require sistering or strong-backs to support the loads and additional fasteners. These are usually designed by the EOR or sub-trade as long as the truss design engineer has anticipated the superimposed loads.

Aside from showing the MAGNITUDE & LOCATION (not shouting) of point loads or uniform loads on the truss design drawings, the truss design engineer would almost never give any specific attention to HOW the loads were being connected to the truss and whether or not the bottom chord was compatible with the connectors used. Secondary/localized strengthening of the bottom chord to accept fasteners or specialized hangars would fall to the EOR or to the sub-trade.