Distribution of heavy truss loads on wood framed walls

[Antnyt23]

I was curious to know how others treated the distribution of a heavy truss load or concentric load on a framed wall and its distribution to the base of that framed wall.

Do you consider the concentric load to distribute into a distributed load or do you apply a single concentric load on the base bearing of the wall at that point? Is it based on wall height and/or amount of load?

I'm more familiar with the distribution for concentrated loads on masonry walls and how it distributes.

Any literature or code references on this would be great if someone has the info...

 

[Guastavino]

By "concentric" do you mean concentrated? Concentric implies that the load is centered on the wall.

As for how it distributes, it depends on the details. Is it resting on a continuous double top plate? If so, you'll get some continuity, albeit not complete. Also, at each floor level below (if applicable), there could be a ledger (if platform framed) which would spread the load out more as you move down the structure. Depending on the spacing and size of the load, I would consider adding a stud pack under the wall at the bearing locations if you can't justify the top plate spreading out the load.

As for specific code references, I don't know any specifics off the top. It's not like CMU running bond though. And if you look at CMU stack bond, it's clear as to why (look in the MSJC commentary).

I'd recommend you post about the truss spacing, loads, wall height, wall stud size, wall stud spacing, or maybe a sketch too.

Lastly, in the real world, the wall sheathing and gypsum sheathing will further distribute loads, but I don't think you can calculate anything with it, and I wouldn't want to if you could.

 

[Antnyt23]

Sorry concentrated, I was typing fast and must have autocorrected to that.

I was more wondering just in general at this point. Your statements make sense and I appreciate your judgment on the real world distribution vs. how to actually calculate it. Was kind of my thought process.

I guess I was picturing wood trusses resting on a wood framed wall, which beared on a floor truss system or even a footer. Just curious on peoples thoughts of transferring the load to each.

I know I have seen in some cases where a continuous footer is designed with a bump out specifically for the heavy concentrated load from the framing above. I assume from your opinion it is best (especially with wood framing in comparison to masonry) that you carry that heavy load to the component below and do not assume any distribution from the wall construction.

I guess under heavy loads like that you would have additional support in the framed wall which would act almost like a post carrying the load to the structure beneath. So assuming it doesn't distribute is the best route.

 

[jayrod12]

Generally below point loads in wood framed walls I provide stud packs that go continuously down to the foundation. This includes blocking in the stud space to ensure continuity of the vertical load path. I'm sure if I crunched the numbers you could account for load spread at each floor plate but for the cost of a few wood studs, it seems like a waste of everyone's time.

 

[dcarr82775]

Generally I assume no horizontal distribution of a point load in a wood structure for things like a beam bearing on a post in a wall. Once it hits concrete I will distribute it a little bit.

 

[boo1]

solid stud pack to met bearing and column loading, thicken slab per load, strap for uplift if present

 

[KootK]

A few weeks ago, I looked into the same question as it pertains to cold-formed stud walls. I found an industry expert who had studied the issue and was willing to share his results with me. In summary:

1) Stud wall with a steel HSS distribution member over the track.
2) FEM study of load distribution considering one story and rigid support for all studs at the bottom.
3) Loads distributed to just the two neighboring studs and the distribution was pretty modest at that.

I take this as support for the view that load distribution through wood stud walls is pretty poor.

To my knowledge, there is also still no explicit code procedure for live load reduction on stud walls other than to allow it based on the actually tributary load to each stud which adds up to nothing useful. Again, support for the view that load doesn't distribute much through a stud wall.

Back when I was a truss guy, we'd see a lot of issues with girders. Our responsibility ended at the girder to wall connection but, there, we'd routinely use steel bearing plates etc to make things work that the EOR hadn't envisioned. In many cases, it was pretty doubtful that same capacity could be maintained all of the way down to the foundation. And uplift situations were worse still.

Lastly, in the real world, the wall sheathing and gypsum sheathing will further distribute loads

This is true but, in my opinion, is reason to avoid relying on distribution. If the top plate pushes hard against the sheathing locally, the likely outcome is that you'll crush the sheathing and/or fail the fasteners locally.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Antnyt23:
I agree with KootK’s last post. Draw yourself an accurate elevation of a two or three story stud bearing wall, with the wall plates, floor framing, etc., and the footing or found. Then put the truss load btwn. two studs and trace the load path down to the footing, adding the appropriate loads as you go. Will the top wall pls. take the truss load in bending, will the two adjacent studs take the truss load, immediately under the truss? What about these loads at the fl. framing immediately below the roof framing level, particularly compression perpendicular to the grain, at the studs and bot. pl. Do you need vert. blocking in the fl. framing depth under these studs? If there is a rim joist, it acts as a much better beam than the wall pls. do, for a little lateral distribution of loads. Do this process all the way down to the footing, and at some level you will need 3x4 studs or some such.

Do this same exercise, but put the truss load right over a single stud. Most everything (the thinking) above still applies. But, now you might ask is there any distrib. to the two adjacent unloaded studs? That’s a beam (the double top pl.) on springs problem. The springs being the axial stiffness of the three studs, and Koot’s contact says there isn’t much distrib. and he and I generally agree with that. You can check this same beam on springs problem at a rim joist and the lateral distrib. is still pretty small.

I have seen some real night mares where the truss girder reaction ‘ball’ was dropped btwn. the truss guys and the Structural Engineer, if there was one. These reactions are a significant problem, and must be tracked all the way down to the footing, often their own footing, and whoops, a big post in the middle of the living room. Often the Architects or designers plans are not well dimensioned and they have one of those God awful roof framing schemes that only a truss company, with their software, could possible do or love. The truss guys don’t often pay much attention to the framing layout below their bearing point, so these shop drawing have to be reviewed with some care to be sure you have a load path for these large reactions.

 

[Antnyt23]

Great. I appreciate all the info and definitely in concurrence now that the stud walls will not provide any distribution. I will definitely pay close attention to any large girder loads I come across and make sure the correct load path is taking and accounted for. This was just the type of thing I wanted to confirm and get a little better insight on. Thank you very much.