LVL Cross Grain Bending

[XR250]

Have a client who wants to add a second story to a house AND raise the ceiling 12". The contractor is proposing the detail shown. The LVL would act as a distribution member so I can support the second story loads down to specifically reinforced points in the existing craptacular foundation. it would also serve to provide the needed 12". Boise does not list any numbers for cross grain bending but they did say 20 psi is OK for direct tension perp to the grain. If I assume I can nail the plys together to achieve composite action, the section modulus is 3.5X3.5/6/12 = 24.5 in^3
The flexural tension would then be 75 x 15"+/- moment arm/24.5 = 46 psi. Higher than 20 psi but does not seem unreasonable. One other idea I had was to use a single 24" LVL and sheath both faces in 3/4" plywood which has the availability for tension. Achieving composite action would be required here as well.

Does anyone have any other ideas?

Thanks

 

[Agent666]

Couple of things come to mind in no particular order:-

1) Do things improve if you had part of the beam poking above the floor to lower the lever arm. i.e. in best position locating the middle of the beam at the middle of the floor joist (dependent on the same load acting from the wall above). 6" sticking above and below joist, bolt to top and bottom of the joist, or some other appropriate connection?

2) you need to think about how the moment couple being generated is resisted, the short 'backspan' to the two nails into the floor need to be assessed, its a propped cantilever with the cantilever portion being longer than the backspan. So the force at the top 'nail supports' will be higher than the applied 75 lb/ft.

3) your calculation appears correct, however you are double the stress allowed and its not clear if the 20psi capacity noted allows for any applicable strength reduction factors which would obviously make things worse again. So clearly it is unlikely to work. You implied this might be ok if I'm understanding your 'it does not seem unreasonable' statement.

4) Adopt a steel member that's more in line with the underside of the trussed floor joist? So wall force transfers more directly into the floor joists.

5) You did not note what the span was between the foundations, but if short enough, span a steel member under the beam to take the face loads to columns supporting the LVL beams. Connect this to the bottom of the beam to tie everything together. Alternatively, can similar be achieved by looking at the bottom of the beam and top plates spanning horizontally (i.e. true situation is probably a bit of load bending the beam like the downwards facing cantilever you are looking at, and partly bottom of beam perhaps trying to span horizontally to the supporting column elements depending on the relative spans involved.

6) Put the LVL or some other transfer structure at foundation level between the existing foundations?

 

[Triangled]

the hinges make it tough...
an early try for me would be to ask my truss guy if he could plate a 24" high 4x4, as the end bearing block, integrally into the truss.
plate 4x4 into truss to transfer moment
design 4x4 to take the 75 plf as a cantilever
blocking panels between trusses in plane of wall for shear || to wall

 

[XR250]

Thanks for the comments.
I was thinking about an idea similar to Triangled's without putting an onerous request on the truss companies. I was going to add a 2x4 x24" "leg" on each side of each truss face nailed into the top and bottom chords. I would also install a 12" 2x4 oriented sideways under the truss to take the gravity load and to tie the two legs together. The load in the nails would be reasonable and it should be easy to field fabricate and avoid any steel (as the contractor has pleaded). I can put the distrubution beam up in the roof flush with raised heel of the roof trusses and post down to the foundation thru the floor system. I really only need it to take the roof load anyway so that is not a deal breaker.

Thank You

 

[XR250]

Screen shot of proposed solution...

 

[jayrod12]

I like this detail better than the last, however didn't you indicate that you need to have the gravity load come down at discrete points? Which is why you had a beam spanning.

 

[XR250]

however didn't you indicate that you need to have the gravity load come down at discrete points? Which is why you had a beam spanning.

Jayrod,

The floor system load is less than the current roof load on the foundation so I was able to move the beam up to the roof to carry that load pseudo independently. The beam supports would carry thru both stories down to the areas where additional spot footings are to be installed. My plan is simply not to increase the load on the current foundation - except at points of reinforcement. I know this might seem sketchy, but these are the hoops I jump through to keep work coming in :> Honestly, it might be easier to simply add more spot footings (essentially underpin the whole foundation) and forget about the beam altogether. We are going to get a geotech out there to evaluate but based on my previous experience in this neighborhood, we are going down 3 ft to hit good dirt. Might end up using helicals instead of footings.

What is irritating about this project is that it would likely be cheaper or a wash to tear this thing down and start over. I have suggested that to the owners but they are hell bent on keeping it as they overpaid for it.

 

[jayrod12]

...but they are hell bent on keeping it as they overpaid for it

I get you. I've been down this rabbit hole a few times. I've successfully lobbied a couple times by having a contractor bid both including demo costs, and when it comes back cheaper, the owner usually wakes up.

I missed the part about you moving the beam up to the roof, I like that idea much better.

 

[McSEpllc]

I have specified roof trusses with the heel member extended up to form a parapet. You could ask a floor truss mfr., if they can do something similar, with a (2)2x4 heel in weak axis extending down.