LVL Beam on 12' tall wood columns.

[Jmeng1026]

I am working on an addition to a building with a concrete block wall 12' tall. The owner would like to remove a 57' long wall and install three 8x8 wood columns with a four ply lvl (57' long) to support the existing trusses. The building is only 28' wide so the tributary width of the roof load is 14'. The columns will bear on an existing foundation wall. What is the best way to provide lateral support for the columns? I will call for a simpson elev. post base to be installed to fasten the bottom of the column to the floor and a post cap on top but how I can provide some kind of lateral support?

 

[1503-44]

IMHO Thats going to be some work keeping it from becoming an Engineering Disaster Forum" subject. Unless you've got a bunch more rabbits in a hat somewhere.

 

[Jmeng1026]

Would it be better to keep a 4' wide or so amount of wall instead of the wood columns. Have about 15' of wall removed and then keep 4' of wall then remove about 15' of wall, etc.

 

[hokie66]

How thick is the wall and how is it reinforced?

 

[XR250]

I assume you are concerned about out-of-plane beam lateral stability at the top of the column? Simpson makes some pretty heavy duty saddles that could be used. Seems pretty straightforward, unless the middle wall is a shearwall...

 

[1503-44]

I'm picturing a plan 57' x 28'. I'm not sure about anything else, other than removing a 57' length of wall and using 3, 8x8 cols on caps. You don't mention dead, live, snow or wind loads and the 8x8 cols are 12' tall, which gives them a rather high slenderness ratio and pretty low carrying capacity. You don't even mention the spacing of the columns. Are they 3 columns placed at 0, 28.5 and 57 ft? That seems pretty far apart, so I'll guess you have some other idea of distance between them. Maybe you have end walls supporting the corners and you will have the oolumns at 57/4 = 14.25' c/c, ie at 14.25, 28,5 and 42.75' along the missing wall. There is so much left to my imagination, that I got worried enough to mention the disaster forum. So tell us how the rest of the rabbits will help. Fill in some info gaps.

Maybe you are relying on 1, 2, or 3 other walls to provide lateral stability for the building's wind loads and you are really just inquiring about providing lateral stability for the columns? That you will need, because I imagine the load/column is more than 1500 lbs, which is what a 12ft tall unbraced 8x8 will get you.

And I note that there are only so many ways to provide lateral stability to columns, especially when the wall is removed. You must know there is basically only one way, a bracing rod to grade, or to eve beam, back to the middle of the column height? You will have to do that in both buckling planes, so your interior space will be disturbed by those braces to the ceiling elevation, or you will have some funny looking braces from the col to ground running away from the building.

 

[XR250]

That you will need, because I imagine the load/column is more than 1500 lbs, which is what a 12ft tall unbraced 8x8 will get you.

HUH? I imagine it is almost 10 times that amount.

 

[1503-44]

My ROT is 500,000/(kL/D)^2
500,000/(12'height x 12"/ft / 8" width)
My max KL/D is 50, and usually that is too liberal. A max KL/D of 38 was suggested by more learned members of this forum when I first mentioned it. And I have to agree. I usually have to up the size, but the 500K and 50 numbers are easy to remember.

You commented in the same thread.
thread507-482441

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[Jmeng1026]

Here is a sketch of what the owner would like to do.

Remove entire 57' wall and install 8x8 columns.

Construct 40' x 57' addition.

 

[hokie66]

That screams out for a steel moment frame to support the existing plus the new, and provide lateral strength and stability.

 

[Harbringer]

I assume you are concerned about out-of-plane beam lateral stability at the top of the column? Simpson makes some pretty heavy duty saddles that could be used.

....uh no. You actually have to brace the beam to something, otherwise the most heavy duty saddle Simpson makes is just a hinge OOP.

I agree that you seem to be removing a large portion of the lateral system however the building isn't that large and you can probably make the new south CMU shear wall work. I don't see a need for a moment frame.

As far as OOP stability of the beam, which direction are the trusses running? If perpendicular to the beam they can brace it (if detailed properly), if parallel to the beam 2x kickers (with strongbacks if necessary) braced to blocking at the roof diaphragm at whatever spacing you calc.

Bracing a wall or a beam to the roof diaphragm is pretty normal wood construction. Have you designed with wood before?

 

[Harbringer]

...load/column is more than 1500 lbs, which is what a 12ft tall unbraced 8x8 will get you.

Yeah....don't listen to this guy.

 

[1503-44]

Yes I agree. I made a horrible mistake.
500000/(144/8)^2 = 1543, but that's f'c = 1543 PSI, not lbs.
I forgot to multiply by AREA.
X 64 = 96 Kips!
Thanks for calling me out.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[XR250]

....uh no. You actually have to brace the beam to something, otherwise the most heavy duty saddle Simpson makes is just a hinge OOP.

I feel comfortable using this csp to resist rotation. He could also just add some kickers from the top of the column to the rafters.

 

[XR250]

I agree with Hokie66, however. You probably should have a braced frame or moment frame in there somewhere. I say that because it is likely that the existing lateral system is lacking (which is what I usually find).

 

[XR250]

Yes I agree. I made a horrible mistake. blushblushblush
500000/(144/8)^2 = 1543, but that's f'c = 1543 PSI, not lbs.
I forgot to multiply by AREA.
X 64 = 96 Kips!
Thanks for calling me out.

Which is why I don't use rules of thumb for anything. I just use experience.

 

[1503-44]

No substitutes for experience.
Thanks again.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[Jmeng1026]

The existing building trusses are running perpendicular to the beam. The beam would support the existing trusses. The addition trusses are running parallel to the beam. Here is picture from chief architect showing the trusses and beam/columns.

 

[hokie66]

That reinforces my opinion that you need a moment frame (or bracing) in that plane. How would any type of diaphragm work end to end through that discontinuity?