Need to add roof load capacity to a PEMB. Suggestions?

[MWPC]

A client needs to add ceiling loads to an existing PEMB. No drawings were available so we measured everything up and ran it on a frame program. As expected the interaction equations (code checks) came out near one. I gave myself a pat on the back for the good modeling effort.

Now the hard part, how to reinforce it economically. Has anyone had good success with any particular approach? We can add columns making the span slightly shorter but I suspect the inner column will pick up a tremendous load. I looked at adding a tension tie but the tie took all of the load unless I used a material with a very low E. I think plating all of the members and reinforcing the connections is out of the question cost wise.

Thanks for any help.

 

[JAE]

One of the first things I do is review the applied loads to see if I can squeeze out any original conservatism - allowing that conservatism to apply to the anticipated added loads.

Many times, older designs were based on snow loads (for roofs, of course) that were in excess of what the codes now require.

For example, in my city, designs back in the 1960's - 1980's used 30 psf. With the new IBC code, the required load is up to 20% less depending on the design parameters.

 

[ArneG]

If you're adding ceilings - you're probably adding offices or walls of some sort below. I've taken care of this a couple times in this fashion:

Make the office walls semi load bearing (just drop them on top of the slab on grade and let the client know the slab might crack - so use carpet, not brittle finishes like ceramic tile).

Then use ceiling joists made of light gage joists to form a platform to support the ceiling, and to run your new mechanical ductwork/equipment on. If you have enough headroom sometimes you can sell the client on this idea by adding a few stairs, dropping plywood down and letting them use the space above for light storage (if you do this, you will need to design the cieling joists for 125 psf light storage load).

If you hang the new ceiling off the original structure you may have issues with ceiling tiles falling out because these metal buildings are weak in strenght and weak in stiffness (they move around alot).

Whatever you end up doing watch your isolation joints between the original structure and the new structure.

Good luck.

 

[MWPC]

Thanks for the replies.

JAE unfortunately the slope of the roof puts it in the catagory where the new code requires unbalanced loads. That would realy kill it.

ArneG it is a maintenance hangar for airplanes. The owner wants to make it into a giant paint booth. This requires a ceiling, additional lighting, ductwork, etc all without additional supports in the center.

I will keep looking.

 

[DaveAtkins]

I agree with JAE. Also make sure that when you design for DL+LL+WL, that you only use 3/4 of the WL and 3/4 of the LL. If the interaction equation still exceeds one, then engineering judgment kicks in. How much overstress are you willing to accept? I would not make anyone do a lot of reinforcing unless absolutely necessary. Reinforcing a steel structure in order to install a ceiling may raise some eyebows.

DaveAtkins

 

[MWPC]

Dave,

I used 3/4(D+S+W) per ASCE 7-88, but D+S governed. Low building, long span.

 

[haynewp]

Are you trying to make it work by an old code?

 

[MWPC]

haynewp,

I am trying to make it work by the current adopted code, IBC2003. When my analysis showed it severely overstressed I went back to the old code to try to verify my work. It agreed quite well. If I cannot come up with an efficient reinforcing method, I think the project is dead.

It looks like ASCE 7-05 has reduced the unbalanced snow load quite a bit. I may look at complete loading by ASCE 7-05 (with reinforcing) and make a pitch to the building official. Does anyone have much background for the unbalanced load changes in 05? Maybe time for a new post.

Thanks

MWPC