Roof Deflections in PEMB

[SmithJ]

All,

I was wondering if anyone could share their experiences on this subject. In PEMB, typically, the clear span frames will deflect more than the Endwall frames that have bearing columns supporting the rafter. As a result, the deflections in the main frames will be more than the deflection in the endwall frame. What effect, if any, does this have on the roofing system?

In really wide span buildings (over 150'), this deflection can be significant. Are there any guidelines or limits for this type of differential deflection between frames?

Thanks in advance.
JS.

 

[haynewp]

I would think the roof systems of those buildings would be pretty flexible. There is an AISC publication listing recommended deflections and drifts of similar buildings, if each of the frames met that criteria I would probably not be too concerned structurally. The purlins are normally fabricated with slotted holes (not slip critical either) or else erection would be impossible, this may add some give for the differential.

I would expect the roof membrane to leak a little sooner from a large difference in deflection. I am not sure, they may put cable bracing in that last roof bay just to help, you might call a metal bldg mfr and ask what they normally do if anything to compensate for the deflection difference.

 

[ArneG]

The metal frames typically have light gage 'Z' purlins on them that span from the endwall to the interior frames with a partial moment connection over the interior frames.

While the differential deflections are quite large, all the members will bend quite easily - there really isn't anything to break.

For example:

At 150', assuming a L/120 deflection for the interior frame yields a 15" deflection. With a 30' span for the 'Z' purlins this results in a 2.39 degree bend in the weak direction of the metal roof deck. I'm guessing I could bend (1) sheet of that 22 to 25 gage roof deck 20 degrees in the weak direction with my hands, and I'm not Superman.

In other words, they deflect a lot - but its ok.

3 big things to remember:

1) Do not attach anything to these buildings, no interior walls, no piping, no electrical, NO ANYTHING - without carefully looking at slip joint detailing.
2) These buildings have ZERO reserve capacity built into them. I was called in to fix a metal building that had roof deflections so severe you could see the roof bow driving by on the highway. During a renovation all the new mechanical was hung off of the 'Z' purlins. No good.
3) They move laterally as much as they move vertically. Watch out when building a CMU enlosure next to or in one of these buildings.