Z-Purlin Rotation - what is acceptable? 1

[Fatboycowen]

I am evaluating a Butler building built in the 60s. It appears that some of the roof purlins have rotated. We measured about 5-6 degrees of rotation on some of the purlins, while others were within 1-2 degrees of being perpendicular to the roof deck.

It would appear that the entire purlin, along it's length rotated, rather than just at the midspan.

The question is this - when this building was built, what were the tolerances used for this? What would today's standards be?

The next question is, what should be considered an acceptable amount of rotation before the purlin's strength is significantly reduced.

Thanks in advance.

Jon

 

[hokie66]

Have you asked Butler? Of the "pre-engineered" building manufacturers, they probably have the best technical staff. I don't know about a standard, but 5-6 degrees is too much rotation. Does that mean the cleats at the rafters are bent?

 

[Fatboycowen]

I have not yet asked Butler, and that was going to be my next step.

The purlins are butt spliced at the girders, rather than lapped. At the girders, there is clip angle. There is 1 bolt attached to each purlin. The purlins essentially rotated about that bolt. If there had been 2 bolts vertically, maybe this wouldn't be possible.

Thanks.

 

[MiketheEngineer]

That is too much. The building has been over loaded at one time or another - unless it was poorly built.

Butler generally makes a pretty good building!!

 

[jgailla]

I've installed several (about 20) PEMB buildings in the eraly 2000's and I've never seen the purlins butted instead of lapped.
If there is more than a nominal rotation of the purlin, the roof has low spots where the excessive rotation is.
I'd suspect the building was improperly built, especially with the one bolt issue. I've not seen fewer than two bolts at a connection.

 

[Fatboycowen]

Here is a photo of one example. (hope it works).

This is just one example. Some other areas are slightly different. Some have the two upper bolts, and one lower bolt. Several have all 4 bolts.

I can't find any correlation between the purlins that have rotated, and those that haven't. They all seem to have similar components, etc.

There is no visible low spot.

According to the building's owner, this was built by the Butler company.

The reason for the investigation is that another part of the building with bar joists showed excessive deflection during last winter's storms. It turned out to be an area beside the Butler building, where there was a 10 foot roof height change, but no additional reinforcing for the drift loads. The owner decided to investigate the whole building.

Quick and dirty calculations on these purlins show that they met the code snow load at the time, but those loads have been significantly increased since.

Thanks for the comments so far.

 

[MiketheEngineer]

And significantly larger snow loads may well do what you are seeing!!

 

[hokie66]

Those bolt holes were there for a reason.

 

[ajh1]

The Butler purlin in the 1960's was a 9.5" deep purlin with 90 degree lips. The 90 degree lips require the simple span purlins you are seeing because the lips don't permit the continuous laps at the supports like the angled lips used today.
The common rule of thumb that I have heard from the experts is if the top flange has moved approximately 1" laterally in relation to the bottom flange you are approaching collapse. I don't know that there is (or was) what would be described as a "tolerance" for this dimension. The 1" value has been determined by a number of researchers as being a point of concern. Current purlin anchorage criteria specifies d/20 as a requirement (d being depth of purlin) at the supports, but those sorts of provisions did not exist in the 60's. In a building this old, the roof panel is a through fastened roof, which is providing a lot of lateral support to the purlin and is certainly helping in regard to the capacity. It would not surprise me if the majority of the rotation is near the supports with the middle of the purlin staying fairly vertical albeit moved somewhat laterally. There is limited restraint to the purlin at midspan which allows the whole purlin to move as a unit. At the supports the bottom of the purlin is more restrained (or should be if all of the bolts are installed)which then results in the top of the purlin moving laterally from the bottom of the purlin. Your picture seems to show the purlin is fairly straight which would indicate that it has not at this point been severely overstressed. Look for kinks or other deformaties especially in the lips.
Since you are observing some of the connections with four bolts, the standard detail would have been calling for that. The simple span connection is in effect two connections, each with two bolts in a vertical pattern. I am not aware of a Butler detail that would permit a single bolt to connect a member.

Overall, keep in mind that this is an approximately 50 year old building so it has performed very well despite as you have noted large snows and increasing design loads on the structure over the years. Cold-formed design provisions have also become more stringent over the years which could, per today's codes, make this building deficient based on today's specs even for the original design requirements. Butler would be happy to provide you with whatever information we can, although admittedly that information will likely be fairly limited for a building of this age. We tend to lean on local people for a building of this age since we have no way of knowing the full physical condition of the building, such as the connection conditions you have noted.

You don't indicate where the building is located, but feel free to contact any of our regional offices with your contact information. They will either have someone that can assist or more likely they will forward your request on to our R&D folks in Kansas City.
The one downside to these forums is that it is hard to take the process off-line since they frown on posting things like email addresses (with good reason).

Al..

 

[dik]

If the purlins are overloaded, then you can likely bet the farm that the building frames are more overloaded...

Dik

 

[chicopee]

Hokie66 may have your answer.

 

[Fatboycowen]

Thank you all for the recent replies.

The week before Christmas, i called Butler and asked for the engineering department. I explained the issue. The engineer i spoke to basically said there wasn't much they could help me with, given the age of the building. When i asked about tolerances, he simply pulled open his metal building book and looked, but couldn't find anything.

 

[ajh1]

Not sure I can add much more to what you already discussed with Butler, but I'm willing to try. Give me a call on Butler's main switchboard number in Kansas City and ask for Al Harrold. I'll let you pull the number from the Butler website

http://www.butlermfg.com

to keep the spammers at bay.

Al..