Metal Building Slotted Secondary

[haynewp]

Does anyone know if metal building manufacturers are still putting slotted holes in purlins and girts without slip resistant bolting? I talked to an engineer at a metal building company a few years ago because I was analyzing the existing roof for new loads. The purlins had slotted holes connecting the purlins to the girders and I asked him how the purlins were able to act as struts to carry the load to the bracing with the slotted holes connecting the purlins to the frames. He was not able to answer my question, just that it was always done this way (he noted even the eave struts had slotted connections to the frames also). Also, how does this justify as bracing points for the frame members if it is not a slip critical bolting?

Any metal building engineers here?

 

[msquared48]

Are these slotted holes purlin to purlin (24" overlap common here), purlin to frame, or both? Girts assumed too here.

Sounds a lot like the common dilema of providing a continuous chord tension tie at the roof diaphragm of a CMU/Tilt building, yet providing for longitudinal expansion/contraction in the walls.

Mike McCann
McCann Engineering

 

[haynewp]

Purlin to frame in the case I was looking into.

 

[msquared48]

OK. Been dealing with metal buildings for years here. Seems to me that the slotted holes are probably to provide some room for misplacements in the anchor bolts so the mainframes can be true and vertical. Any lateral should be transmitted by the x bracing in the roof or the roof diaphragm if it is heavy enough - ususlly not though. The bolts would be tightened not allowing slip even though the holes are slotted. Therefore, it stands to reason to me that the holes are only to facilitate the erection process. Nothing more.

Make sense?

Mike McCann
McCann Engineering

 

[haynewp]

I agreed and so did the metal building engineer I talked to that the holes are slotted horizontally to facilitate erection but the bolts to the frames were NOT specified to be slip critical, and the metal building guy told me that this was the standard. I couldn't see how they justified the transfer from the purlins to the frame with the slotted holes under these circumstances.

So you are saying that usually the metal building mfr. does specify the purlin/girt to frame bolting to be slip critical? That would make sense..

 

[j19]

I believe it would be pretty rare to see a slip critical connection in a metal building. I think that most of the slotted holes in the cold-formed members are perpendicular to the load path. For example, in the purlin to rafter connections, the holes in the purlin would be slotted parallel to the direction of the rafter. Purlins are usually designed as strut members (as well as bending members) so the slots are perpendicular to the direction of the axial load. But, in another example, the holes for the lap bolts (the bolts that are located at the end of each lap condition) are slotted (some manufacturers may supply oversized holes) in the direction of the shear load caused by bending in the purlin. This type oversized hole is required because of the offset in the members when they are nested together. If you don't have a slightly oversized hole in the vertical direction you will never get a bolt to fit. This condition is covered in the 2004 NASPEC (Section E3a) where it says that vertical slots are allowed in this connection.