Cold Formed Z Purlins 2

[landongws]

I am looking at an existing PEB with 8 1/2" Z purlins spanning 25 ft. @ 5ft o.c. Snow load = 30 psf, DL =2.5 psf. The purlins are lapped 2.5 ft. Does the lapping detail constitute continuous beam action? I am having a hard time making these work for the original design loads much less coming up with a reasonable fix for upgrading to 40 psf snow load.

 

[landongws]

"Fantasy Land" - agreed. For my particular project I have added cover plates, doubler plates, flange braces, rod braces, and doubled up the number of purlins to get my calculations to match the new loading. We always struggle with this type of problem. Now with all that has been said are we seeing a large number of failures attributed to "light" design of PEMB's?

 

[dcarr82775]

I have seen (3) in the last 4 years. One a spectacular complete collapse, the other two partial failures of the ......purlins.

 

[ajh1]

A response to a couple of the questions raised:
1) Inflection point as a brace point. Research by Dr. Tom Murray at Virginia Tech shows that on a Z-shaped purlin that the inflection point functions in a similar manner to a brace point. The distinction between AISC and the Z under AISI rules is that a typical AISC section does not directly rotate under load and therefore the deflection pattern is the same on both sides of the inflection point. With a Z-shape the purlin rotates about it's center, and the rotation is opposite depending upon whether top flange or bottom flange is in compression, hence providing some stiffness at the inflection point.
2) Continuous laps generally require a minimum of 2 bolts in the web at each end of the lap plus some form of attachment bolts to the supporting frame, either through the lower flange or through the web into a supporting clip. As noted by others, just the lapping does not do it, you need the bolts. I'm not aware of any manufacturer who would suggest they get continuity without lap bolts. Minimum lap length per certain AISI rules is 1.5d on each side of the support, where d is the depth of the purlin.
3) Since 1996, there are only two ways to treat the bracing of purlins. a) Ignore any benefit from the paneling for lateral support and provide intermediate discrete bracing. Design the purlin based strictly on the unbraced lengths between the discrete braces; or b) Perform a series of base tests to determine the benefit of the paneling. The base test is a two-purlin simple span test in a vacuum box modeling the system as it is to be built in the field. Test is run to failure. From the test result and a calculation to determine full strength of the purlin assuming 100% lateral support, a calculated R factor may be determined. For all subsequent design, the capacity of a particular purlin then becomes R times full allowable capacity. Typical R factors run in the 0.50 to 0.70 range. R-factors must be determined for both gravity and uplift loadings. Purlin systems designed before 1996 may have used other approaches for determining the beneficial aspects of panels (or lack thereof).