hanger rods anchors capacity reduction due to non-uniform load distribution

[RabitPete]

Any coefficients to use when calculating a load on the hanger rods from the structure suspended at 4 points? Client wants to suspend a somewhat large and heavy transformer off the PEMB purlins. I assume the load at 4 corners is never going to be equal and actual distribution will depend on how well the contractors levels it. My thinking is if one of the rods is poorly leveled, the 2nd rod on that side will take nearly 50% of the entire weight instead of ideal 25%. I don't seem to recall anything in the code to address this. Thanks!

 

[bones206]

A good industry resource is ASHRAE Practical Guide to Seismic Restraint, specifically Chapter 10 in your case (snip below). They don't make any allowance for weight distribution in hanger rods besides relative distance to the CG. That being said, it's not a bad idea to add a little extra factor of safety.

 

[JLNJ]

I’d be more worried about the strength and stiffness of the PEMB purlins. They are usually designed for supporting a small sprinkler line and a light and that’s it.

Plus, the levelness of an object doesn’t affect its load distribution. If you and your son are both carrying a ladder, one at each end, the amount of load your son carries does not depend on how tall he is.

 

[RabitPete]

Purlins are exactly what I am worried about, anchors in particular. We used Sammy Xpress XP35 anchors for similar applications in the past and had to add longer strut channels across several purlins to help distribute the load.

the levelness of an object doesn’t affect its load distribution. If you and your son are both carrying a ladder, one at each end, the amount of load your son carries does not depend on how tall he is

In this example, what if I release my right arm? My left will see 50% of the ladder's weight, while my son will still carry 25% of the weight in each of his arms. Same thing will happen if strut channels on both sides don't form a plane. Rigid load on top will find its 3 points to rest on instead of distributing the weight equally

 

[bones206]

I like to attach Unistrut to cold-formed purlins using P2784 fittings. I've never used anything like the Sammy Xpress anchors, but the P2784 is the "safest looking" purlin attachment product that I've come across. My approach with the purlins themselves is to sister on a cold-formed C-channel sized for the additional loads.

 

[RabitPete]

Unfortunately this is one of the newer buildings with fiberglass insulation between purlins, so using something like P2784 or adding a sister channel would require removing a section of the vapor barrier.

 

[Settingsun]

How critical/how worried are you? Crane sling rules are 50% (two diagonally opposite corners) if no mechanism to prevent it.

 

[Tomfh]

I’ve had a situation where a 2 meters wide smoke duct (which was VERY heavy) ended up suspended off a single row of purlins, despite us asking for a spread overs minimum of 3 purlins. The way they arranged the hanger rods and struts focussed all the load onto a single line.

 

[dik]

Careful with PEMBs... they have little or no reserve strength. If new, any mod will void any warranty you may have had.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[RabitPete]

The weight in question is about 700 lbs, so if equally distributed between 4 hangers, its only 175 lbs per rod/anchor which I can live with, spread over 2 purlins each will see 350lbs. Its worth mentioning that location is only 2 ft away from the portal frame, and not in the middle of a span.

 

[bones206]

So are those Sammy anchors IBC approved? I'd never heard of them before, but I'm interested in knowing what failure modes are covered by the listed capacity rating. I imagine a designer still has to perform local strength checks on the purlin itself.

 

[RabitPete]

They claim ultimate pullout at 1,783 lbs, so that's probably the only failure mode covered.
And then they also list several UL test loads: 1500 lb (4" Pipe), 185 lb (Luminaire), 250 lb (Luminaire), 416 lb (Conduit & Cable) and FM test loads: 940 lb (2" Pipe), 1475 lb (4" Pipe)
Transformer probably falls under "Conduit & Cable"
We specified Sammys in the past for hanging lights and electrical bus ways, with some disconnects being around 200 lbs and I detailed it with 2 additional strut channels spanning across 2 purlins to support each disconnect