Load On Strut

A

Alwayslearningelec

Senior Member
Location
NJ
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Estimator
I's like to put (8) 4" PVC Coated RGS on one of these Durablock racks. They each have 4 #600 XHHW. Seems the load limit is 1000 lbs. How can I determine how much load these conduits/wire create?

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Is this a small project that isnt engineered? Where I worked, all electrical support was a complex design if it was hung or had large conduits spanning hallways or open spaces because of seismic calculations. Static load is one thing, wind has more factors, and seismic is a major pain. We got so many designs that were unusable but earthquake stable...
 
suemarkp said:
Is this a small project that isnt engineered? Where I worked, all electrical support was a complex design if it was hung or had large conduits spanning hallways or open spaces because of seismic calculations. Static load is one thing, wind has more factors, and seismic is a major pain. We got so many designs that were unusable but earthquake stable...
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It is a very large project and engineered. All they said for supports of roof conduits was to use Durablock.
 
Add the weight of all the conduit and wire, and divide by the number of supports. (You can find online calculators for weight per linear foot. So you could also calculate the weight per foot * distance between supports.)

Some specs ask for a safety factor - like maybe 3. So you could load each support up to 333 lbs., and not the full 1000.
 
I would think the metal strut assembly would be plenty strong depending on the size bolts you use for the strut connectors. The weak link looks like that rubbery support base -- it may squish flat if overloaded. But I also wonder if the underlying roof is the weaker link. How many pounds per square foot can it take as a dead load and are they instructing you where those durablocks should be placed on the roof (hopefully near an underlying truss)? If you are at or over 1000 lbs per block, that is about like parking small cars on the roof along that whole run.

If this is engineered, you shouldn't have to ask these questions and the engineers should have already considered the strength and placement of the durablocks.
 
suemarkp said:
I would think the metal strut assembly would be plenty strong depending on the size bolts you use for the strut connectors. The weak link looks like that rubbery support base -- it may squish flat if overloaded. But I also wonder if the underlying roof is the weaker link. How many pounds per square foot can it take as a dead load and are they instructing you where those durablocks should be placed on the roof (hopefully near an underlying truss)? If you are at or over 1000 lbs per block, that is about like parking small cars on the roof along that whole run.

If this is engineered, you shouldn't have to ask these questions and the engineers should have already considered the strength and placement of the durablocks.
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Don't think the supports were engineered.
 
I found the following

4" PVC RGS is 9.73lbs per ft.
#600 XHHW is 1.973lbs per ft.

So 4" w/ 4 #600 is roughly 18lbs per ft.

The supports I posted are to be 7' apart. So 18lbs x 7'=126lbs x 8 conduit = 1,008lbs. So that is the weight load on the piece of strut? Wouldn't it be less since the strut width( the portion the conduits rest on) is only ~2"?
 
Alwayslearningelec said:
The supports I posted are to be 7' apart. So 18lbs x 7'=126lbs x 8 conduit = 1,008lbs. So that is the weight load on the piece of strut? Wouldn't it be less since the strut width( the portion the conduits rest on) is only ~2"?
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All of the weight of the conduit has to be supported. Any conduit weight not directly supported by the strut is transferred through the conduit itself to the area that is supported. For a deeper dive, look up the concept of 'tributary area' in structural engineering.

When you have something like conduit supported on trapezes the _approximation_ is that each trapeze supports an equal portion of the conduit weight. So if the supports are 7 feet apart then the _approximation_ is that each support holds the weight of 7 feet of conduit. The approximation ignores the stiffness of the conduit but is usually good enough if you have proper safety factors.
 
winnie said:
All of the weight of the conduit has to be supported. Any conduit weight not directly supported by the strut is transferred through the conduit itself to the area that is supported. For a deeper dive, look up the concept of 'tributary area' in structural engineering.

When you have something like conduit supported on trapezes the _approximation_ is that each trapeze supports an equal portion of the conduit weight. So if the supports are 7 feet apart then the _approximation_ is that each support holds the weight of 7 feet of conduit. The approximation ignores the stiffness of the conduit but is usually good enough if you have proper safety factors.
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Thanks so per my post #12 it would be 1,008lbs per support?
 
steve66 said:
Add the weight of all the conduit and wire, and divide by the number of supports. (You can find online calculators for weight per linear foot. So you could also calculate the weight per foot * distance between supports.)

Some specs ask for a safety factor - like maybe 3. So you could load each support up to 333 lbs., and not the full 1000.
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Yes, the "ultimate load capacity" is the limit above which failure is very likely to occur. So a safety factor needs to be applied to obtain an acceptable working load. And even before the ultimate load limit is approached, a yield limit where the material begins to plastically deform will occur.

I would not be comfortable with that structure without some diagonal bracing to add rigidity and prevent possible collapse.
 
Can you divide into 2 groups of conduits on different supports (side by side), and also space the supports closer together?

Although, with that much weight, it might be worth an RFI to make sure the engineers have done their homework.
 
Is the intent to support all the conduits from a rack like shown in the first post, or to support the conduits individually from the blocks without the racking? Is the capacity of a single block 1000lb, or is that the capacity of the assembly shown?
 
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