Hanger Supports

[nonsinusoidal]

I would like to know if anyone of you may be able to guide me on how to determine what the mechanical loads would be on a hanger supports system. The hanger supports would be for a bridge crossing with fiberglass duct. The fiberglass duct system will contain electric power cables. The hanger supports will be made from angle brakets. If I have a crossing span lenght of 56' and to support the one duct system I choose to have seven spans of 8' each, I am not clear on how to calculate the loadings(distributed load?) on the hanger supports. For the moment, let us assume that the weight of the conduit and power cables per 1000'is 2000 lbs. Can anyone advice on this? Thank you in advance.

 

[ishvaaag]

(2000/1000)x8=16 lbf (service level) per support is quite a small load. In the first supports you will see somewhat higher a reaction than that.

Anyway, my main concern would be to ensure that the long term behaviour of the containing duct allows to think it remains straight for the operational life of the installation. If not, I might think of a rigid tube. Bridges are stout structures and the difference of loads shouldn't make much difference, but one must also check.

Live loads: Will snow fall or ice hang from the duct? I also would consider if someone would accidentally hang by whatever the cause from some point of the duct.

Wind: Normally a hanged duct also will see some wind action, that will vectorially compound with weight loads. Wind at bridges is taken quite strong, check the code for a proper value; however, the round shape diminishes much the acting forces. Also, for particular settings of the duct, shaking by the wind and/or resonance may make the wind loads grow even bigger.

Fatigue: Vibrations by moving traffic and wind will cause that the connections be subject to fatigue. So the detailing should contemplate number of cycles for range of stresses, etc. Normally this will resolve to the selection of a proven detail and some limit to the working stress and/or the range of variation for the loads.

 

[racookpe1978]

Check your weight assumptions:

"For the moment, let us assume that the weight of the conduit and power cables per 1000'is 2000 lbs."

How many cables? 2 lbs per ft "might" be correct for a "small" single power cable, but if you have more than one, your assumption is dead wrong. The plastic will add more weight.

What about separation and insulation margins - what voltage spacing??? - between cables? Heat load from I^2R losses (resistance) if more than one cable is in the closed duct under worst case (summer, direct sunlight) on a closed plastic duct?

How long will the plastic be exposed to sunlight degradation every day?

Simple suspended pipe hanger supports - see any pipe hanger supply catalog - will suffice for the supports. But that isn't your entire problem.

 

[blackmaddog]

The support loads are calculated as follows:

w = total load/ft (plf) (dead + live + snow + ...)
l = length between supports (ft)(ie 8ft in your case)
R = support reaction (lb)

For back to back simple spans (fiberglass duct individual 8' pieces) R = w*l

For 2 spans (duct continuous over one support) R_max = 1.25*w*l

For more than 2 spans R_max = 1.134*w*l (this is simplified a little from exact numbers. See the attached table for exact values.)

Hope this helps.

 

[nonsinusoidal]

Thank you everyone for the quick responses.

The Epoxy fiberglass conduit meets NEMA TC-14-2002 and it is UV Stable per UL 1684 AND CSA-C22 NO. 211.3-96.It has been approved by NEC to be utilized (RTRC) underbridge crossings in class I div 2.

I apologize for the weights given out of no where. The actual numbers are as follows:

1. 6" Epoxy fiberglass conduit: 117 lbs per 100'
2. There will be Three Medium concentric neutral underground power Cables in the single run of conduit. A 6" Conduit was selected to meet NEC 40 percent of cross sectional area occupied by the three concentric neutral underground power cables. The weights of each of the power cable is 4090 lbs per 1000'. This information has been evaluated with respect to the electrical design of the three cables within the conduit and that part is within our current electrical standards.

Back to the Hanger support loads,it appears that "blackmaddog" has provided the appropriate equations for calculating the mechanical loads that will be imposed on the hanger supports. There will be wind loadings (where can I find an appropriate wind load factor) that I will account for but not sure how to add those loadings to the above given equations? The hanger supports will be attached to the concrete beams and not to the deck of the bridge. This is another issue which I have not provided any information to all of you to assist me to calculate the appropriate hanger supports. For now, I would just like to completely understand the "HOW TO CALCULATE THE MECHANICAL LOADINGS ON THE HANGER SUPPORTS." Please advice of any further information which can be helpful to me to accomplish the above task. Thank you very much once again for all of the valuable information you have provided.