Angle Braket Loading Calculations

[nonsinusoidal]

Hello Guys:

I am in need of assistance. My task is to build a support made out of Steel Angle Brackets as a means for supporting a duct system. So far I have not advance to a point that I can proudly proceed with my task. Can anyone help me determine the allowable loading for a given Steel Angle Bracket. Lets say that I would like to utilize a L 5" x 3 1/2" x 3/8" bracket to support a vertical load laying evenly on top of it. The steel angle bracket is to be supported at both ends by means of other two sets of angle brackets (vertical positioned) which are to be attached to I-Beams. I have available to me the eight edition of a manual of steel construction book. Can anyone be so kind and provide me guidance on how to utilize the values given in such manual. I also do not clearly understand the given data in that books. Please advice.

 

[Ron]

How long is the angle that will be used as a beam? Will the 5" leg of the angle be horizontal or vertical? How "tall" are the two angles used as "columns"? Where will the duct sit? Does it produce a concentrated load or will the load be distributed across the angle length?

A sketch would be helpful.

 

[nonsinusoidal]

Please see the attached drawing. What I actually have is a 56’ span of 2H x 5W duct system. The duct system will run between two concrete box beams. It will be supported by the steel angle brackets at points B, C, D, E, F, and G (support spacing is 8’). At the abutments, the conduits will be connected to steel conduits which will be inside the concrete walls of the abutments. There will be cables in each of the conduits. The loads per each conduit and three cables per conduit are 15 lbs per lineal foot and 7 lbs per lineal foot, respectively. With the given data, I am not sure if the load should be treated as distributed or a as concentrated load in a given point. In addition, how is the load per steel angle support calculated? Please advise as to how to determine the appropriate size for the steel angle support brackets shown on the attached drawing. Thank you in advance

 

[Ron]

I would analyze as a series of concentrated loads, since you have so few of them and they are less than 12" apart.

The top and bottom angles will act compositely like the top and bottom flanges of a beam, due to the stiff rods between them and the fact that you will not have lateral wracking due to the confinement at the box beams.

Given that, the likely control will be shear at the top and bottom angle ends. The loads are relatively small (350 lb+/-)at each load point, so you have less than 2000 lbs total load on the system. If you treat the system as a beam, the tension in the bottom angle will not likely be critical but check it to be sure...then check the shear in the angles at the ends. The bolts into the box beam will be in shear as well, but shouldn't be an issue if you are using 1/2" or so diameter expansion or epoxy anchor bolts.

 

[rb1957]

your beam is supported at 8 points, 2 ends and 6 intermediate points, evenly spaced. assume each support reacts 8' of load (yes, i know, the end two react only 1/2 of that). but 8'*22lbs/ft = 176 lbs, reacted on two attmts; no?

 

[rb1957]

thinking about it, could those weights are probably per item, so the result would be 10x ? ... 15lbs per ft seems pretty heavy for conduit, but 1.5lbs/ft seems very low ??

 

[desertfox]

Hi nonsinusoidal

First off I would find the second moment of area of your compound beam using the Parallel axis thereom, I estimate this to be 329in^4 for your configuration ignoring any contribution from the vertical rods and assuming the angles take all the bending.
Once you have the second moment of area you can use the standard bending formula to find the stress in the angles ie:-
[σ]= M*y/(I)

where M= max bending moment

y= distance to extreme fibre from neutral axis

I= second moment of area

This should give you a start.

regards

desertfox

 

[chicopee]

I should point that your free body diagram is incorrect. Your concentrated loads s/b facing downward, and your end reaction forces upward. You should also consider the unifromly distributed weight of your angles.

Nonetheless your problem can be solved either with uniformly or with concentrated loads and the differences in bending stresses being the prevailing type, should not be significantly different.

I would also draw a shear and moment diagram to get a better idea where and which max stress should be analyzed.

You should however be more concerned about deflection and that should be your main focus of the analysis. If you determine your Z value based on some allowable deflection, you'll find out that stresses will be well within your allowable working stresses.

 

[nonsinusoidal]

Thank you all for the information you have provided.


DESERTFOX:

It has been a while since I had to perform calculations to find the moment of inertia (second moment) of a compound beam. I dug out my statics and mechanics of materials book and read the section on "Deformation of prismatic member possessing a plane of symmetry and subjected at its end to equal and opposite couples acting in the plane of symmetry." As I read to the derivation, it gave the formula below that you provided:

"?= M*y/(I)

where M= max bending moment

y= distance to extreme fibre from neutral axis

I= second moment of area"

Can you help me on the calculations to determine the second moment of a given angle support and how to apply the parallel axis theorem? Does the parallel axis theorem is to be utilized to translate the moment of inertia to the neutral axis? How is the angle support ought to be treated to calculate the second moment since a square beam is not geometrically shaped as an angle bracket? Please advise and thank you in advance.

 

[Gobiman]

Check out:

http://www.roymech.co.uk/Useful_Tables/Sections/Angles_dim_prop.htm

 

[desertfox]

Hi nonsinusoidal

I'll try and help over the next couple of days, at present I am working away.
Try looking in your text books for the parallel axis theorem it should come under either its own heading or compound beams.

desertfox

 

[desertfox]

Hi

try this link and go to chapter 15 of this online book

http://books.google.co.uk/books?id=...cG_Dw&sa=X&oi=book_result&ct=result&resnum=10

desertfox

 

[rb1957]

are you analyzing the conduit or the supports ?

i figure you need to to both, but your post started with the supports and without seeming to get to an answer you've gone off onto the conduit.

this can be a very simple problem. you've found a static textbook, great. the analysis starts with assumptions. very simplistic and reasonably accurate is to say that the support react their nearby load (a span of load for the intermediate supports, 1/2 span for the ends). Alternatively you can use more complex (and accurate) methodologies like "3 moment equation" ... google it if it's not in your text. this'll tell you the reaction at your supports, which should bring up a host of questions.

next calculate the maximum moment in the conduit beam; if you use the simplistic reactions it'll be PL/4 (where L is a span and P is the load of a span) ... this is conservative (bigger than the more accurate solutions, like "3 moment equation").

So now you can analyze the conduit beam. Important assumption, how are the 10 conduit going to react the moment ? are they going to bend together, as if they are one welded assembly, or are they going to slide against one another ? i think it's conservative if you say one tube reacts 1/10th of the moment.

but that's enough for now ...

 

[Mecha20]

where is the load concentrated ir distributed? do i treat it as a cantilever? what sort of load would it be subjected to?

 

[rb1957]

mecha, are you trying to answer the post ? or asking your own questions ?

if the latter, i think you should start your own thread, and not hijack someone else's.

 

[Mecha20]

rb, how can someone answer the question without those relevant information? should we answer in vocabulary? if so, we should have opened a sco sci forum....i wanted to believe that the question required a direct answer, not any run around....

 

[trainguy]

nonsinusoidal,

Have you considered asking a structural engineer? I wouldn't try to design an amplifier from a handbook.

tg

 

[desertfox]

Hi nonsinusoidal

The formula for parallel axis theorem can also be found here

http://www.roymech.co.uk/Useful_Tables/Maths/Mathematics.html#Parallel

axis Theory

desertfox

 

[desertfox]

Hi nonsinusoidal

I have uploaded a file showing how I got the second moment of area for your compound beam.
This assumes all the bending would be taken by the angles and is conservative as it takes no acount of the vertical studs.
This is only a small portion of the calculations your faced with, you need to look at bolt loads connecting your beam to its supports etc

desertfox

 

[nonsinusoidal]

Hi Desertfox,

Thank you for the uploaded file that shows how the second moment for the compound beam was found. I understand the second moment for each of the angle brackets was found and then it is translated to the neutral axis. My question is how did you now where the neutral axes was to be found at 17.0984/2? Please advice