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Up Topic Welding Industry / Technical Discussions / simple weldment query
- - By vscid (*) Date 04-20-2008 17:18
I am using a welded structure which is isoceles triangle shaped, with the 2 equal sides as box tubes and the base of triangle as an I beam. A 2500 lb force act on the I beam at the center. How would the forces be transferred at various joints? I wish to select an I beam and I wish to know the exact force acting on it. Should I design the I beam for 2500 lb force or would it be lesser because of the distribution of the forces?
Attachment: forceanalysis.JPG (0B)
Attachment: 1207forceanalysis.JPG (0B)
Parent - - By chall (***) Date 04-20-2008 19:43
This is a simple "Statics" question, not a simple weldment question.  The general anwser is the load will be distributed based on the trigonomic value at each conntection point.  I'm rusty on the specifs, so I won't try now, but I do know that much.

Charles
Parent - - By 803056 (*****) Date 04-21-2008 02:01
As Charlie stated, this is a statics question. You need to provide more information.

Where are the supports? If the base, i.e., the beam is supported at both ends, let's say by cement blocks, and the connections are considered to be pin connections, the two diagonal tube could have no load other than their dead weight.

However, if the two tubes are hanging from a hook and the load is hanging or supported by the beam, you need to know the lengths of the members to derive a numerical solution.

A sketch of the conditions saves a lot of typing and additional questions.

Best regards - Al
Parent - By vscid (*) Date 04-21-2008 02:57
803056 ,
Please see my sketch attached.
The top and bottom horizontal beams are I beams.
The triangular elements are square tubes welded to the beams.The distance between the suports is 50 inches.
How would the force analysis be? How do I select the weld size and the square tube size?
Parent - - By DaveBoyer (*****) Date 04-21-2008 02:32
In addition to what the others have mentioned, You need to know how much deflection You are willing to live with, there will always be some.
Parent - - By Bob Garner (***) Date 04-21-2008 15:50
You would design the I-beam for the 2500 lb. force in the center.  In addition, the angle of the diagonal pieces will cause a compression load in the beam so you will have combined bending and compression in the I-beam.  The amount of compression is related to the angle of the diagonals.  If you let me know the angle, I can figger a compression load for you. 

Also, will the diagonals be welded to the I-beam such that there is no flexibility in this connection.  If so, you may be inducing some bending in this connection and the diagonals, depending on the ratio of stiffness of the I-beam to the stiffness of the diagonals.

Bob Garner
Parent - - By 803056 (*****) Date 04-22-2008 20:43 Edited 04-22-2008 21:14
Most joints are assumed to be pin connections, i.e., rotation is permitted, to simplify the calculations. Assuming the joints to be rigid adds a layer of complexity to the problem. The truth of the matter is that no joint is truly rigid or truly pinned, i.e., full rotation is permitted.

The easiest solution is to assume pinned connections and remember the sum of the forces are always zero.

The attached sketch shows a solution for the conditions shown. The actual joints may not be pinned, so you would have to analyze the moment couples at the joints unless you do design them as pin connected joints to allow rotation.

You should check my math. There is no way you know if I'm right unless you check my work.

Best regards - Al
Attachment: StaticsProblem22-April-08.dwg (34k)
Attachment: StaticsProblem22-April-08.wmf (15k)
Parent - By Bob Garner (***) Date 04-23-2008 17:29
Al,

Your math looks good to me; it is based on the diagonals being at a 45 degree angle.

Bob G.
Up Topic Welding Industry / Technical Discussions / simple weldment query

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