composite beam design

[vscid]

Hi,
I know how to apply the s = My / I equation for a I beam or for a rect. beam, but how do design a composite beam where the rect. beam is welded to the bottom flange of the I beam as shown in the fig.? What approach would I use
Thanks!

 

[desertfox]

Hi vscid

First find the centroid of the composite shape.
Then use the parallel axis theorem to work out the "I" for the composite section.
Try these site's they might help.

regards

desertfox

http://en.wikipedia.org/wiki/Second_moment_of_area

http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture28/461_lecture28.html

http://www.rocw.raifoundation.org/m...neeringmechanics/lecture-notes/lecture-15.pdf

http://www.mae.ncsu.edu/courses/mae206/rabiei/ch09.ppt#276,35,Parallel

Axis Theorem

 

[vscid]

Is a beam with 3 supports, instead of the usual 2, with the 3rd support in the center of a beam, still considered a simply supported beam? What is a common practice to analyze its bending moments? (See pic attached, assume a support is present at the center of the beam)
Thanks!

 

[desertfox]

Hi vscid

Your attachment doesn't have any pic with it just a blank page.
A beam with 3 supports is statically indeterminate, although it can still be solved by using some property of the beam itself for example knowing the deflection or slope at given points on the beam.
Upload your link and lets see if we can help.

regards

desertfox

 

[vscid]

I have tried attaching again.

Assume a 3rd support in the center of the W beam (besides the 2 at its end). Also, the length of my beam is 54 inches. The load acting on this beam is about 2500 lbs, about 40 inches from the left end of this beam. (so its offset) Basically, would I really need a support in the center? How do I know that? My supports are welded box tubes.
Thanks!

 

[hydtools]

Run your calculations without the center support. If the stresses and deflection meet your safe criteria, you don't need the center support. Done.

If without the center support your outside your criteria, add the center support. Use virtual work to calculate the load taken by the center support. Work is force x distance. The distance is the unsupported deflection at the center support location. The force is the force required to return the beam deflection to zero.

Ted

 

[vscid]

To start, with I have the following design (as attached).
The upper and the lower horizontal beams are I beams.
The triangle is formed by welded square tubes.How do I do force analysis of this weldment? Would the 2500lb load add completely on the lower I beam? Or would it be reduced because of the distribution of forces? Also, how do I select the size of my tubing and weld sizes?

 

[desertfox]

Hi vscid

Is there anymore information you can give us because so far we seem to be jumping from one diverse question to another ie:- from calculating a I value for a composite beam to a statically indeterminate beam and finally to a triangle of forces problem. Moreover you have given no feedback on the previous questions that have been answered by hydtools and myself, so let me ask, is this the final state of your problem or is there yet another diverse question.
Well to answer your last sketch it is a complicated situation and depending on how long the beams are and distances to there supports, you may well have significant bending in those beams.
If the lower beam is merely supported by the two welded legs and assuming that the upper beam as no significant deflection or stress then I would calculate the forces in those legs using triangle of forces which would give you the tensile loads in the welded legs on which to size your weld and tube.

regards

desertfox

 

[vscid]

desertfox,

Let me connect all my questions that I asked:

The reason I asked about the composite beam is that, my lower beam would be composite, made of an I beam and a rectangular beam. My load would be bolted onto the lower rectangular half of the composite beam.
As, you said that a beam with 3 supports would be statically indeterminate, I am trying to see if two supports would work , hence the 2 inclined square tubes.
I was thinking of using chain members instead of those welded tubes, but cannot as I cannot tolerate any swing(play) in the load I am lifting.
The distance between the supports would be 50 inches.
Can you explain a bit more on how I solve using the triangle of forces? Do you mean that I resolve in the X and Y components? What will be my reactions at the welded joints? Also, for what load should I design my beam?
Please do not hesitate to ask, if this is not clear.

 

[desertfox]

Hi vscid

You split the 2500lb into two 1250lbs and they would be vertical at the ends of the two tube supports, then resolve
that vertical load into a resultant load using trignometry to give you a load running parallel to your support.
Why do you need two I beams? why can't you lift using the top one in your sketch.
Isn't there anyone else were your working that can help you with this as you seem to be out of your depth?

regards

desertfox