Built up section question

[7202cdz]

I will design a simple support beam to support linear load. In stock W24x84 beam will be used. The moment and deflection are overstressed (limit). The beam shear capacity is adequate. I will add double angles to form a built up section like truss in order to increase section modulus and section moment inertia (space under beam is ok and this is our prefer option). I am wondering how to design the web members. Is it same as standard truss and the web members shall take all the shear force?
Please see attached sketch and please someone provide me help on this issue. Thanks

 

[msquared48]

Well, I would be conservative and design it that way, for to reduce the deflection, the bottom chord will have to be stressed, meaning the diagonals too through the shear transfer mechanism.

Mike McCann
MMC Engineering

 

[jberg]

Why couldn't you use a WT section attached to the bottom flange? Depending on the loading, I would think it would be a much simpler fabrication.

JWB

 

[7202cdz]

Thank you all. It will be great to have someone give me advising on how to calculate shear force on web bracing members or any reference books.

Thanks.

 

[rb1957]

the lower chord is reacting only tension, yes? (assuming down loads)

the vertical members react the vertical component of the diagonals, yes? (so the lower angles see only axial load)

i'd model the effective bending section as the lower angles and the upper chord of the W24, or more simply two axial members at the centroids of the lower angles and upper chord; this'll give you a 1st estimate of the axial load in the lower angles. Then over the span of a diagonal you know how much to load in the lower angles increases, ie the load delivered by the diagonal. so each diagonal delivers a different load, so there is a nett up load onto the W24 at each diagonal/vertical intersection ... yes? on the angle the diagonal/vertical member intersections balance in the vertical direction ... yes?

but this load won't build up as assumed (continuously), instead it builds up with discrete steps (where the diagonals and the load) ... yes? so i'd now draw a FBD of the W24, adding the loads from the diagonal and vertical members to see the moment inthe W24 ... yes?



Quando Omni Flunkus Moritati

 

[BAretired]

You are wasting material. The W24x84 cannot function as a bending member, so it will act merely as the top chord of a truss. But you don't need that much material to serve as a top chord. You would be better advised to build a truss with double angles in the top and bottom chords.

Save the beam for a project where it can be used more cost effectively.

BA

 

[SteelPE]

How overstressed are you with your beam..... and how much deflection are you getting/looking for? I agree with BA, this seems like a huge waste of material and fabrication time.

 

[structSU10]

Yea, why cant you stitch weld the angles to the section to create the built-up section for bending?

 

[BAretired]

Another possible method is to use a castellated beam, but that involves a considerable amount of cutting and welding.

https://www.google.ca/search?q=cast...vKHeX5iwK-7IHwCQ&ved=0CC0QsAQ&biw=785&bih=464

BA

 

[dhengr]

BA:
The castellated beam idea is a far better approach than the sketch he has shown, it uses the W24 much more efficiently, and probably doesn’t cost that much more to accomplish. Another possibility would be just to bolt a 36" deep bar joist under the W24, and split an increased cap’y. in proportion to their relative stiffness’. At the ends of the bar jst., since the W24 is o.k. in shear and bearing, just fab. a bearing saddle for the bar jst. and fix that to or over the WF.

 

[paddingtongreen]

Another way would be to slit the beam into two 12" tees and make the truss chords from those.

If you have to work per your sketch and if you must analyze by hand calculation, you must first analyze the beam alone under full load to find the deflection. Then analyze the truss under full load with the members pinned at all joints and calculate that deflection. The loads will be applied to the beam and to the truss in inverse ratio to the deflections and the new effects added together, i.e. the top chord will have bending from its beam portion of the load and compression from the truss' portion of the load.

You may have to iterate this because the members of the truss will be too big if you selected them for the truss to carry all of the load first time round.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[BAretired]

dh:
I like the castellated beam idea too, but it has never been popular in my neck of the woods, presumably because of the high labor cost.

BA

 

[paddingtongreen]

There was an outfit that automated them, I think in the late seventies/early eighties but they couldn't sell enough to make it work. I wanted to use them, I had done so in the UK well before. The problem was acceptance, they were new and didn't look as strong, especially in big fossil power plants.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[dhengr]

My experience with castellated beams was about the same as Paddington’s, but I would have guessed there were several different outfits that tried to make them work and sell. Now, there’s a new entrant in the field,

http://www.newmill.com,

New Millennium Building Systems; proving that good ideas never die, they keep getting reinvented, and some day may finally fly, with new equipment, methods and materials. Paddington... do you suppose that the fact that this worked a few times in the UK was because material was more expensive and labor was cheaper as percentages of the total cost of the job? My experience was with lighter steel roof and floor framing than you would find in power plants.

 

[7202cdz]

Thank you all for your time. I try to design this member as composite beam such as cover plate weld to top and bottom of flange or shear stud floor. For adding bottom double angles (or other structural shape) to move W24 section neutral axis downward (and add more section area) to increase moment inertia and St and Sb. The composite action between W24 and bottom angles will based on shear force (transfer through web member). I use coverplate method to decide the shear flow for intermediate (VQ/I) and end web members (MQ/I). I am not sure how to calculate the exact shear value in order to make the bottom chord and top W24 fully composite action. I agree MSQUARED48 that it is conservative for design like standard truss diagonal bracing.

Thanks again

 

[BAretired]

You have not explained your problem in a way that anyone can help. What is your span? What is your load?

BA

 

[dhengr]

For the life of me, I can’t figure out why it is damn difficult for these OP’ers. to provide the needed info. to have a meaningful discussion of their problem. Do they all assume we can see the plans and details they are looking at, from here? Do they even know what info. is important and needed? Then they show some kinda crazy scheme which they can’t explain and don’t know what to do with, and ask for help. Then they don’t really read the responses from others for its full meaning, and continue charging down the wrong path. It’s a good thing that the rest of us are just here for the typing practice. I think it should be an Eng-Tips forum requirement that they tell us where these things will be built so the rest of us can avoid those structures.

 

[7202cdz]

Thank you DHENGR, I am sorry if I did not explain question clearly. In my question, I mainly asking how to calculate the force on web members. I read all feedbacks, and also get help from these feedbacks such as MSQUARED48, RB1957, PADDINGTONGREEN, and Others. Again, I am looking for how to calculate the web force for this type built-up section, simple support beam under linear load. Thank you again.

 

[BAretired]

If you know the change in chord force at each panel point, the web forces arise naturally from statics. There is nothing mysterious about it. But you are still wasting material, a fact which you do not yet seem to comprehend.

BA

 

[paddingtongreen]

Are you saying that you do not know how to find the forces in the web members? I am often surprised to see people performing sophisticated tasks but don't know basic analysis.

You need a mentor and a book on structural mechanics.

In the truss, with all the members pinned, each panel would be a mechanism without the diagonal. Calculate the reactions, and starting at one end, calculate the force in the diagonal from the vertical component (the reaction less any imposed load at that joint) you now have the reaction force and the diagonal force, from those resolve the forces in the chords. Work your way along reducing the shear by the imposed load at the previous joint. This is called the method of sections.


Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin