Built up section question
Built up section question
(OP)
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
Please see attached sketch and please someone provide me help on this issue. Thanks






RE: Built up section question
Mike McCann
MMC Engineering
RE: Built up section question
JWB
RE: Built up section question
Thanks.
RE: Built up section question
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
RE: Built up section question
Save the beam for a project where it can be used more cost effectively.
BA
RE: Built up section question
RE: Built up section question
RE: Built up section question
https://www.google.ca/search?q=castellated+beam&am...
BA
RE: Built up section question
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.
RE: Built up section question
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
RE: Built up section question
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
RE: Built up section question
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Built up section question
RE: Built up section question
Thanks again
RE: Built up section question
BA
RE: Built up section question
RE: Built up section question
RE: Built up section question
BA
RE: Built up section question
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
RE: Built up section question
BA
RE: Built up section question
And, you still resist giving us some of the particulars on the problem, like span length, total depth of the truss, and actual loadings. These would be easy to show on your sketch. Is this new construction or is the W24 in place already, and now, to be reinforced. If your sketch is in proper proportion, the span is about 80', the truss is about 5' deep, and the panel points are about 5' o/c, why not show this. You would be surprised at how important that type of info. is for an experienced engineer who is asked to comment on your problem. They need that info. to make judgements about the feasability of the concept, and what might control its design, etc. They look at your sketch and wonder how you are going connect the verts. and diags. to the bot. flg. of the W24, they sure can’t go through it as your sketch shows, although I suppose your thought process probably was that you wanted their work point to be at the center of the W24.
If you have really read the responses for their full meaning, I’m a bit surprised that you haven’t picked up on the fact that most of the experienced engineers here think your approach to the problem is a little bit crazy, not impossible. A very inefficient use of material and overly complicated for what we think you are trying to do. Another useful piece of info. would be why you are forced to use the W24 for the top chord. Our initial reaction is that that is a difficult way to handle a simple problem. Probably the best way to use the W24, would be to split it, as someone suggested and use the WT’s as top and bot. chords for a truss you design and fit-up the normal way. Although, the W24 may not yield the best sized WT’s.
RE: Built up section question
dhengr, the transition from material to labor being the more expensive part of the cost had started in the early fifties when I started working. To be honest, I think there is more than a touch of culture involved. The USA goes big, unnecessarily in some cases where the UK the culture is less bombastic (I don't mean bombastic unkindly, I just can't come up with a better word). I did start out with a mix of work on refinery design, drawing from small access platforms to 1000 ton cat cracker structures, I enjoyed those, they were riveted girders, very pretty.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin