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how many beam sizes to use

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发表于 2009-9-9 17:30:31 | 显示全部楼层 |阅读模式
how many beam sizes to use
i'm new to structural design/analysis and so my question may sound naive, sorry.
when you design reinforced concrete, do you use a rule of thumb about how many beam/girder types to use for a given floor area?
let me clarify. think about a structural floor plan: the beam schedule will specify several types of beams -- each having unique width, depth and reinforcement combination. for simplicity, assume there're 2 beam types - wider and deeper beams towards the center of building and less wide and deep around the perimeter.
obviously, there's a differential cost involved in forming 2 beam types, rather than using a single beam throughout the floor (overdesigning). that is, you may be saving on material cost (concrete and steel) if you customize the beams more for a particular loading. on the other hand, the more individualized layout incurs higher cost of design, increased and more error-prone labor and rental/purchase of forms.
clearly, there's a balance between the demands of material efficiency and labor efficiency. what guidelines do you use?
thanks in advance.
i never seem to have enough fee to "finesse" the design.  for the example you give, i would probably design just two beams (as you indicated)--a spandrel beam (it will need to be stiffer if it is supporting masonry veneer), and an interior beam.
daveatkins
material is cheap compared to labor cost in most developed parts of the world. but no cost can compare to errors out of over-design.
depth is the best variable to increase for cost effective reinforced concrete beams.  it is limited by floor to floor height and mechanical clearances or penetrations.  use the same maximum depth for all beams and vary the width and reinforcement as needed.
i believe it a matter of engineering judgment.  usually, i like to keep it simple, so i stay with the same beam size if possible.  on the other hand, if there are a significant number of   
check out crsi.  they've published some documents that outline good practices (with cost in mind) for proper sizing/detailing of concrete beams relative to depth, column interface, etc.
i also agree with civilperson that you should use a constant depth if possible and vary the width/reinf.

the beauty of a good design is the design concept and load pathes can be identified by walking through an existing building without the aid of original design drawing at hand.
for simple layout, 2 beam size should be adequate. for more involved use of space, 3 is a good number to stick to. for same depth beams, try to uniformly use the width of the heaviest beam to simplify the fabrication of stirrups and stocking, and minimize the time wasted in finding correct stirrups for a specific beam (woops, the fabricator missed 1 specific size of stirrups, now what?). re  
thanks to everyone for your replies.
i think the main idea is to use as few as is reasonable.
mikhelson,
this is a skill that you will learn with experience, there are no simple rules as every building is different.
the general rule for scheme design is to make it as simple as possible without ignoring the constraints. basically as structuraleit said this usually means to use as few as is reasonable.
what i would do is first look at the worst loaded beam and design that. look at the next loaded beam, is it similar with similar loads, if yes then leave it same as the first and keep working this down until the answer is no and then design a second type of beam e.t.c. dont analyse all these beams just compare them rationally and then perform one calculation for each type (possibly with more than one load case).
for a really simple structure it could be 1 or 2 types of beam, for a really complex one it could be 20, normally it should ideally be 3 or 4 types per floor.
i second csd72, one of the techniques that i use is to design a few beams and get capacities, rather than design each and every beam.
it works out better in steel design more than in concrete because computer programs are set to optimize to the gnat's ass and before you are done, you are looking at 127 different beams for a floor.
in concrete beam design as well, you don't want to vary the reinforcing on the beams a lot. this makes placing complicated and increases the chances of mistakes.
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