How many beam sizes to use 11

[mikhelson]

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.

 

[DaveAtkins]

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

 

[kslee1000]

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.

 

[civilperson]

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.

 

[BAretired]

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 members which would benefit from a different size, or if there is some merit in using a different beam size in order to meet architectural requirements, that would be the way to go.

You cannot expect to get rigorous responses to this question. It is pretty much a matter of engineering judgment.

Best regards,

BA

 

[JAE]

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.

 

[kslee1000]

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?). Remember the material (additional concrete to maintain larger uniform b) is chepper than labor, and is much cheaper when delay as a result.

 

[mikhelson]

Thanks to everyone for your replies.

 

[Lion06]

I think the main idea is to use as few as is reasonable.

 

[csd72]

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.

 

[slickdeals]

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.

 

[csd72]

One more thing that I forgot to mention, when reducing longitudinal reinforcing from one beam to another, if they both have the same cross section then it is conservative to reduce the area of reinforcement in proportion to the moment (e.g. halve the moment, halve the reinforcement). As long as it still meets minimum reinforcement criteria.

Just be aware that this is only conservative when comparing a lower moment to a previously designed higher moment but is unsafe the other way around. You also need to compare like to like so ensure that the effective width is the same for T beams and that you are not comparing a T beam section to a rectangular e.t.c.

This doesn't help with shear reinforcement though as this needs to be recalculated using the new longitudinal reinforcement.

It is useful for those more complicated arrangements.

 

[jike]

Please keep in mind that formwork is at least 50% of the cost of the structure, therefore the emphasis on keeping as few sizes as possible and varying the amount of reinforcing. If that doesn't work, then widen the beam.

Try to keep all beams to the same depth (if at all possible) so mechanicals can be run underneath efficiently. Don't forget what you are shooting for is a low cost building for the client, not necssarily the lowest cost structure. Don't loose sight of the main goal.

 

[MiketheEngineer]

Once I change beam depths - I usually stick to one size fits all. It is too easy for the field crews to get similar height beams in the wrong place.

 

[mikhelson]

Several people mentioned the ideal of keeping the depth of all beams consistent. If ceiling tiles and mechanical conduits can be suspended at any height and apart from the cost of forming, exactly what are the advantages of same depth for all beams & girders?
Thanks

 

[jike]

Building cost is a function of the floor to floor height of the building. As the floor to floor increases, the vertical runs of piping, conduit, elevators and walls increase. By keeping structural floor depth (if possible) to a minimum, it will help the overall costs. Spandrals can often times be deeper since they do not cause problems with mechanicals. If a deeper beam is needed in the interior and it can be hidden in a wall, this may not cause a problem either but needs to be coordinated with the architect and the HVAC guy.

 

[mikhelson]

jike:

I understand that we want to keep the depth of beams to a minimum. But once we agree on the depth of the deepest girder, is there any reason to keep the depth of other beams and girders same -- even though they could be more shallow? Thanks for your posts.

 

[jike]

I would try to keep everything the same depth (if possible). This translates into the same height of shoring and same form depth. We are trying to keep forming as simple as we can.

We use to build girders deeper than wide; now we often build them wider than deep.

 

[hokie66]

I agree with simplification, but disagree with the goal of keeping all beams the same depth unless the spans are all the same. Services can often be routed around deep beams. There is an advantage in having a difference in depth of intersecting beams, as the bottom bars don't clash. The answer to this question depends to a large extent on the type of formwork system used. If there are prefabricated forms, then standardization is essential. If the formwork is all field cut plywood, it is only necessary to consider making best use of the sheets of plywood.

 

[MiketheEngineer]

I don't keep all the beams the same depth just the same size for a given depth -- just all W8's are the same, W10's are the same, w12's are the same, etc, etc. Almost any construction guy can figure this out... Well not sure...

Usually 2 to 5 depths will suffice - often only 2 or 3...