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.

 

[hokie66]

Huh? This is about a concrete structure.

 

[kslee1000]

Keep "size changes to minimum" in mind, but be practical - always think about, and listen to other trades before make decision, if you have such luxury.

 

[RHTPE]

Consider the following:
- Consistent beam depth throughout is always better for the formwork contractor. Actual beam side height (ideally) should evenly divide into a 48" wide plyform sheet (i.e. 12", 16", 24").
- Spacing of the beams is also a factor. When the beams are closely spaced (i.e. <12'-0 c/c) it may make more sense to construct the formwork deck thoughout at the beam bottom elevation (and block-up for the slabs). Now beam depth changes will adversely impact field productivity.
- ALWAYS consider how bars can be interwoven at beam/girder/column intersections.
- Beams/girders equal to or wider than the columns make for easier forming.
- Deeper spandrel beams can be okay, but ideally some lumber + plyform thickness increment.

You might want to check out:

http://www.cecoconcrete.com/pdf/design_main.pdf

 

[mikhelson]

RHTPE:
Thanks for your advice. I'm surprised at how the contractor suggests to keept beams at constant depth and vary the width instead. Surprised because depth obviously has a much greater impact on the moment of inertia of the crossection, so to compensate for the lesser depth it'd take a much greater increase in width. Of course, increasing reinforcement is another option.

Thanks again for your post.

 

[GalileoG]

It's the increase in moment arm that you get with depth that is important. Your beam is cracked so you won't be benefiting from the beam's moment of inertia, unless we're talking about deflections.

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[RHTPE]

mikhelson:
One of the great advantages of reinforced concrete structures is the tremendous freedom in variability - which in turn presents a dilemma for the designer when presented with choosing the appropriate member sizes. You are correct that depth affects member stiffness much more than width. It also presents another challenge to choose an overall member depth best suited to mitigate long-term creep deflection. Beams do not always present a problem with creep deflection - I have seen long-span beams cambered to combat immediate + creep deflection that never deflected the calculated amount, which resulted in additional construction headaches involving finishes (picture a long corridor with chair rail that follows the ups & downs of a cambered floor structure). Slabs, however, will show immediate & creep deflection much sooner. I have seen far too many structures with a slab depths offering the required strength which resulted in unacceptable deflections (say 3"+ in a 30' x 30' bay). Now one could try to camber the slab for this, BUT your concrete contractor will fight this because of the increased construction effort (read: labor $$).

Best advice: find a depth (for your most typical spans) best suited to satisfy serviceability concerns, then try to maintain that depth throughout. Having been involved in concrete formwork & shoring design for over 40 years, I can honestly say that that will result in the most efficient construction methodology. And do NOT make beams shallower for a roof level - if the same plan arrangement works for the roof, keep beam & slab thicknesses the same as the floors below. Just reduce the reinforcing if there is no chance the structure will ever be expanded vertically.

RHT_PE

 

[hokie66]

Just two comments about RHTPE's last post:

1. Cambering a cast in place beam is almost always a bad idea.
2. 3" deflection in a 30 ft span means the slab is not deep enough.

 

[RHTPE]

hokie66

You are correct - the slab wasn't thick enough. I have seen a few office/residential structures with 30' bays and 9" (2-way flat plate with drop panels) or 10" (2-way flat plate) slabs. All exhibited severe long(er)-term deflections - on the order of 3".

The excessive deflections can be attributed to many things I suppose, but in most cases the key items were: underdesigned slabs and poorly executed shoring/reshoring procedures.