ACI 318 Span/depth ratios

[frv]

I've got a question regarding interpretation of table 9.5(a) of ACI 318. The table gives minimum thicknesses (based on span) to avoid having to perform more detailed deflection calcs. The tables are based on the length "l".

The question I have is whether the length "l" can be taken as clear distance between the supports for a member built integrally with the support.

I'm unclear on this question because if you look at the definition of "l" in chapter 2, you are directed to section 8.9. Section 8.9 then proceeds to define span length of members not built integrally with the supports as clear span plus the depth of the member (not exceeding cl to cl of the supports). Does this then imply that when the member is built integrally with the support that you can take the length for purposes of Table 9.5a as the clear distance?

For example, say I had a beam spanning between girders. The girders are two feet wide, spaced at 20 ft. on center. Would "l" be 20 ft or 18 ft?

 

[ishvaaag]

ACI 318.11 seems to support your idea:

"8.9.3 — For beams built integrally with supports,
design on the basis of moments at faces of support
shall be permitted."

No spec is (here) given for deflection calculation but since for slender beams deflection is more dependent on moment than shear, one could think this would be a go for clear span, at least if "design" comprises a proper check of deflection (or can be assumed per the code is not required.

If on the contrary by design what is here implied is more than anything the selection of adequate rebar, certainly for the beam itself the reinforcement would be enough in the clear span, and the code is permitting less rebar than conventional analysis to axes (or at least for notional spans bigger than the clear) would be requiring.

The kind of clause seems to imply that forces derived from analyses to axes are conservative enough as to permit the exemption clause, that hence would mean for most cases that to assume deflections from analyses to axes also would over-represent the deflections, hence, you might take the clear span also for deflections.

However it remains from 8.9 still some doubt when states later that

"8.9.4 — It shall be permitted to analyze solid or ribbed
slabs built integrally with supports, with clear spans
not more than 10 ft, as continuous slabs on knife edge
supports with spans equal to the clear spans of the
slab and width of beams otherwise neglected."

So it seems to want to put a limit in length for solid or ribbed slabs when wanting to use clear span.

Other than that, maybe one should look to the specifics for checking deflections ... if I find something interesting will post later.

 

[DST148]

@frv : Yes. For span to depth ratios using Table 9.5(a), l is clear span for beams built integrally with supports.
You may also refer to solved example 10.2 given in PCA Publication - Notes on ACI 318-05.