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Clevegar (Structural) (OP)
19 Jan 04 17:02
(I posted a similar question in AASHTO forum)

Reality check is requested by engineer who doesn't often stray into the AASHTO code.

I want to find the design live load bending moment in my rc deck with main reinforcing perpendicular to traffic. Following

S=10 ft span between longitudinal members
P=16,000 lb wheel load
M = ((10ft+2)/32)*16,000lb = 6,000ft-lb per unit strip of slab, and I will choose my transverse flexural reinforcing based on that force.

or, if my reinforcing is parallel to traffic, following

S= 10ft between transverse members
P= 16000lb
E= 4+0.06*10 = 4.6ft
M= (P/E)*S/4 (assuming the maximum loading occurs midspan)
 = ((16000/4.6)*10)/4 =8696ft-lb per unit strip and that is what i will use to choose my flexural rebar.

Am I doing this right? Many thanks in advance.

Qshake (Structural)
19 Jan 04 18:59
If you have a typical slab and girder bridge where the girders are the only support for the slab, the main reinforcing will always be perpendicular to the traffic.

You will often find that the cantilever moment controls rather than the interior (between girders) moment.  However, at a girder spacing of 10 feet it is a good idea to check the interior.

Generally, you must check...

1.  Cantilever Moment, Wheel Load.
2.  Cantilever Moment, Collision Load (impact into the barrier).
3.  Interior Negative Moment.
4.  Interior Positive Moment.

Usually, the only time you have reinforcing parallel to traffic would be the case for short spans where the slab is supported on floorbeams.


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Clevegar (Structural) (OP)
19 Jan 04 20:23
Qshake, thanks for the reply. I've read many of your's in the past and you are a great resource.

I really could use someone specifically addressing the code articles I sited, in order to be able to get through a particular problem tomorrow. Any advice out there?
koodi (Civil/Environmental)
19 Jan 04 21:59
No your not doing it right.  Span will not equal the clear span in most instances.  Also your wheel load needs to include impact (assuming service load design).

Everything else looks right, just plug and chug, it's empirical.  
Clevegar (Structural) (OP)
20 Jan 04 11:44
"Everything else looks right, just plug and chug, it's empirical."

Thanks Koodi, that was the answer needed (I did not want to complicate my question with issues of determining span, load combinations, etc. Those are all clearly given in the code and not the problem. I think it's hard for engineers to assume anyone knows what they're doing until shown. I never do myself).

thanks again,


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