Reinforced concrete - continuous vs. simple support

[enfsgi]

A concrete beam spans between two other concrete beams. Can it be designed as a simply supported beam? Bottom steel designed for beam moment, top steel provided for stirrups, concentrated loads to supporting beams. Or should it be designed as contimuous? Top steel to resist moments at supports, torsion in supporting beams.

 

[TXStructural]

Simple support unless you will be able to put significant torque into the supporting beams AND they have the required stiffness to get redistribution and stay serviceable.

You will never be wrong to assume simple support for mid-span reinforcing. Alternatively, given continuity on the ends, you _could_ design a span as two cantilevers, if you really want. The convention is somewhere between, and is based on strength and compatibility of the elements.

I jokingly tell people to "design for M=wl^2/8, and put half the steel in the top and half in the bottom, continuous throughout, develop the top at the ends and the bottom at midspan, and you're good to go." It's not quite so simple, but it gets you close without being excessive. Theoretically, in a continuous beam, once you know how much bottom mid-span reinforcement is required for your simple span, you could nearly divide it in any proportion between top end and bottom mid-span.

 

[spats]

I would design it as a simple span beam and provide nominal top reinforcement at the supports, extending to about 20% of the span.

 

[asixth]

The support beams will have some torsional rigidty and hence attract some negative moment over the supports. Also if the beam you are designing has a back span, be it only a thin slab, it will still provide some negative moment over the supporting beams. Without known too much about your design I would say design the beam assuming it is simply supported and provide minimum flexural reinforcement at the support beam locations and extend to 30% of the span. Also should provide some additional hanging ties at the support beam locations to allow shear transfer because the load is not applied at the top of the beam.