Design of Concrete Beams in Two Directions 2

[4inch_orange_doorhinge]

When designing mild reinforced concrete beams do you give consideration to the change in capacities as the effective depth changes in each direction? Perhaps I'm not understanding how rebar is laid out in the field but theoretically would one beam have more capacity than the beam in the perpendicular direction? Any advice would be helpful from an analysis and/or construction standpoint.

Thanks!!!

 

[dik]

Yup... with the lesser beam reinforcing being located within the reinforcing of the greater beam... hopefully it was designed this way... YOu might ask.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[4inch_orange_doorhinge]

Thanks for taking time to answer Dik. Can you elaborate on that? I'm afraid I don't quite understand your response.

 

[dik]

With minimum concrete cover to reinforcing being similar for both beams, there will be a conflict in placing rebar. Generally the beam with the greatest moment (reinforcing) will be placed using this minimum concrete cover. The reinforcing for this other beam will have to be placed beneath the main beam top reinforcing and above the main beam bottom reinforcing. When you have multiple layers of reinforcing it gets a little more complex.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoelTXCive]

Structure Magazine had an article that covered how to detail (see attached). I do not think they discuss capacities though. My assumption is that the depth to reinforcement is reduced in one of the beams.

 

[MIStructE_IRE]

I would really question the lap/Anchorage off the bottom bars in that sketch!

 

[dik]

For a non-seismic zone, it could be OK.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

1) For the positive moment reinforcement in gravity beams, it is fairly common to assume that it can be draped as shown in Joel's detail such that you still retain the full flexural depth. This may be problematic at shorter beams with larger diameter bars where the bars are not so amenable to the draping.

2) For the negative moment reinforcement, your maximum demand is at the location where the bar elevation is compromised and draping is not an alternative. There, I don't see that you have much choice but to take account of the change in flexural depth.

3) For systems with well proportioned beams, I'll often just assume that the beams in both directions will have their top and bottom reinforcing in sub-optimal layers. Because beams are relatively deep elements, the penalty for this is often fairly minor. This way, if the rebar detailer screws up the bar placement order and I don't catch it in the submittal review, it's not a huge deal.

4) As a minor corollary, in a seismic moment frame, you'd still have to consider the reinforcement at the maximum effective depths for the sake or determining over strength plastic beam hinge capacities.