Categorising this type of RC Beam 1

[nicolas77]

Hi,

How do we call a beam where the web and bottom bars are cast first (cast-in-situ) and the top of the beam is cast at a later stage together with the slab. The beam overall height and width is 450 and 150 and the 450 is 300mm. web and flange is 150 (same thickness as the slab). The casting is done in two distinct/separate stage days or weeks apart. So far in handbooks they have singly versus double reinforced beam and rectangular and T or L shaped beam. How to intepret that beam, how to categorise it? should it be singly rectangular with the dimension of the web taken in consideration? Thanks.

 

[Luceid]

Are the "web" bars stirrups? or are you talking about skin reinforcement - horizontal bars around the faces of the beam?

 

[nicolas77]

Usually beams reinforcement can be divided into three parts: 1. the bottom or main tensile part which is the very minimum, the essential with the binders and at very least two top bars to hold the stirrups together making out singly-reinforced beams. 2. the skin reinforcement in case of the height of the beam being greater than 750mm. where rebars are placed at the sides of the reinforcement to prevent cracks. 3. the top bars which can either be just here to hold the stirrups together part of a singly reinforced beam or being there on purpose, part of the design making the bar a doubly-reinforced beam. In all cases, including the particular one I am asking, we have the minimum tensile bars at the bottom, stirrups acting as traverse reinforcement and top bars to hold the stirrups together. In my particular case, the same hold true except that the lower part of the beam is cast first with the bottom bars and part of the stirrups. The remaining top is cast together with the slab along with the top part of the stirrup at a later stage. The web or lower part of the beam is even used as formwork/support for the shuttering of the slab.

I have consulted many books on the subject but none of them discuss this methodology used in casting beams. Using AI, I was told that this is called "stage-cast" or composite beam.

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Luceid reading in: should you have anything, please let me know. Thanks.

 

[BulbTheBuilder]

How do we call a beam where the web and bottom bars are cast first (cast-in-situ)

There's no particular name for this beam. Just a beam with concreted casted on different days

The casting is done in two distinct/separate stage days or weeks apart.

Generally speaking, this is not a good practice, but it is done sometimes depending on the design concept. From the one I observed, the beam was designed as rectangular not flange and a ribbed pre-tensioned beam were installed.

So far in handbooks they have singly versus double reinforced beam and rectangular and T or L shaped beam

Reinforcements are provided at tension zones since concrete is good in compression but not in tension. If it's a simply supported beam, then you have tension only at the bottom. For this scenario, you can use only singly reinforced beam (whether T or L or rectangular). If you have a continuous beam with both + and -ve moment (sagging and hogging) then you need doubly reinforced beam. The bottom rebars for +ve moment(sagging) and top rebars for -ve moment (hogging)

should it be singly rectangular with the dimension of the web taken in consideration? Thanks

I'd recommend considering the web only with no flange (including the portion of the slab in the web) in your design. So, if your beam is 450mm deep and your slab is 150mm thick, use 300mm rectangular deep beam for your design. Why? You might not the composite action of the beam and slab, which means you beam system might not behave as T or L.

The web or lower part of the beam is even used as formwork/support for the shuttering of the slab.

Design as rectangular.

I was told that this is called "stage-cast" or composite beam.

Stage cast is a technique of pouring the aggregate first and later adding grout to the concrete member. I consider it as a traditional method and haven't seen it that in North-America (US to be specific. I am guessing you're on a different continent since you're using ACI metric codes)

we have the minimum tensile bars at the bottom

Is minimum required per design or you just provided minimum reinforcement?

 

[nicolas77]

Thank you very much. You confirm many things I thought about those beams. Again thank you a lot for answering.