## Shear at support of continuous beam

## Shear at support of continuous beam

(OP)

I've often used this site for advice but this is my first time posting here! I'd like to hear your take on a debate a friend and I have had regarding shear at the intermediate support of a continuous beam.

The argument is about whether one should design the shear at the central support of a continuous beam for the total reaction under the support or for the maximum shear calculated from the shear diagram.

By way of example see the attached images. A beam consisting of 2no. 5m spans, all supports pinned. 10kN/m UDL applied to the beam (RC beam 300x800mm deep). Based on this the shear force is +/- 31.1kN at the intermediate support. The total reaction at this support is 62.2kN. Should the beam be designed for an applied shear of 31.1kN or 62.2kN?

The argument is about whether one should design the shear at the central support of a continuous beam for the total reaction under the support or for the maximum shear calculated from the shear diagram.

By way of example see the attached images. A beam consisting of 2no. 5m spans, all supports pinned. 10kN/m UDL applied to the beam (RC beam 300x800mm deep). Based on this the shear force is +/- 31.1kN at the intermediate support. The total reaction at this support is 62.2kN. Should the beam be designed for an applied shear of 31.1kN or 62.2kN?

## RE: Shear at support of continuous beam

BA

## RE: Shear at support of continuous beam

Beam shear is 1/2 of the total reaction in this case....31.1 kN.

Per BAretired, in many codes you can reduce the beam shear further by taking the shear at a distance d from the face of support. (d = design depth of the beam from compression face of beam to centerline of reinforcement).

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## RE: Shear at support of continuous beam

Glad to hear i'm not going mad. I thought this was structural engineering 101 and yet my friend who is a much more experienced engineer was adamant that you should design for 62.2kN in this case. How do you think I should try and explain this to them? I've tried a number of ways but am evidently not persuasive enough! They tried to provide an analogy to slab punching shear (i.e. you use the total column reaction when calculating the shear stresses in this case) which I can only see being relevant for a sort of face shear failure of the support and I can't ever see this being a governing failure.

## RE: Shear at support of continuous beam

another day in paradise, or is paradise one day closer ?

## RE: Shear at support of continuous beam

## RE: Shear at support of continuous beam

Thats exactly how I tried to counter argue that point. If you make a flat slab so narrow that it can no longer develop a punching shear plane on all 4 sides then you are left with a continuous beam of sorts with the two shear planes. I just don't understand where they are coming from and why I am unable to convince them! Super frustrating

## RE: Shear at support of continuous beam

He may be a more experienced engineer but his experience does not seem to be in structural analysis and design. I don't know how you should explain it to him; you could suggest a review of his old text books.

BA

## RE: Shear at support of continuous beam

## RE: Shear at support of continuous beam

It's just the beam never sees the full amount, it only sees the half from each side.

## RE: Shear at support of continuous beam

Kootk's suggestion should enable him to either prove himself wrong, or invent some new branch of physics where sum of the forces in the vertical direction do not equal zero.

## RE: Shear at support of continuous beam

## RE: Shear at support of continuous beam

## RE: Shear at support of continuous beam

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## RE: Shear at support of continuous beam

## RE: Shear at support of continuous beam

Incidentally, say there’s a simple beam 5m long with a 10kN point load at midspan, giving us a simple rectangular shear force diagram of 5kN above and below the line... What shear force would he use for shear design at midspan? 5 or 10kN?!

## RE: Shear at support of continuous beam

Good question.

Or even a 10 m long simply supported span with a uniform downward load of 10 kN/m and an upward point load at mid-span of 62.5kN?

Doug Jenkins

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