Reinforcing Deep Beams
3

Question

Reinforcing Deep Beams

Ginger (Civil)

(OP)

7 Dec 00 18:12

Having read some papers on the design of deep beams (span/depth ratio < 2.0), the design guidance given is that the beams act as strut/tie structures.  A compressive strut is formed from the load on the top of the beam to the supports at each end, and there is a tension zone running along the bottom of the beam acting as a tie.
If reinforcing the beam using rebar, it is advised that the main bars are placed near the bottom of the beam in the tension zone  AND additional horizontal rebar is placed in the web of the beam to prevent the struts bursting and to prevent shear cracks running top to bottom of the beam by intercepting the cracks with this additional rebar.

I am considering using steel fibre reinforcement in place of conventional rebar to give me a reinforcing presence throughout the entire beam depth and this may prevent the bursting and shear problems occurring.  has anyone attemptd this?

Regards

Andy Machon
Andy@machona.freeserve.co.uk

Qshake (Structural) · Answer 1 · 2000-12-07 19:35:53

No, I haven't attempted this but have read a good deal on deep beams. I suggest that you look up Kong as a reference on the behavior of deep beams. Theory of elasticity also will give you some idea of the stress distribution. Most of the reinforcement in the bottom of such a beam is of no consequence and more focus is usually necessary at the edges (ends) where reinforcement lacks proper embedment. Most of the beam is reinforced like a wall.

Personally, I'm not a fan of fibre reinforced concrete but would welcome more dialogue on this issue.

Ron (Structural) · Answer 2 · 2000-12-08 04:59:19

Andy...be careful with the the fiber premise. Yes, it does enhance the properties of the concrete, but it is discontinuous, so its direct tension capability is limited, moreso than continuous rebar. It does provide a higher modulus of rupture for flexural considerations and it does provide a good stress distribution within the matrix. Another "feature" is that the fracture mode occurs at much higher stresses, and though touted to be a "ductility enhancer" the potential for a more brittle (i.e. catastrophic) failure is there. I am refering to steel fibers. I would not consider polypropylene fibers for an application such as this.

JAE (Structural) · Answer 3 · 2000-12-08 10:09:48

Qshake: I would be interested in getting feedback from others on fiber reinforced concrete - specifically slabs-on-grade. Perhaps a new thread?

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Reinforcing Deep Beams
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