Actual Photos of Shear Diagonal Crack & Design Strategy

[p8psk]

This thread is about the standard shear diagonal cracks and not study of one occurring at midspan so let me post this separately to avoid confusion.

Please post all the shear diagonal cracks you have ever seen.

Why does these occur less frequency than flexurable cracks?

Now about design strategy.

Supposed your factored sheared (1.2 DL + 16 LL) is 20 kips (or about 90 kN). Should you find concrete sizes that will have Vc that completely enclose it.. that is.. finding concrete dimension with computed Vc of say 25 kips or 100 kN? Or do you make the concrete sizes smaller such that for example Vc is only half or 10 kips (only 45 kN) and the rest taken up by stirrups?

If this is the case, then when your load approach half of the factored load.. there should be diagonal cracks already where the stirrups will finally be engaged.. or do you design it such a way that you don't expect any diagonal cracks?

Please share your design strategy (and pictures if you have seen some).

Thank you.

 

[Ron]

Please post all the shear diagonal cracks you have ever seen.

Not practicable. I've seen hundreds.

Why does these occur less frequency than flexurable cracks?

In my experience that has not been the case. I've seen more diagonal shear cracks than flexural cracks. Just the support conditions will show this to be the case. If a beam is under-reinforced for diagonal shear that will likely be on both ends of the beam, whereas flexure issues generally occur at or near the center of the beam, given reasonable construction placement of the rebar.

 

[p8psk]

In my experience that has not been the case. I've seen more diagonal shear cracks than flexural cracks. Just the support conditions will show this to be the case. If a beam is under-reinforced for diagonal shear that will likely be on both ends of the beam, whereas flexure issues generally occur at or near the center of the beam, given reasonable construction placement of the rebar.

Here's a mystery. The tensile stess in bending or modulus of rupture of 4000 psi concrete is about 475 psi. This is very small stress compared to the available strength of the materials. Here's the weird part. In most buildings and malls I saw (including my house). I haven't seen any flexural crack even at midspan. Does it mean the loading is so low it doesn't even reach the tensile stress in bending? Or maybe those buildings without midspan flexural cracks have overdesigned beam such that even at service load (in a mall), there is just no flexural crack (not even hairline crack)? What do you or others think about this odd behavior?

 

[Ron]

The modulus of rupture as you have noted for 4000 psi concrete is about 475 psi; however, that depends on the aggregate size and other mix design factors. It is measured by a 3 point load test on a short beam. It is for UNREINFORCED concrete. The tensile stress in a reinforced concrete beam is taken by the rebar. The concrete contributes only a very small amount of the tensile strength of the beam in bending.

What you have described is not "odd" behavior. It is normal behavior for reinforced concrete.

 

[p8psk]

The modulus of rupture as you have noted for 4000 psi concrete is about 475 psi; however, that depends on the aggregate size and other mix design factors. It is measured by a 3 point load test on a short beam. It is for UNREINFORCED concrete. The tensile stress in a reinforced concrete beam is taken by the rebar. The concrete contributes only a very small amount of the tensile strength of the beam in bending.

What you have described is not "odd" behavior. It is normal behavior for reinforced concrete.

I know it's taken by rebar. I wonder what strain/stress values before the concrete cracking occurs.. even so.. it is still at low loading... even halfway from ultimate load. I mean, at low load, the stress/strain of rebar and concrete is proportional.. at low loading, the concrete should crack and only rebar take over... however in malls I don't see any flexural cracks even if the load above are parking lots.. any idea why..

 

[Ron]

any idea why..

Yes. The actual loading has not reached the design load.

 

[p8psk]

Yes. The actual loading has not reached the design load.

Is this the normal where you must anticipate the actual loading to be less than the design load?

I have a friend working in a structural company. They seldom see cracks. So whenever they see cracks. They immediately demand retrofit saying crack shouldn't occur even hairline... the cracks is in the picture:

The retrofit is:

I don't know if their retrofit even works. Have you seen similar technique. But based on the crack width.. is it not normal flexural cracks? They seldom see any flexural cracks that when they do. They panic.

 

[p8psk]

Yes. The actual loading has not reached the design load.

To continue with the above. In the book "Design of Concrete Structures" by Dolan & Co, an example used in page 79 is thus...

For 60,000 psi longitudinal bars

in the uncracked tension load, the stress in the rebar is 2870 psi corresponding to modulus of rupture of 432 psi (out of the 475 psi modulus of rupture)

in the cracked tension load, the stress in the rebar is 22,300 psi corresponding to modulus of rupture of 864 psi (out of the 475 psi modulus of rupture)

at the nominal moment capacity, the stress in the rebar is 60,000

Since most beams don't have tension cracked.. does it mean at service load, you are using only the tension bars stresses to be at the 3,500 psi range (instead of even half of 60,000 psi)? But this doesn't equate to any normal service loading.

What is the recommended rebar stress level at service load?

Notice that in reinforced and unreinforced concrete.. they crack similarly at 475 psi modulus of rupture.. that is.. even if there are rebars in the reinforced concrete taking the rest of stresses.. just the same, you should see the crack in both.. no?

 

[hokie66]

The crack shown in your photos does look like a typical flexural crack, but accentuated by the stress riser created by all that rubbish on the beam soffit. A classic case for the necessity to clean the forms before concrete placement. I don't know if their retrofit works either. May help some. But that steelwork is certainly not fire rated.

 

[BAretired]

I have a friend working in a structural company. They seldom see cracks. So whenever they see cracks. They immediately demand retrofit saying crack shouldn't occur even hairline... the cracks is in the picture:

If they seldom see cracks, they are not looking very carefully. Hairline cracks are normal behavior and do not require remedial measures unless more load is to be added to the beam.

BA

 

[p8psk]

Thank you