existing concrete beam - shear problem 6

[n3jc]

Hi guys. Im having some problems with existing concrete beam. There is gonna be an increasment of the load on a beam so I have to check if existing reinforcement is sufficient.

Existing bending reinforcement (top/bottom) is sufficient but im having trouble with a shear control at both ends of a beam.

I dont know what to do since there is no access to bottom or top of the beam. The only accessible part of the beam is marked yellow - see attached picture.

What are my options?

 

[BeFEA]

Hello n3jc!
Is this an underground foundation beam?
First off, depending on the design code you are referring to, this might not be a "beam". I would call it a "deep beam" or a "wall". Since it is a deep beam, I think you should check it using a strut-and-tie model. The shear force diagram you provided is suitable for beams, but deep beams have a different shear resisting mechanism.

Also:
1) are you sure you are not over-simplifying your problem by assuming the beam as simply supported?
2) try to check the distance between the support and the location in which shear forces are lower than the shear resistance. If the distance is smaller than the effective depth of the beam, you might want to check what your design code says. Normally (at least for continuous beams) you should check the shear force at a distance "d" from support (d= effective depth).

If the deep beam check is not satisfactory, than increasing the width is one of the most effective methods against shear. It will not be easy to ensure proper composite behavior of the existing and new concrete, but I don't see any other option.

 

[MotorCity]

If you are following the ACI code, you are permitted to use the shear force located at a distance "d" from the support, not at the support as you have shown. Not sure if that will help you, but it will at least reduce your design shear force.

Do you know the size and spacing of any stirrups in the beam?

A few options to increase the shear resistance are increasing the beam size (doweling to it) or scabbing on steel members.

 

[n3jc]

thanks for answer and elaboration!

Is this an underground foundation beam?
No, it isnt, its a beam between two floor slabs.

About deep beam - that was the first thing I thought but according to Eurocodes its not since the ratio between span and beam height is bigger than L/d = 3.

Existing stirrups are fi 8/20 cm (8 mm diameter, spacing between them is 20 cm). Shear bearing capacity for this is 250 kN. But max shear force is around 360 kN.

I was thinking about adding a steel plate to the side of a beam but I have no idea how to connect it - calculate it... I also wonder how smart it actully is to add a steel plate only on one side of a beam?

 

[rapt]

Don't forget also that you have an extra shear requirement for the part of the load that is applied at the bottom of the beam. Hanging shear reinforcement is required in addition to normal shear reinforcement for this load in addition to the normal shear reinforcement.

You could drill full depth of the beam and add vertical rods to increase shear capacity. These could be stressed bars. This is often done to strengthen bridges for shear.

 

[jayrod12]

You didn't answer the question whether your shear diagram is face of support to face of support, or centre to centre.

Depending on the configuration at each end of the beam, there could be a case for analyzing the beam shear at d from the support, which may just get you into a passing range.

 

[atrizzy]

X2 on the suggestion that you need to check at a distance from the support. For your deep beam that may well reduce your effective shear span to 4.4m. I suspect this will bring you down to about 215 kN (based on a very rough proportioning.

 

[KootK]

Keep in mind that the "d from support" shear improvement business doesn't apply to load coming in through the low slab. If that's a bearing wall on top of the beam, you're likely in good shape. Heck, the wall itself might arch over some. On the other hand, if the low slab is plaza/traffic, you might still have problems.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[n3jc]

I heard for this 'd from support' shear force reduction, but I dont know why? whats the science behind it? is it because of load path? Im searching for this in EC 2 but cant find it...

You didn't answer the question whether your shear diagram is face of support to face of support, or centre to centre.

Its centre to centre. Supports are 30 cm wide.

tnx for answers guys!

regards

 

[KootK]

Draw a sketch showing the end of the beam and your last potential shear crack on the interior of your bearing support. You'll see that any load applied to the top of the beam will go straight to the bearing via diagonal compression struts rather that utilizing the stirrups or diagonal concrete tension in any way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

n3jc

If you cannot find the "d from the support" rule in Eurocode, you did not look very hard. Took me 2 minutes. Clause 6.2.1(8)!

As for the reason why, I think you need to read a good text book (or your university notes) on concrete shear design. If you do not understand the reasoning, you should not be designing until you do, and it I a basic of concrete design. We are not here to teach you basic concrete design logic!

 

[dik]

Shear is considered as diagonal tension in the concrete and strength is predicated on the tensile capacity of concrete. That's the reason for d from support. As the angle witin the d distance becomes more vertical, the actual shear strength of the concrete increases. For short distances the concrete shear failure strength can exceed 400 psi. Another reason for using d is that if you have a heavy point load within that distance, you should investigate it to do a proper reinforcing job.

Dik