existing concrete beam - shear problem
existing concrete beam - shear problem
(OP)
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?
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?






RE: existing concrete beam - shear problem
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.
RE: existing concrete beam - shear problem
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.
RE: existing concrete beam - shear problem
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?
RE: existing concrete beam - shear problem
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.
RE: existing concrete beam - shear problem
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.
RE: existing concrete beam - shear problem
RE: existing concrete beam - shear problem
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.
RE: existing concrete beam - shear problem
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
RE: existing concrete beam - shear problem
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.
RE: existing concrete beam - shear problem
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!
RE: existing concrete beam - shear problem
Dik