Failure of Bending moment and shear force

[Alfalah]

Hello every one,
As we know that failure of bending moment in beam act on 90 degree and failure of shear force in beam act on 45 degree so what is mechanistic behind this theory? any threads will be much appreciated.

Sabawoon

((S.M.N))

 

[KootK]

In locations of high shear and low moment, the crack that forms is essentially a diagonal tension crack due to the inherent weakness of concrete that way. Hence the 45 degree diagonal-ness.

In locations of high moment and low shear, it's just a straight forward flexural crack as you'd expect to see in any brittle material under axial stress. Transverse to the longitudinal axis.

Most real world cracks are at locations where there is flexure, shear, and possibly axial load. These cracks are hybrid animals exhibiting some features of both shear and flexural cracks.

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.

 

[rb1957]

bending creates normal stresses, shear creates shear stresses ... combine them according to your favourite failure theory, and simmer gently until ready.

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

 

[StrEng007]

The reason why you see these different types of cracks goes back to your mechanics of materials fundamentals. When a beam is loaded (let's call it a simple span beam), the highest moment in the beam will be at the mid span while the highest shear in the beam will be at the supports. Recall that the location of maximum moment is equivalent to zero shear in this beam.

Mohr's circle indicates that each finite element of the beams cross section is going to experience a certain magnitude and direction of shear and normal stresses, which will vary with the location of this element along the beam. Since the tiny elements at the mid span of the team are experiencing only bending, that equates to normal(tensile)stresses on the elements at the underside of the beam. This causes the beam to crack in a single vertical plane, due to the concrete's lack of tensile capacity.

As you transition away from the center of the beam, the combination of bending and shear will induce both normal and shear stresses on those elements. Using Mohr's circle, those principle stresses are resolved to a maximum stress and direction which is seen roughly as the 45 degree angle we're accustomed to.

 

[Alfalah]

Thank you so much to all of you for your useful threats.

((S.M.N))

 

[msquared48]

That cracks me up.

Mike McCann, PE, SE (WA)