Beam shear 5

[Fatalxception]

Does anyone know why you can neglect the shear in concrete and wood beams that occurs up to a distance "d"(depth of member) from the face of the support but you cannot do this with steel beams?

 

[JAE]

The shear in concrete has a failure mode that involves a critical stress where tensile and shear stresses combine to form a diagonal crack extending from near the edge of support at the bottom of the beam upwards and away from the support. This forms a kind of arching action. Any load located closer than "d" is essentially resisted more by the compressive arching portion of the beam instead of through direct shear. Thus, the design of the shear capacity, and the resulting stirrups required is not directly affected by the closely located load.

In wood, it is similar but more affected by the horizontal shear reaction to the load as it comes near the support.

The above is fairly non-technical - you might want to re-visit some concrete or wood textbooks to get a total grasp of the effect.

 

[vonlueke]

Then it stands to reason, if a steel beam is used in a similar scenario, sitting on top of (bearing down on) a support, and loaded in a similar manner, and having a similar cross section (rectangular), then steel, being relatively far stronger in shear than interlaminar shear strength of wood and shear strength of typical reinforced concrete beams, should resist the support face shear even better, and thereby allow neglecting in the same manner. Perhaps it's not elaborated in some of the steel codes only because you rarely have a steel beam of rectangular cross section sitting on top of supports in a similar scenario without features nor details that may concentrate the shear stresses. Hope this helps.

 

[dlew]

Generally shear does not control the design of steel beams.
To calculate the shear at the end of the span instead of at a distance "d" from the support is easier and does not penalize the design.

For deep plate girders, where the shear may control, AISC allows to use the "tension field action" that follows more closely the actual behavior of the shear stresses near the supports and at the point of application of concentrated loads.

The latest ACI 318 now allows to use, as an alternative, the equivalent method for concrete, the "Strut-and-Tie Model", to design for shear in deep beams, pile caps, and other similar concrete elements.

 

[ishvaaag]

If you see the limit states failures of steel double tee fixed at the ends you see a strut and tie triangle form in the web. This one way or another is embedded in the checks, explicitly sometimes.

Hence one way to look to your statement is that a check can be set at lower levels of solicitation that satisfies the requirements wanted of design.

 

[rapt]

You are talking about 2 different shear failure modes for the different materials. The code approximation allowing a check at d from the support for reinforced concrete is simply there because, under most common design situations, the shear will not be critical between this point and the support face due to the fact that the angle of the compression strut is rising so they let you out of checking for shear in this region in these cases. That does not mean there is no shear there, just that it will not be critical in those cases.

This is explained very well (with diagrams) in ACI Clause R11.1.3.1.

In normal situations, a flexure shear crack will start in the top surface at the critical section for shear and extend diagonally to the support face at the bottom of the member. At points closer to the suport, the angle of the compression strut will rise and the shear capacity will increase faster than the uniform load shear will increase.

So, for uniform loading and correctly aligned supports, it is permissable to design for a critical section at d from the column face and assume that the design between the column face and the critical section requires the same reinforcement as at the critical section.

If heavy loads are applied between the support face and the critical section, then the shear must be checked at the support face (fig R11.1.3.1 (f)), also if there is no diagonal compression into the support. This shear check will be similar to a Deep Beam shear design and failure would be by direct shear rather than flexure shear as would occur at points outside the critical section.

In steel design, I presume you are checking for direct shear all of the time, rather than flexure shear as you ar in concrete design, as a direct shear failure can occur at any cross-section, so you need to check it at the support face and all points between.

British, Eurocode2 and Australian codes all allow for increased shear capacity in the region from 2d to the support face (this assumes a 30degree compression strut rather than the 45degree compression strut implied by the critical section being located at d from the support), but do require a check at the support face for non-uniform loads in the region of the support.