Shear on simply supported beam

[precast78]

Guys, I am having another engineering discussion with my coworker. Let's say you have a concrete beam simply supported 10 ft long. You have 10 kips factored point load that can be anywhere on the beam. The beam has 12" d distance. What Shear load would you design the beam for? Let's neglect the dead load from the beam.

 

[Lomarandil]

I was just arguing this (with myself) the other day. I came up with 9 kips. Assuming the 10k load falls just outside the critical section, the near reaction is Pb/L, which is the maximum shear in the beam.

(neglecting semantics like AASHTO where you'd need to offset your critical section by dv instead of d, etc.)

 

[rb1957]

9 kip says "imagine the load is applied 10% from the end" ... right ?

why not be conservative at use 10 kip ? the difference seems small ...

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

 

[precast78]

rb1957, what if the beam is 2+ ft deep?

 

[KootK]

- 9 kip for the diagonal tension shear check.

- >9 kip for loads closer to the support. At that point it would become either STM or a combination of shear friction and direct bearing rather than diagonal tension.

That's the theoretical answer that I believe we're debating. In practice, just 10 kip of course.





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.

 

[BAretired]

The critical section for shear in a concrete beam is located a distance 'd' away from the support according to ACI 318. The beam would be designed for a shear varying from 9k at 'd' to 5k at midspan each end. Stirrups required between the support and 'd' would be spaced the same as required at 'd'.

A more interesting question is what is the maximum concentrated load which could be placed between the support and distance 'd' from the support?



BA

 

[Tomfh]

In AS3600 there is a clause that allows you to as much as double the concrete shear capacity provided the concentrated load is close enough to the support. In the commentary it explains this is quite conservative and in reality the concrete is several times stronger again close to supports.

For the original question, I would use 10 kips. Life's too short to go into any more detail than that...