One way shear

[slickdeals]

I have an interesting question pertaining to one way shear in flat slabs or raft foundations.

I have seen calculations/methodologies that compute the one way shear capacities of a bay and then compare that to the shear in the CS + MS. The idea is that the column strip cannot alone fail in shear without the middle strips failing too.

Let's take this a step further. Assume that you have a situation where there is a tremendous load on a column/middle strip such that the shear exceeds the capacity of the bay. In that case, what width do you assume for the one way shear?

1. Is it the width of the entire building and the total shear across all the strips?
2. Is it bay width + some additional width that you feel comfortable or satisfies the requirement?

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[ash060]

I usually go with the bay width for whatever loads I am analyzing. Although, I have to admit that I have never had a one-way shear problem on a two-way slab, something else always controlled whether it was punching shear, deflection, or moment.

But if one way shear is an issue can you thicken the slab in the area of concern?

 

[DST148]

In general agree with ash060.
Option #1 - No.
Option #2 - Yes. The additional width is debatable.
One crude approach could be.... The effective width for shear in case of one way slab with a concentrated load depends on the location of the load from the supports. One can calculate one-way beam shear for normal loading using the bay width / design width. Then idealize the unusual loading into series of point loads, calculate one way shear using effective width for each of those idealized point loads, and then superimpose with the shear from the normal loading.

 

[JoshPlumSE]

I've alwyas used the shear capacity of the column strip to resist the shear in that strip. I'm not sure if the code ever explicitly tells us to do this. But, it just seems like common sense to me.

Though it is an interesting idea. Perhaps, it would be impossible for a 1-way shear overstress to casuse true failure without somehow propogating into a punching shear failure. Though I have to admit that I feel better checking the two failures independently.

 

[msquared48]

Your latter comment Josh was my first thought too - that the punching shear may control instead in that case.

Maybe we should induce an earthquake to test our hypothesis?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[PicoStruc]

The two-way shear (punching) highly depend of the column size for defining critical section while this is not case with one way shear.

Thus, a separate one way shear check is usually required because you can solve punching problem with shear reinforcement, drop panel, or increasing column size and still have one way shear problem !

But, i agree with you that in usual good design, punching will govern the design.

 

[hokie66]

If one way shear controls because of a big load, it is really a one way system, at least in that area. I would only use the width of the column strip.