Reversal effect on cantilever beams

[Tayaban]

Hi everyone,

Does cantilever beams affected by reversal stress caused by earthquakes and how it is affected?

Thanks in advance.

 

[DETstru]

I'm not sure where you're located or what codes you are under but in the US, ASCE 7 requires any horizontal cantilevers in Seismic Design Categories D-F to be designed for a minimum net uplift force of 0.2 times the dead load (along with self weight) of the cantilevered element.

This is the minimum design requirement. If you have a configuration or load path that induces a net uplift greater than the amount stated above you have to design for that.

 

[KootK]

The vertical component of earthquake acceleration creates the upwards demand described by DETstr. The critical results of that are often a need for a tie down connection at the support adjacent to the cantilever and the reconsidering of the lateral torsional bracing scheme under reversed moments.

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]

this'd apply to any structure, not just cantilevers, no?

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

 

[hokie66]

It's not really load on the cantilever that is of concern, as the cantilever is not connected to the ground. Rather, it is the up, then down load on the supporting column that can cause flexural reversal.

 

[wannabeSE]

With load combination including (0.9 - 0.2Sds)D, I don't see how there will be a stress reversal on a cantilevered beam since Sds would need to be greater than 4.5. Unless there are some unusual loading, I would not expect a bending stress reversal (- to + positive moment).

 

[Tayaban]

Thanks everyone,
In addition does the cracks occurs diagonally or in vertical motion for cantilever beams if earthquake occurs?

 

[KootK]

With load combination including (0.9 - 0.2Sds)D, I don't see how there will be a stress reversal on a cantilevered beam since Sds would need to be greater than 4.5.

The uplift requirement is it's own load case based on an assumed upwards acceleration of 1.2g. So it's a net uplift of 0.2g as detStr mentioned. More info here: Link.

In addition does the cracks occurs diagonally or in vertical motion for cantilever beams if earthquake occurs?

Not quite sure what you're asking here. Can you elaborate? In a concrete member, you can have diagonal cracks or vertical cracks depending on the predominance of moment or shear at the section being considered. For a concrete cantilever under 0.2g uplift, it's likely to be a fairly vertical flexural crack near the support I would think.

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.

 

[wannabeSE]

Thanks KootK. Somehow I missed, that section in the code.