Lateral analysis - Detailed vs. Simple

[designrider]

Lateral analysis - Detailed vs. Simple

When performing the analysis for a Lateral Force Resisting System, typical practice in my office is to calculate both wind and seismic loading conditions for every resisting element independently, then proceed with the design of each element using the controlling load combination by comparing the calculated wind and seismic loading. Thus some resisting elements may be designed for wind, others for seismic.

However, I am aware that others typically only calculate the base shear for both wind and seismic and then proceed in designing the resisting elements only for the loading conditions generated from the controlling base shear force. Thus all resisting elements are either designed for wind, or all elements are designed for seismic.

I would argue that calculating both wind and seismic forces in detail is a more code compliant design (US codes), but in terms of safety I don’t regard designing for the worst base shear to be an inappropriate choice. Any comments on this?

 

[JAE]

You are correct that you should "proceed with the design of each element using the controlling load combination by comparing the calculated wind and seismic loading."

It is incorrect to compare base shear values and then design for either wind or seismic only.

However, in some cases, the engineer can be able to determine, by sheer observation, when one or the other will primarily control.

The problem is in seismic load combinations, there are many times controlling detailing requirements that would control that are sometimes difficult to just "observe" and know that they won't control the design. Brace connections, collectors, etc. that require magnified seismic forces are one example.

Many engineers, in much of the central US, for example, have not historically bothered with seismic. In the UBC, some areas were in a "zone 0" which avoided seismic altogether.

 

[hokie66]

You have to consider both, but as JAE says, one or the other may obviously dominate. Sometimes seismic controls in certain directions and wind in others. And the loading pattern is different because wind is greater as you go up. With equal base shears, wind will produce higher base moments.

 

[WpgKarl]

Another point to consider - as far as I know if your structure requires seismic as one case for loading in addition to wind, temp, and it doesn't end up governing - (say wind governs all your cases) I believe you still need to detail all the steel as per the seismic requirements (in reinf concrete), even though wind governs the design .

 

[ahmedhegazi]

I think
For members design (not detailing) this method you are speaking about is correct in case you use Equivalent static load method for seismic analysis

Using Equivalent static load for seismic analysis means there is no vertical irregularity.

But if you use response spectrum or time history, base sure comparison is not always correct and it will need some engineering judgment and data from previous projects under same loading conditions.

 

[designrider]

I am very supportive that sound engineering judgment can allow you to recognize that wind or seismic will “obviously dominate.” But in the case that the Lateral force calculations are very close…

I agree with Hokie66 in regards to also comparing “higher base moments,” but would anyone dispute that if you have calculated that seismic has a higher base shear AND moment it will control also all the way down to the level of detailing and thus it is ok to only proceed in designing for seismic?

I recognize this assumption cannot be made if wind is calculated to have a higher base shear OR moment because of JAE comments regarding “magnified seismic forces” for certain detailing.

 

[csd72]

There are always exceptions to the rule.

For example: the roof of this seismically controlled building may be lightweight and therefore not attract much seismic load, but the wind on it will be more significant.

Or a wind controlled building may have a heavily loaded plantroom that is controlled by seismic.

If you are not at a level that you can confortably make these asessment, then you should calculate both cases.

You will probably find that after one or two jobs you will get a feel for it and will no longer have to fully calculate it.

csd