Capacity Design

[GalileoG]

I am sure you are all thinking by now..."Dang, not another thread by Clansman" but I got to ask this one because it has been bugging me for a while now.

Say I have a one storey building with concentrically braced frames on each end. The frame is subjected to either wind or earthquake. Now, I am curious, when designing the SFRS of the building, how would you determine that the member selection is governed by wind or seismic capacity design prior to carrying out the design? It almost seems like you have to check the design for both. I suppose I'll rephrase the question, is it possible for me to determine from the onset of the design process whether seismic capacity design or wind will be governing my SFRS selection without having to run the design for both?

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[JAE]

Dang, not another thread by Clansman.



You have to check both through with the applicable load combinations. The brace might be controlled by wind for the brace itself, but the connections might be controlled by seismic.

Unless of course you live in LA or something. But even then, a wide, light building might have more wind force on a brace.

Sometimes you can tell from the general loads but with the special detailing requirements, overstrength factors, etc., you should check it all through.

 

[asixth]

For a one-storey steel framed building, I would think that wind would govern.

For strength, the ultimate design of the concentrically braced frame will be a function of which loading (wind or earthquake) has the highest loading.

If you are performing a simplified static seismic analysis, which I think is all this design warrants, the seismic force applied should be 10% of the weight.

As for wind, the combined windward and leeward pressure coefficients should be 1.2 (0.7 windward and 0.5 leeward), multiplied by the wind pressure.

Whichever gives you the greater force should govern the design.

 

[Ron]

I agree with JAE...I've tried to use experience and "gut feel" on many an occasion to predict my analysis approach, but end up checking multiple ways and have found that I was only right part of the time in my prediction.

 

[mudflaps]

Clansman,
Time to write a spreadsheet to determine base shear for wind and seismic. It's pretty simple to write and Oh so very useful. Just use the biggest base shear for all your calc's.

Old CA SE

 

[MiketheEngineer]

Check both - I have had some dseigns that were controlled by wind for the second loor - whereas siesmic controlled the first fllor -- course each floor had a different R for the walls. No easy way around it...

 

[JAE]

mudflaps,
The idea of getting base shear for wind and seismic, comparing the two, eetermining the max. and discarding the other isn't correct.

Each load combination of the code must be checked along with the overstrength factors, detailing requirements (which I'm sure you know well being from CA). In cases where wind and seismic are close in values, there are all sorts of conditions where one controls here, the other controls there.

It's not an one-or-the-other choice. You have to look at both throughout the design.

 

[mudflaps]

Well - - OK, maybe I over simplified it a little.

My spreadsheets also, depending on geometry and mass, indicate a wind vs seismic cross over from time to time. Most of my work is wood, masonry and steel schools and custom houses. (Boy I'd sure like to have another school job right now.) With wood the combination factors don't come into play if you set things up right and can get your architect to eliminate all the "B*** S***" design. With masonry walls seismic almost always governs so the worst case factors are already in our spreadsheet modules. Steel, well, just put a 2.8 factor on all the drag connections and don't take short term increases and you're done. Course you COULD check every load combination at every location but you'd kill the engineering budget.

Most of my stuff goes thru the California DSA who just accepted a wind analysis system that looks a lot like the old 87 UBC wind analysis. So much for the new code being "Right". It's just the one we're stuck with right now. At least until the practicing engineers start volunteering for the code committees again.

 

[JAE]

mudflaps - that makes sense.

I just keep seeing a lot of posts here where folks get the idea that once you determine the main wind force and the seismic demand that you then pick the larger and forever ignore the smaller.