Wind Load on a Building with a Soft Story 1

[Ron247]

I know I always see "soft stories" mentioned as related to seismic activity but wouldn't the failure mechanism also be an issue with wind? I know the seismic forces probably control in a high earthquake area like California, but what about similar older construction practices in coastal areas where wind is the villain and not seismic?

I was just curious if others see a reason to review an older building closer when you see that soft story concept.

 

[molibden]

Same problem with wind and seismic. Both are dynamic loads, but for simple stuff we use static loads for design. Soft story is an issue in both cases.

 

[KootK]

I believe that the soft story issue is much less of a concern for wind than it is for seismic. In seismic, soft story is primarily a concern because it concentrates inelastic, damage inducing strains in certain parts of the building, normally in the joints just above and just below the soft story. This isn't an issue for wind because we do not design buildings to drift into the inelastic range in wind design. Sure, there are other things to consider with a soft story besides seismic:

1) Possible lack of redundancy in the soft story lateral system.

2) Possibly a greater propensity for P-Delta moment magnification in the soft story lateral system.

3) Scour and and whale carcasses drifting into your lateral system in coastal applicatoins.

 

[Ron247]

Thanks KootK and molibden for responding. I run into this at times with older houses which most likely were not "designed" by an engineer. They were built under a 40 to 50 year old code. The most common problem I run into is a "daylight" basement home that has lateral soil pressure on one side only and a double car garage door one of the shear walls. 80% I look at are racked to some degree but not collapsed.

I was just curious if anyone has run into one that appeared racked due to wind.

 

[HTURKAK]

Soft story is vertical irregularity and an issue for seismic design. The concept of strong column - weak beam will be violated and “weak column-strong beam” design can result in the undesirable story mechanism, which also known as a soft story.
The picture depicts storey mechanism vs sway mechanism.

 

[human909]

I believe that the soft story issue is much less of a concern for wind than it is for seismic. In seismic, soft story is primarily a concern because it concentrates inelastic, damage inducing strains in certain parts of the building, normally in the joints just above and just below the soft story. This isn't an issue for wind because we do not design buildings to drift into the inelastic range in wind design. Sure, there are other things to consider with a soft story besides seismic:

Exactly my thoughts. But this is hardly my area of expertise so I figure I'd wait for better informed comments.

 

[kostast88]

I have designed steel coastal projects (right on and into the sea) and the soft story mechanism was not a concern. As KootK pointed out you will design for full elastic behavior under wind loads. Soft storey mechanism under earthquakes is the development of plastic hinges = inelastic behavior , so out of the question for wind loads.

Make sure to increase your wind a bit (say 10%) than normal to account for gusts if you are right next to the sea. Check metocean data beyond codes, every location has it's own unique micro climate. And allow for some corrosion on steel elements. You should be fine with these.

Bigger beam sections than columns are not a problem in non-seismic areas (for low rises).