Lateral Deflection

[531234]

Most building code requirement for lateral deflection of buildings is to limit the storey drift to H/500. However it is unclear if this limit is based on the worse orthogonal direction or the resultant of the two orthogonal directions (resultant of X and Y deflection). What is you thought on this?

 

[jayrod12]

Wind can't blow full force in each direction. So what components do you use for the resultant?

I can't comment on the applicability of this thought to seismic however as I'm not experienced with any seismic design.

 

[531234]

I am using 0.71 for both X and Y direction (based on wind direction 45 degree on both X and Y axes)

 

[jayrod12]

I guess that kind of works if your building is square. However if the wind were blowing directly on the corner of your building I would expect the total wind pressure to be lower than that due to the smaller projected area. You will also have a deeper diaphragm when looking it.

Seems like a ton of additional factors to work out and account for just for it not to govern in the end.

 

[531234]

Well the building doesn't need to be square. If the wind direction is 45 degree of X and Y axes then the wind component of each axes is the Sine of 45 times the total wind.

 

[KootK]

In examples that I've seen based on ASCE7, drift is always taken to be the maximum relative lateral displacement in any direction divided by the story height. That makes sense to me as the primary intent is to limit P-delta stability issues.Note that the calculated lateral displacement should include torsion effects in addition to direct shear.

Any load case that you evaluate for strength should be evaluated for drift. Conversely, any load case not evaluated for strength need not be evaluated for drift. Let your governing building code be your guide in that regard.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[wannabeSE]

I don't know about codes that limit drift from wind. The IBC and ASCE 7 do not have requirements for wind drift. ASCE 7's commentary provides some recommendations. Engineers limit the building drift to prevent damage to cladding, walls and other non-structural components. ASCE 7 seismic drift limits, but that is a different issue.

Without code requirements, engineering judgment should be used to consider non-orthogonal load cases for drift. If there is something that is sensitive to drift or the building has extreme torsional irregularities, it might be worth looking at the wind load cases in ASCE 7-10 figure 27.4-8. It includes case combining 75% from both direction as well as some torsional load cases.

 

[531234]

Thanks for all your comments.

 

[bookowski]

For wind I have always seen drift calculated for each of the load cases, i.e. including orthogonal and torsion cases etc. As others have mentioned there is generally no code drift limit for wind so it's up to you.

 

[CBEngi]

The National Building Code of Canada requires a engineer to evaluate 75% wind load acting in both directions at once.

 

[531234]

AaronKen, where in the NBCC can I find this 75% requirement? I checked it in NBCC 2010 and can not find it.

 

[PicoStruc]

531234, the 75% both direction is in building code commentary 'Partial Load' Fig. I-16 in NBCC 2005.

BUT BUT BUT in paragraph 36, it is clearly stated that unbalanced load do not apply to low height building designed using fig. I-7. Are those unbalanced load and partial loading the same things... I am honnesly not sure how to interpret this (does this mean case partial load case C & D or partial load cases B,C & D ???)

 

[CBEngi]

@531234 I only have NBCC 2005 at home (PDF) its clause 4.1.7.3C which states that you need to consider the wind loading as I described above, I'm almost 100% its in NBCC 2010 as I was looking it up a couple weeks ago. I do this check mostly when looking at existing structures, as it would never really govern the design of a new LLRS.