wind sways/deflection critera, aisc dg#3 & 'pre-eng' metal buildings
wind sways/deflection critera, aisc dg#3 & 'pre-eng' metal buildings
(OP)
AISC Design Guide #3 (DG#3) suggests a 10 year return period wind pressures (~75% less) for checking element
Deflection and building sway/drift. This seems like a effective ‘1/3 allowable deflection’ increase, contrary to overall structural engineering practice.
The ‘AISC Code’ is not prescriptive, the ‘ACI Code’ is a bit more but still lacks definition as far as drift is concerned. It seems like the 'pre-engineered' metal building industry (MBMA) had a lot of input into DG#3.
It points outs that a lot of ‘special detailing’ is required to accommodate deflections/sways, but does not address solutions to critical areas like corners. ‘Special detailing’ for large movements will not done by Architects unless they are made aware that it is required. In addition, I do not think that they would like 1+” masonry control joints at building corners to accommodate sway movements etc by flexible frames.
Also most criteria are significantly less than sway / deflection values mandated by 'Seismic' Codes that accept
'Damage’ to non-structural elements as a criteria vs. 'Wind' Codes that are based on a 'no damage' basis to Elements. A lot of the listed values are also less than the few ‘Code’ mandated requirements that do exist. ‘Codes’ apparently do not allow ’10 year’ pressures to be used.
‘Codes’are mandated, while ‘Commentaries’,‘Recommendations’ and ‘Design Guides’ are not.
ASCE 7-98 'Standard' also now clearly (rather than hinting as a in previous editions) says that allowable stresses should not be increased for ‘dead + wind’ load conditions unless it is based on the materials characteristic (e.g. wood) for taking short term loads, since combinations are based on probabilistic theory of simultaneous loading. Steel, masonry and concrete do not quailfy here.
Why are ‘Codes’ not more prescriptive in wind deflection / sway?
Building owners and the public need to be protected in what they ‘buying’ when they get a building designed to DG#3 vs an ‘Engineered’ building designed to ‘Code’.
It also seems like people who use & reference DG#3 are not 'playing on a level field' with the rest of the structural engineering profession.
Any feedback/thoughts?
Deflection and building sway/drift. This seems like a effective ‘1/3 allowable deflection’ increase, contrary to overall structural engineering practice.
The ‘AISC Code’ is not prescriptive, the ‘ACI Code’ is a bit more but still lacks definition as far as drift is concerned. It seems like the 'pre-engineered' metal building industry (MBMA) had a lot of input into DG#3.
It points outs that a lot of ‘special detailing’ is required to accommodate deflections/sways, but does not address solutions to critical areas like corners. ‘Special detailing’ for large movements will not done by Architects unless they are made aware that it is required. In addition, I do not think that they would like 1+” masonry control joints at building corners to accommodate sway movements etc by flexible frames.
Also most criteria are significantly less than sway / deflection values mandated by 'Seismic' Codes that accept
'Damage’ to non-structural elements as a criteria vs. 'Wind' Codes that are based on a 'no damage' basis to Elements. A lot of the listed values are also less than the few ‘Code’ mandated requirements that do exist. ‘Codes’ apparently do not allow ’10 year’ pressures to be used.
‘Codes’are mandated, while ‘Commentaries’,‘Recommendations’ and ‘Design Guides’ are not.
ASCE 7-98 'Standard' also now clearly (rather than hinting as a in previous editions) says that allowable stresses should not be increased for ‘dead + wind’ load conditions unless it is based on the materials characteristic (e.g. wood) for taking short term loads, since combinations are based on probabilistic theory of simultaneous loading. Steel, masonry and concrete do not quailfy here.
Why are ‘Codes’ not more prescriptive in wind deflection / sway?
Building owners and the public need to be protected in what they ‘buying’ when they get a building designed to DG#3 vs an ‘Engineered’ building designed to ‘Code’.
It also seems like people who use & reference DG#3 are not 'playing on a level field' with the rest of the structural engineering profession.
Any feedback/thoughts?
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RE: wind sways/deflection critera, aisc dg#3 & 'pre-eng' metal buildings
It is my opinion that more than H/400 drift at probabilistic level 50 years windstorm should not be allowed, and that the need of special detailing and corners, ridges etc should be clearly detailed in codes.