emporary wind loads

huangyhg

temporary wind loads
are there any approved national (u.s) standards for determining wind loads on temporary structures?  say, for example, that during renovation of an existing two-story building, the second floor diaphragm will be removed and replaced.  while the second floor is absent, the existing exterior walls must either span from the ground to the roof, or be temporarily braced.  it would be economically advantageous if bracing could be eliminated or designed for a reduced loading.  but what wind loads should be considered?  it seems logical that the full 50-year return period wind load could be reduced if the construction period will only be a few weeks.  but i have not found authoritative guidance for this condition.  i have seen some design guides for tilt-up and masonry construction that give some recommendations, but they are not what i would consider an approved national standard.

this is the type of thing that your professional liabliity insurance always warns you about-  stay away from construction means and methods.  tell the contractor to hire an se to design the bracing to meet all applicable codes, including osha requirements.
if you are designing this for the contractor-which should only be done for a separate fee, under a separate contract than that for the building design- do not, under any circumstances, try to save money.  this is not the place to look for and economical design or to play against the odds of a 50-year wind.  no matter what reference you may find, no matter what logic you may apply, if the wall fails despite being braced per your design, you will be held accountable for reducing the wind loads.
osha section 1926.706(5)(b) states, for the case of masonry but it has similar provisions throughout,that a wall "shall be adequately braced to prevent overturning and collapse unless the wall is supported to prevent overturning or collapse.  the bracings shall remain in place until permanent supporting elements of the structure are in place."
that's it.  the wall must be braced to prevent overturning and collapse.  if it fails, then it violates the osha requirements.
be conservative.
one clarification:  osha does not apply directly to design professionals.  however,if you are designing this for a contractor, he will include in your contract that it must meet osha requirements, thereby pulling you into osha's net...  if the contract does not state the osha requirement explicitly, in court the contractor will argue that you knew what the design was intended for, and therefore should have complied with all osha requirements.  this is a reasonable arguement that will stand up in court.
in australia we would generally design for 80% of the long term design wind velocity.  this is in accordance with a provision in our wind loading code for buildings 'of a temporary nature and which are to be used for less than 6 months'.  (we are warned that this does not allow for tornadoes etc).
there is a reasonable argument that goes like this:
wind loads are all based on probabilities anyway, and we rarely design for the highest credible wind velocity.
the common acceptance of a 50 year wind as the basis of long term wind load (for a structure with a design life of 50 years or more) is based on acceptance of a certain level of risk.
it is reasonable to accept a similar level of risk for short duration critical stages of construction, by designing for (say) a 2 to 5 year wind.
designing for the full 50 year wind under conditions which may only apply for two weeks or so would be somewhat like designing the final structure for a 500 year wind (and unreasonably conservative).
if you are in an area where tornadoes are fairly common, then the 5 year wind might well be a minor tornado anyway.
british standard bs 6399 part 2 is probably not much use to you in the us, but just for interest you might like to know that it contains a statistical factor to be applied to short term structures:...
"the seasonal factor ss may be used to reduce the basic wind speed for buildings which are expected to be exposed to the wind for specific subannual periods, in particular for temporary works and buildings during construction.  values which maintain the risk (probability) of being exceeded of q=0.02 in the stated period are given in annex d."...
annex d has a whole table of figures.  for example the one month value of ss varies from 0.98 in january down to 0.62 in july.  clearly this is specifically related to the geographical and meteorological situation in the uk and could not readily be exported.
but the principle given in bs6399 supports your (taro) plea for a rational approach to short term wind loads based on good wind data.
taro,
i am not a lawyer, but...
in the usa, the aisc's "code of standard practice for steel buildings and bridges" calls for the designer to identify on the contract documents the lateral-load-resisting system for the completed structure, and also any special conditions during erection.  (para.7.10)
it looks to me this paragraph put the onus on the designer to call the attention of the contractor to any weakening of the structure during refurbishment.
as a designer, i would make sure that the structure is safe at all times, even if that means providing for temporary bracing while the structural diaphragms are removed.  
aef
on our cd's, we do call attention to provide temporary bracing, but we stop short of saying what it should be, as that falls under means and methods.
the responses regarding design responsibility and liability are well-intentioned, but do not really address the technical aspect of the question.  the primary issue is whether there is an approved national standard in the u.s. for calculating design wind loads on temporary structures.  please disregard the specific original example of a building renovation project.  the temporary structure might be construction scaffolding, a parade grandstand, a big-top circus tent, or any other structure that will only be subjected to wind loads for a short duration.  the references to australian and british standards are helpful, but i would like to find a u.s. equivalent.
taro,
i do not know of any national standard in the usa  that specifies a temporary wind.  
i doubt that a national code would address that subject.   it would have to define "temporary", consider the season of the year, and the type of weather forecast available at the site.
i have seen operation manuals for large equipment, as cranes and launching pads, where operating winds were established using the probabilities that a wind of certain intensity occur while the equipment is in operation.     
different sites may have different criteria.   in hurricane-prone regions, high winds occur only during the hurricane season and could be forecast, while in tornado-prone regions, they may occur with no notice.
sorry this reply does not answer your question categorically.
good luck in your search!
aef
there is a us model code technical reference that is consistent with the liability/osha/safety issues that i posted previously.
boca-1999, section 1609.1.2, states that "adequate temporary bracing shall be provived to resist wind loading on structural components and structural assemblages during the erection and construction phases."  boca section 1609.1 states that "wind loads on the building's main windforce-resisting system shall be determined in accordance with section 1609.7."   there are no provisions for wind load reduction for temporary conditions, and it is explicitly stated that the bracing must resist the wind loads.
taro's question related to a very short-term condition, with a duration of only a few days. in such circumstances, it would be reasonable to forget the code for permanent buildings and look at somethink more transient, such as codes for designing cranes. the typical crane code nominates a design wind speed of 15 or 20 m/sec for a crane in service, and applies the normal factors of safety to this. 20 m/sec corresponds to something like the "once a fortnight" 3 second gust, and produces wind loads of about 1/4 to 1/3rd normal design pressures.
when i have a critical operation, such as lifting a large roof or assembling several components to form a real structure, and i know it will only take a day or so, i use a design wind speed of 20m/sec. this is sufficient to identify the critical items, and to analyse the real cost of extra bracing. if the cost is significant, and the contractor is prepared to accept a few days delay, then that becomes the design.
the next step is then to ensure the wind experienced with the structure in that state is no more than the design will allow. i use the weather forecast for "on water" winds, as this tends to be more consistent. i also note that forecasters quote "mean hourly" wind speeds, and do not forecast gusts with any degree of accuracy. therefore factor the forecast by at least 1.6 for gusts. all this has to be incorporated in erection procedures, and the construction manager has to be brought up to speed on how to get and read a weather forecast.
it has worked for me for the past 15 years, with only the occasional shouting match. when all else fails, stand the project manager in the back of a pick-up, and drive it at 40 mph, and say "do you expect your crew to work in that!"

russell keays