glazing in non-windbourne debris areas

huangyhg

glazing in non-windbourne debris areas
according to asce 7-98, section 6.5.9.3, it says that glazing in the lower 60 ft.  of buildings shall be impact resistant glazing, or such glazing that receives positive external pressure shall be assumed to be openings.
my question: in an area that is not a wind bourne debris region, do you have to consider non-impact resistant glazing as openings?  since section 6.5.9.3 only specifically requires glazing in wind bourne debris areas to be impact resistant, i imply that for areas that are not wind bourne debris areas the glazing does not have have to be impact resitant to avoid being considered an opening.   
here in upstate ny (albany area), we have v-90mph, and are not a wind bourne dbris area.
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ahh, the old "opening" debate.
for the most part, i agree with you.  fixed windows in non-wind borne debris areas are not openings.  but if a window is operable, i generally consider it to be an opening.  i consider doors to be openings, using the same logic.  it could be open during a "design wind event."
interestingly, when i see pre-engineered metal building calculations, the designer usually considers the building enclosed, even when it has gigantic overhead doors.  so i guess not all engineers agree on this.
daveatkins
daveatkins,
i too am interested in this subject.  i recently started thread 176-105697 in asce (civil) code issues forum:  asce 7-02 wind loads definition of enclosed structure.  
not much activity there so far, but maybe you'd like to chime in.  my question there involves in part a real building i'm currently working on.  metal building, low rise, 275' x 75' x 28 eave.  has on one end 2 overhead doors 16'x16' plus 1 overhead door 20'x20'.  other openings on other walls also.  this building is an enclosed building, it is, i tell ya!  to get the full story i'd need to get you a 3 page write up with description, a spreadsheet for some calcs & some sketches of the bldg.  probably all off line.  can you/would you be willing to look at that and try to answer my questions?
chichuck
the following is and excerpt from an e-mail i sent on a project that i am complete with its design. the e-mail i sent to the architect regarding glazing in the first 60 feet. please keep in mind that this is local to florida.
asce 7-98 and the florida building code. fbc 1606.1.4 requires that all exterior glazing in the first 60 feet of a building in a "windborne debris region" to be impact resistant, or protected by covering (shutters, plywood) or the building shall be designed considering these glazing are open.

fbc figure 1606, which is the map for the wind born debris and the wind speed map, indicates that up to 120 mph wind speed zone to be debris region. of course the critical requirement here is our project located within 120 mph 3 second gust zone or is it out of it?

it is my opinion that our project is out of the 120 mph zone. i base this on the following:

1. fbc figure 1606 is not clear enough to define the end and start of this region.
2. fbc figure 1606 leaves it up to the local government (county/city) to establish the end of the region.
3. based on a copy of a map that was developed for the fbc by the university of florida, i interpret that our project to be outside of the wind debris region. assuming that my understating of the building location is accurate.

i suggest that the building official give us a written determination of not only the wind speed; but that we are out of the wind debris region.

if this is the case, the fact that we are located out side the wind debris region will cause savings since impact resistant windows are not required.

i have maps for all of the counties in florida. i find them to be of great value when it comes to showing wind zones in more details since they are detailed to the level of streets.
daveatkins, the assumptions i make regarding what is and what is not an opening are similar to yours.  if it is an operable window or a door, i consider it an opening.  if a wall has lots of non-operable glazing , say like aluminum storefront, and is not in a wind bourne debris region, i now consider that not to be openings.
regarding large doors in metal buildings, i came across an interesting thread about the hurricane that devasted the air force base in florida.  it said that the airplane hangars had been designed assuming enclosed buildings, assuming that the doors would be closed during a wind event.  however, the wind blew the doors in, which spiked the internal pressure, causing the hangar to "pop".  in my opinion, the pre-engineered building industry takes their designs right to the edge, and sometimes over it.  i consider those buildings to be on the flimsy side, but that is a topic for another thread someday.  i guess you could also argue that had the doors not failed, maybe the rest of the building wouldn't have.  when you consider the cost of the contents of a building relative to the cost of the building itself, it seems conservatism is warranted.  try telling that to a penny pinching owner or architect though.
chichuck,
if you are comfortable with your calculations, i have no reason to disagree.  the code is a little "goofy" in that buildings with lots of openings can still be "enclosed", as long as the amount of openings on the various sides are balanced (that is what the code formulas check).  for example, when i design mini storage buildings, they generally are enclosed, even though they are almost all doors.  this is because you have equal amounts of door openings on opposite sides.
daveatkins
daveatkins,
i concur with your comment, it does look a little goofy.  but i think there is a reason for it.  i think the point is, if there are a lot of openings in the windward wall, that will allow a lot of air into the bauilding.  if there are not enough openings in the rest of the building, then that pressure cannot be released, so then the code definition of a partially enclosed building applies; and you have to use larger cpi numbers. (cpi = +/1 0.55 for partially enclosed building, vs cpl = +/1 0.18 for enclosed.)  on the other hand, if there are enough openings in the remainder of the building, that air can get out of the buildng, so pressure inside does not build up.  this is what happens in my building, and perhaps is what you are seeing as well.  the pop effect on the hanger described by bjb is an example of a building where there are enough openings on the windward wall and not enough on the other surfaces to relieve the pressure.  that would be a partially enclosed buildng.  please note though, that this is true only for wind blowing toward the open hanger door. for wind in other directions, that hanger is probably an enclosed building.  
like you said, the code is a little 'goofy' here.  but it does accurately reflect (what i think is) the intent of the code.  i myself cant figure out a better way to express it that is more elegant, more precise and less convoluted.
all of this is slightly off the subject raised originally.  on that, my opinion is that even though bjb's buildng is albany ny, it is prudent to consider those windows as openings and determine if it is open, partially enclosed or enclosed.  but the asce standard does not require it.  
chichuck
chichuck, i agree that your proposal would be prudent, but in my opinion, for the area that i practice in it would be too conservative.  if the opening were operable, i definitley would consider them openings.  the glazing in question though is part of a storefront.
before new york adopted the 2000 ibc it had its own "custom" code.  to determine wind pressures for design all you had to do was look in a table and just pick a value based on how tall your building was.  as an example, no matter where you were in the state, if your building was 25 feet or less in height your wind load was 15 psf.  there was no consideration of mwfs versus components and cladding, and no consideration of open versus enclosed versus partially enclosed.  it didn't even matter if your building was located on the ocean shore.  i am not aware of problems with building failing from wind load in new york because of the old unsophisticated wind analysis the code permitted.  this was also when we were allowed to increase allowable stresses by 1/3 when dealing with wind or seismic load.  another point is that i don't reduce my wind moments when they are acting with other transient loads, as is permitted by the asd load combinations of the 2000 ibc.  to sum it up, based on my experience in the locality that most of my buildings are in, and since asce does not require it in a non-wind bourne debris region, i do not consider non-operable glazing to be openings.  with that said, i certainly respect your opinion to the contrary.
bjb,
are you saying that if you consider your fixed windows to be not openings, you have an enclosed building, and if you consider them to be open (say, broken) that you have a partially enclosed building?  is that you you say it is too conservative to consider them as openings?
the reason i ask is that it takes an awful lot of openings (either blown out doors or broken windows) on the windward side to change from enclosed to partially enclosed, and bump the internal presssure coefficient from +/-0.18 to +/- 0.55.  that said, since you mention a storefront, meybe you do have enough openings for that.  but on many (dare i say typical) metal buildings, i really don't think that happens.
regards,
i do have a storefront on one elevation, and the rest of the building is basically masonry.  it's a little more complicated than that because we're also doing alterations and additions to the existing building. i'm just saying that it's my interpretation of asce 7-98 that in an area that is not a wind bourne debris region, non operable glazing such as a storefront does not have to be considered as openings if the glazing is not impact resistant.  because i am not in a wind bourne debris area, i do not consider non operable glazing to be an opening, especially if it is storefront.  if i were to be designing in a wind ourne debris area, i would consider all non-impact resistant glazing in the lower 60 feet of the building as openings, per 6.5.9.3 of asce 7-98.
the code uses the term "glazing" which in my opinion is not a fixed or an operable window!
glazing is 鈥済lass鈥?if you will. glazing can be glass storefront, glass door and or glass window. i would imagine that glass block is glazing as well.
the portion of the code regarding glazing came about after andrew. we learned that if glazing is breached in a structure, the internal factors are totally different than those for an enclosed structure. this will yield higher wind pressures.
this has nothing to do with windows being operable at all. believe me, i went through three hurricanes in my immediate area in the last two months. i rode one of them at home. the last thing i want to do is open the window with horizontal raindrops coming and missiles (other peoples shingles, trees, tree limbs, screen porches, parts of pemb) are flying in the air.
this is my two cents worth.
regards