Wind Loading - Almost totally open walled gable roof 1

[RHTPE]

I need to call on the collective wisdom of this group regarding a building undergoing renovation.

I have been asked by a contractor to evaluate a partially demolished structure for stability. What is to remain are all of the steel pipe columns, the original roof (essentially flat, 3" thick wood decking) + a subsequently installed gable roof, 12/5 pitch, wood trusses, 4 bearing points at each truss line with plywood sheathing AND a masonry gable end wall with a double door opening AT ONE END ONLY. There are steel beams supporting the wood decking running perpendicular to the ridge, and none running parallel. There are no shear walls of any sort. The columns bear on piers or foundation wall pilasters 14" below the ground level slab. The ground level slab (about 3" thick) is supported by bar joists over a crawl space. This slab was cast against and in full contact with all of the steel pipe columns supporting the roof. The plan dimensions are 63' X 113'. The ridge is 113' long. Building height is 27' at the ridge, 14' at the eaves.

For wind perpendicular to the ridge, I would analyze the wind forces as an open pitched roof (ASCE 7-10 Figure 27.4-5).

For wind parallel with the ridge I have a quandary due to the gable end wall at only one end of the structure. This is obviously not a closed building. Wind blowing against the building from the open gable end will spill around the other closed end and will generate outward force from inside and a suction force on the outside. Wind blowing against this gable end will generate forces towards the interior as well as some degree of suction on the inside face of the gable end wall. I am not totally convinced that examining it as an enclosed building in this case is completely correct.

I am modeling the columns as fixed cantilevers from the plane of the floor slab and not as pinned, simply because the floor slab was cast against an around the pipe columns. This seems to be giving the EoR some heartburn. I feel it's a valid approach. By modeling it this way I find the structure to be stable and see no need for supplemental lateral bracing, at least in the direction perpendicular to the ridge.

Perhaps you all could chime in with some advice & opinions. Please, let's not get into a debate of the merits of this method of remodeling - it is what it is and there's no changing it at this point in time.

My questions:
How would you analyze the wind forces parallel with the ridge?
Do you agree that assume the bases of the columns as fixed is valid?


Ralph
Structures Consulting
Northeast USA

 

[dhengr]

RHTPE:
A plan, with some dimensions, sizes, notes critical to the design, etc. might have been helpful, since we can’t see it from here, although the pictures are helpful too. Let’s see if I have this right, you have some 14' high cantilevered pipe columns, size, number, spacing, connections, etc., all unknown. These are supported at the base by a 3" conc. slab, with unknown bot. forming material, on bar joists, and joist arrangement and how the fl. slab is really supported laterally is also unknown. Doesn’t the whole lateral load on the bldg. have to go through the floor system before it gets to the foundation? Below this lateral slab support is a 12-14" long pipe column back-span which is supported on top of piers, some in walls for improved stability in the plane of the found. wall, but not so much stability perpendicular to the wall. And these piers and pilasters are still cantilevered several feet above the actual ground level, in a crawl space. What are the found. details which give the piers and walls lateral stability? What is the column base reaction at the conc. slab and at the top of the piers? Wouldn’t the reaction at the top of the piers be pretty high, with the small back-span? What are the gravity loads on the pipe columns? Isn’t this a temporary stability issue during this construction, and then something of a permanent stability design problem in the new design?

The bldg. may never see the code prescribed wind loads, so it may never blow over, but it sure looks to me like a bunch of toothpicks standing on end waiting to be pushed over. I agree with Ron, pick the worst wind load and go with it for this temp. work. I also agree with BA that the beam/column connection looks kinda shaky, without serious study. Your salvation may be that the part of the bldg. you are working on is attached to other bldgs., but I’d still study the two unsupported wall lines of the bldg., and probably put some cable x-bracing in for the time being. It’s not a matter of fighting about who’s fault it is at this stage. It is a matter of making it safe as you continue to weaken it, by more demolition, and keeping it from rotating about its two lines of attachment/support.

 

[MikeE55]

Haynewp, The definition of partially enclosed I was referring to is in the ASCE 7, chapter 6, wind loads. I mistakenly said the IBC definition - the IBC just refers to ASCE 7. Sorry about that.

 

[haynewp]

You're right, it doesn't fit "open" or "partially enclosed". If you consider it a building, then anything that is not "open" or "partially enclosed" is considered "enclosed" by definition, which is ridiculous in this case. It doesn't seem to fit in ASCE 7 very well, I would look at it several ways including as "open", "partially enclosed", and "other structure" wind.



BUILDING, ENCLOSED: A building that does not comply
with the requirements for open or partially enclosed buildings.