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

 

[BAretired]

Would really need to see the layout, but could you treat the single gable wall as a sign and treat all exposed trusses as partially shielded members?

Doesn't sound like the columns are fixed, but they may be restrained enough to provide the required wind resistance.

Why not put in a few cable braces for good measure until the new construction is completed?

BA

 

[RHTPE]

BA: Thanks for commenting. The space above the flat roof is not exposed to direct wind forces. So looking at the gable end from the north, you see plastic cover truss from wood deck up to rafters. Looking at the gable end from the South, you see a masonry gable end wall from floor to ridge.

My first goal is to determine if it can sustain the loads as-is per my client's wishes.

From there we determine what additional 'suspenders' are appropriate.

See the attached sketch. The trusses are fully sheathed and shingled.

Ralph
Structures Consulting
Northeast USA

 

[msquared48]

Ig uess I would have to ask the question, if the columns are not fixed, at least to some extent, how is the building standing up in a wind?

Temporary cables sounds like a real good idea.

Mike McCann
MMC Engineering

 

[haynewp]

OK, I think you have a closed gable roof with only one full height endwall with no openings. You are treating it as open in the direction perpendicular to the ridge but by ASCE 7-05 (I don't have 2010) all the walls are not at least 80% open? I think the building qualifies as partially enclosed.

1. For the parallel to ridge direction, I would use the MWFRS for partially enclosed (or enclosed since the internal coefficient cancel out) for the enclosed gable roof above the low roof level plus wind on the single endwall below low roof level treating it as a sign. Add the wind on all exposed columns as well.

2. Same as #1 but treat the wall below the roof level as a partially enclosed component and not MWFRS so the coefficients don't cancel out. Add wind on all exposed columns.

I guess by Code if the building qualifies as partially enclosed you could treat the single wall as MWFRS but that would be less conservative than #2 above by treating it as a component.

Have you checked bearing on the 3" floor slab from the column horizontal reaction and passive pressure against whatever ground is abutting the crawl space?

 

[RFreund]

I would agree with haynewp and the use of partially enclosed wind loads. However, I don't think the internal would cancel as there is only (1) gable wall (maybe I am miss-understanding here). Also I would feel better if I could put some sort of calc/analysis together to verify the concrete/column interaction. Possibly try to treat it as an embedded steel pipe or something along those lines.

EIT

http://www.HowToEngineer.com

 

[haynewp]

The internal coefficients cancel for the gable roof surfaces if it is enclosed (as I understand it).

My item #2 suggested treating the wall as components and cladding. But I agree, you could use MWFRS and just not cancel the internal for that 1 wall.

 

[RHTPE]

haynewp: The triangular space formed by the gable roof is enclosed - the underside by the original flat roof, one end by an existing masonry wall to grade, and the other end by plastic sheeting between the flat roof and the gable roof.

The problem with this project is the lack of anticipation of the (wind) exposure conditions that arise between the existing condition before work started and the various phases that will occur during reconstruction. Unfortunately I get to assist one subcontractor (demo) who didn't read the specs before taking the job (go figure). This wing stood for almost 30 years exposed to some pretty intense winds from thunderstorms with no major issues. If there were an Exposure A, this would fit.

The forces involved at the bottoms of the columns, i.e. compression against the slab edge and shear in the anchor bolts is acceptable.

It's the typical 'dance' between what should have been known at bid time and included in the contract and what has been found in the existing building that was not shown in the contract documents. Combined with the Engineer of Record's attempt to shed as much liability as possible relating to undiscovered issues. I'm trying to accommodate my client's needs without compromising my responsibility to keep it safe.

Ralph
Structures Consulting
Northeast USA

 

[TDIengineer]

If I am understanding this correctly, it would appear your Gabled end becomes more like a parapet. If the flat part of the structure is solid beyond the gable, then I could see some negative pressure from that surface and from the inside face of the gable.

I would be conservative with that element and the loading.

 

[BAretired]

A sketch is worth a thousand words.

BA

 

[RHTPE]

BA: How about a photo? This one is looking South - the masonry gable end wall is the far end of the building you see in this picture.

Ralph
Structures Consulting
Northeast USA

 

[RHTPE]

BA: P5300027 shows the gable end masonry wall at the left. The interior walls will be completely demolished.


Ralph
Structures Consulting
Northeast USA

 

[RHTPE]

BA: This pic shows the opposite end. The perpendicular walls at the far end will be demolished. The gable end at the far end of this picture is sheeted with plastic from flat roof decking up to the gable roof.

Ralph
Structures Consulting
Northeast USA

 

[BAretired]

Yes, the photos are very good.

I would be concerned about the connection between the typical beam and column. Are there stiffeners in the beam over each column? If not, the columns are not continuous through the beams and there seems to be nothing much to prevent the beams from racking. Very poor detail.



BA

 

[MikeE55]

Am I right in stating that if you check the definition of a partially enclosed building (IBC definition), then an open structure with a wall on one end does not qualify as partially enclosed? This doesn't seem to fit the definition of enclosed, partially enclosed, or open.

 

[haynewp]

Is that coming from IBC 2012? I don't see a definition in 2006 which is all I have right now.

It looks like the building is somewhat old, I would be surprised if the existing roof members and connections work for uplift as partially enclosed.

 

[Ron]

Geez those are some spindly columns!
I would look at this as partially enclosed and open, taking the worst case of the two.

 

[BAretired]

Ron, the columns may look spindly, but if there are no stiffeners in the beams over the columns, the structure may be close to a mechanism after removal of all walls parallel to the ridge.

BA

 

[RHTPE]

The original flat-roofed wing was built in the early 50s. There were no real lateral force resisting masonry walls in the building. Almost all of the interior partitions were of 4" CMU reaching to just above ceiling height and none were in line with the 3-1/2" pipe columns in either direction. The original exterior walls were CMU reaching to the bottoms of the windows. See the photos in the linked document.

For what it's worth, the tops of the columns have a sizable cap plate with a 4-bolt connection. Not a full-moment connection by any stretch but still pretty stiff. We have gotten past the "who should have known what" stage between owner, design team, GC & sub and will be working towards a realistic solution soon.

Ralph
Structures Consulting
Northeast USA

 

[BAretired]

The cap plate and 4 bolt connection may well be adequate, but unless there are stiffeners, a short length of beam web does not offer much stiffness.

BA