10psf wind load 1

[PostFrameSE]

I asked some brief questions about this issue in the past and several of you responded, which I appreciate. I've been challenged on this another way now and would like your feedback.

Is it possible to make the AVERAGE wind pressure equal to 10psf and meet the requirements of Section 6.1.4.1 of ASCE 7? For example, let's say we have a 10' tall building with a 10' projected roof area. If the wall pressure was designed for 15psf and the projected roof area designed for 5psf, would that be acceptable?? Or does 10psf need to be applied uniformly across the entire vertical projection?

Thanks.

 

[msquared48]

If the areas are the same above and below the plate line, I guess that would work. But that would be a special case.

This is a moot point though, because I believe there is a 10 psf minimum for the projected area of the roof anyway, when E through the end are zero.

Mike McCann
McCann Engineering

 

[akastud]

I do it by applying both the exact wind pressure on the projected area and then comparing my different reactions (at the diaghragm above and below) and if either of them is less than applying 10 PSF on the entire projected area, you use the 10 PSF reaction(s).

akastud

 

[haynewp]

For MWFRS:
"The wind load to be used in the design of the MWFRS for an enclosed or partially enclosed building or other structure shall not be less than 10 lb/ft^2 multiplied by the area of the building or structure projected onto a vertical plane normal to the assumed wind direction."

So splitting up the wind as 5psf on the upper part of the projection and 15psf on the lower does not seem to me to meet the intent. What if you concentrated 100psf in the first 2 feet and applied 0psf over the rest of the frame. Would that seem acceptable?

 

[msquared48]

Additionally, don't forget about the "2a" distance from the ends where the locally higher loads have to be applied. Unless you are conservative with your estimate, I would not average on thinking this thru more. There are too many variables here. Take the more exact approach.

Mike McCann
McCann Engineering

 

[haynewp]

If the more exact approach leads to a net load less than the 10 psf on the total vertical projection then 10 psf still controls.

 

[DaveAtkins]

I agree with haynewp. Check what happens using, for example, 8.5 psf on typical walls, 12.8 psf at corners, and 0 psf on the roof (example only--you must use the correct loads for your situation). Then check 10 psf on the entire wall and roof. See what controls.

DaveAtkins

 

[PostFrameSE]

Thanks to each of you for your replies. Here is my dilema. I do mostly single story buildings with 3.5/12 roof pitches. The 10psf nearly always controls, by a large margin. The problem we're having is trying to figure out how a competitor is seemingly ignoring the 10psf provision, and the prospective client doesn't care. So....giving our competitor the benefit of the doubt, we're trying to determine if we're looking at 6.1.4.1 incorrectly.

Doesn't sound like we are. What recourse do we have? We are going to be priced higher because we're following the codes/standards, and our competitor isn't, and he's going to get the job. I'm not going to bend rules just to get a job, but an unlevel playing field is no fun to play on. That is why I want to make sure I'm thinking about this section of the standard correctly.

Haynewp took my proposal to the extreme, which really helps flush out how best to look at it, and really points to the fact that it probably ought to be evenly distributed from top to bottom.

Thanks again for each of your posts!!!

 

[DaveAtkins]

I see the dilemma, PostFrameSE, but what difference is using 10 psf WL making? Extra nailing at the roof diaphragm or shear walls? Additional shear walls required?

I have done a lot of one story wood structures, and usually "standard" construction (1/2" or 5/8" OSB on the roof, 7/16" OSB on the walls, nails spaced at 6/12) is sufficient.

DaveAtkins

 

[PostFrameSE]

Dave, The one story projects we do oftentimes are 2 and 3 times longer than they are wide, and they're 16' tall and taller. The lateral load that is applied to the heel of my trusses, for post / diaphragm interaction analysis, can double when using the 10psf on the vertical projection compared with calculating the pressures in each zone.

Most often these are steel-clad wood diaphragms. We're talking lots of stitch fasteners and supplemental chord force fasteners to keep the building from ripping apart!

That is why this is such a big deal. Also, there is the need for a lot more shearwalls on the end bays or the need for intermediate shearwalls, if possible.

Thanks.

 

[DaveAtkins]

I see--you are doing a post frame building. I feel your pain--I know that you get large chord forces which necessitate a lot of nails or even lag screws in the purlin to purlin connections. Do you have the Post Frame Building Design Manual? It has a method for distributing the chord force among all the purlins, not just the edge purlins.

DaveAtkins

 

[PostFrameSE]

Sorry, I hadn't checked back after I wrote my last response. I do have the PFDM, but even with distributing the load through all of the purlins, the ones on the outer edge are still requiring many additional fasteners.

Thanks for your response.

 

[Structures33]

I'm not familiar with Post Frame Building Design but it sounds like your problem is a very common one. It's good that you've investigated to see if you're missing something, but sometimes we're not missing anything.
It's really unfortunate but there are many engineers and firms that will allow deliberate disregard for the code requirements. To further agitate the issue, in many cases, the building officials don't care about enforcing code as long as an engineer is taking responsibility. I have had many conversations with building officials about my interpretation of the code, which they have agreed with, but they also said that when an engineer wanted to overwright the code and seal a different requirement, that it is ok. Obviously this isn't for every code requirement but it happens on ones that can be open to interpretation. Whatever you choose to do, feel good about having sound engineering practices - and a conscience.

 

[DaveAtkins]

PostFrameSE,

My only other suggestion would be to consider dividing the building in half, and treating the roof like two separate three-sided diaphragms. This makes the chord forces go away, but now the walls which are perpendicular to the wind load direction must be designed as shear walls.

DaveAtkins

 

[PostFrameSE]

Thanks Structures33 and Dave,

I'm not sure I follow how the chord forces go away Dave. Are saying to cut the building in half so that my roof diaphragm is cantilevered off of my end shearwall???....or are you saying split the building lengthwise along the ridge?? Either way, I still have chord forces to contend with don't I? Maybe I'm not following correctly.

 

[Lion06]

I think the chord forces go away becusee it is now a 3-sided torsion box (well, 2 of them I guess). The diaphragm will rotate as a rigid body instead of acting like a horizontal beam (which has chord forces).

 

[PostFrameSE]

Thanks StructuralEIT,

That makes sense. I'm not sure I want to try and analyze that, but I see where they're going now.

 

[DaveAtkins]

Yes, StructuralEIT is correct--that is what I meant.

Maybe, in the end, this is one of those clients that you just can't work with.

DaveAtkins

 

[KeySol]

PostframeSE-

Maybe your competitor is using teh IRC to design these structures. I know sometimes when i look into an engineered design of a single story building it is very expensive. But when i use the IRC or prescriptive method of construction it seems to just "work out". This could be one option to look at...

 

[DaveAtkins]

But you can't use a residential code to design a building accessible to the public.

DaveAtkins