Uplift Question RE carport

[PianoGuy]

Hello -- I am new to this excellent site.

Situation: my contractor designed a simple 2-car carport to be attached to the front of our 5/12 pitched existing garage/structure (ES). The carport will have a traditional shingled roof, built on common trusses, and the whole structure will be supported at the ES as well as on four posts sitting on concrete footings. D = 20', W = 20' and 1st post out from the ES is at 10', 2nd post located at 20'.

I have an AST in Mechanical Engineering and my brother has been a working Civil / Structural Engineer for decades. Our independent calculations RE uplift in a 90mph zone agree. But a gal at the county code office has taken issue. We used standard uplift concepts RE psf on the roof etc. and in accord with how the load path would dictate the amount of concrete required at each footing.

The lady at the code office is not using any of this, but instead is insisting that the truss mfg. spec on uplift reactions (460 lbs at each end of the truss connection to the top plate) is the guiding principle. Thus, she has taken the 460, divided this by 2 (for something of an average), multiplied the 230 by 10 (1/2 the D dimension) and comes up with 2.3 kips. The foundation pad then, at this 10' post requires 16.9 cu ft of concrete weighing 2,535 lbs! Our calcs, with a safety value, indicate 1,600 lbs. required.

In addition to this she is saying that the existing 26' long by 1' wide, by 18' deep footing at the ES (which weighs 5,850 lbs) would have to beefed up as well.

So, off the record and with promises that I will treat any input as opinions only, I have a few questions:

1) does this seem reasonable?
2) is the use of truss uplift reaction specs the new way to figure forces required at the foundation?
3) any tips on presenting a case to the code folks? The code lady is saying I need to hire a licensed engineer to present the case.

Thanks for taking the time!

PianoGuy

 

[bylar]

You mentioned how she got her number. How did you get yours?

 

[dik]

Design the anchorage as you see fit and pay the contractor to put in the extra 'beef'...

Dik

 

[msquared48]

Seems like the 460 figure gives a uplift figure of about 19 psf or so. Is this the figure for uplift due to the 90 mph wind that your brother figured?

The net uplift is what needs to be resisted, which also incorporates the weight of the structure into the picture. Depending on what you used, I would expect that to be 7 to 10 psf, giving a net uplift of 9 to 12 psf to resist, if the 19 psf figure is correct.

I believe that the 460 psf figure is a design minimum, of the manufacturer's choosing, to allow the truss to be used in higher wind load areas without being re-designed, and not necessarily a true picture of your application. You could call the manufacturer to verify this and go back to the Building official.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Sorry, in the last paragraph I meand the 460 to be a design maximum, not minimum.

In retrospect here, what the BO seems to be saying is that ec=very component in the structure has to be designed to the maximum allowable load of the members they support. This approach is not required by code and is incorrect and leads to a very over designed structure, which you are apparently seeing.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[PianoGuy]

Thank you Mike for your interest and reply to our situation.

Here is our analysis.

Conservative Allowances with a 1.5 Safety Factor (SF)
-- UL pressure 26 psf used (where factoring w/ Exposure C, 24 is called for).
-- Deduction for Structure Download (DL) factored in at standard 10 psf; thus net value for UL used is 16 psf to be resisted by structure / foundations.
-- Roof area (A) = 20 x 20 = 400; and 400 x 16 = 6400 sqft
-- Resistance to be neutralized at 4 posts (4” x 6”) plus the wall and foundation of existing structure (ES). ES is up to code for wall and foundation footing. Wall is sheetrocked on the inside and sheathed on the outside. Foundation underneath weighs 5850 lbs.
-- SF of 1.5 to be used (could have used SF 1.5 at the outset to raise the 26 psf UL to 39)
-- Assuming that each footing must resist (6400 lbs/6 “posts”)*1.5 SF = 1600 lbs, the footings must weigh at least 1600 lbs each.
-- Concrete weight (per quote from county code) weighs 150 lbs per cuft.

So the footings must weigh at least 1600 lbs. We figure to need 1600/150 = 10.67 cubic feet of footing to get this weight. We can use 2'-8" square by 1'-6" deep foundation for a volume of 2.667^2*1.5 = 10.67 ft^3 which meets the criteria, frost line taken into account.

All other fastening, lateral bracing, Simpson hardware, etc., are not at issue at the country code dept.

It is our understanding that nothing about moment arms or truss UL max reactions need to be considered; and, as you have indicated, truss reactions “aren’t necessarily a true picture” of our application.

Thanks again for your interest!

 

[Ron]

Have you considered wind both parallel and perpendicular to the ridge? Assuming a gabled roof, you will need to consider overturning as your possible greatest uplift condition. It seems like you have considered the parallel condition, but not the perpendicular one.

I agree with Mike that the truss designer has probably given a max. condition, not necessarily your condition.

Good luck.

 

[wannabeSE]

Piano Guy,
The code official has given you two alternatives: hire and engineer or use more concrete than you think is neccessary. With that said, there is an error in your calculations. The load is not evenly distibuted to 6 places. The tributary area at the corner posts is 50 sq ft while it is 100 sq ft at the edge posts. Also, 10 psf dead load may be too heavy. If it is a conservative estimate for downward forces, then it is unconservative when used for uplift.

 

[PianoGuy]

WannebeEIT,

Thanks for your input.

Yes, I am well aware of the two alternatives, which is why I hoped to get some other points of view. Mostly I am interested in the use of max truss reactions as the guiding principle for analyzing UL, hence anchorage.

I called Ballard Truss and talked with a tech; he was as helpful as possible, but couldn't answer the question. He put me on to the design engineering team. I will call them Monday and see where it goes.

It is a good thing that the max truss reactions spec isn't any higher than 460; if it were, say 600, then the code office would be asking for quite a bit more concrete for the same exact structure.

The 10 psf DL as it relates to UL is accepted here at the code office.

I will consider your observation RE the sq. footage at the posts. If I understand correctly, the outside footings where D = 20' from the ES should be heavier than at posts where D = 10' and where D = 0'. If I have missed your point please clarify a bit more.

Thanks again for your interest.

PG

 

[msquared48]

I would also be concerned about the lateral across the front of the carport, wind parallel to the trusses.

Might need Knee braces or a pole footing, or cantilever the roof diaphram of the canopy off the existing residence. Your brother should be aware of this. You are at the limit for cantilevering the roof diaphragm though.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

PianoGuy, Want you need is two truss designs. One designed for components and cladding wind for your truss uplift connection to the beam. The other designed for main frame wind for your beam and foundation uplift connections. You should check the wind criteria on the truss design to check that it meets your addition requirements. If you can scan and upload a truss design I would be happy to review and note what wind conditions it was run under.

Garth Dreger PE
AZ Phoenix area

 

[PianoGuy]

Thank you Mr. Dreger for your interest. Your specific technical insight RE our question is exactly what we had hoped for. Don't know where it will all end up, but we very much appreciate your particular insights and review.

Our drawing and spec sheet copy of the Ballard Truss Inc. common truss (designed to span 20') is not dark enough for clear scanning, and some of the text is unclear in any case. I will call Ballard and have them email an attachment of the drawing and specs to me.

Our plans call for 9 of these common trusses, and for 2 gable end trusses, one of which will attach to the gable end of the existing structure.

Hope to upload some time today.

Thanks again.

Kind Regards,

Nick

 

[PianoGuy]

Hello Mr. Dreger,

Please see PDF attachment for Ballard Truss. I don't know how useful it will be; I spoke with their tech dept. and asked about the two truss designs you suggested might exist (one for components and another for main frame). They refer to only one, that which is attached. Perhaps it will be of some use.

This is my first time uploading a file from my computer. I followed "Step 3 Attachment" by typing in the full address; but it is not clear as to whether that function is only for web links. I hope the PDF comes through.

In any case, thank you for your time and interest!

Kind Regards,

Nick

 

[PianoGuy]

Just noticed that the file won't upload using personal PC protocols. So, I will upload PDF via Engineering.com

Hope you get it.

Nick

 

[woodman88]

PianoGuy, Okay the truss is ran as
1) Code IRC2006, which is okay.
2) The 10 psf dead loads have been reduced to 6 (10psf*.6) psf dead load top and bottom chords, per code.
3) Enclosed building, which should be changed to open as you are adding a carport.
4) MWFRS gable end zone, which will give a higher uplift then the interior zone. You need to have this changed for your interior trusses.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

#3 and #4 change a lot of the results here. They need to be rerun as Woodman says, and the new values for uplift calculated.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[PianoGuy]

My sincerest thanks to all who have given of time, interest, and expertise in guiding me towards the next course of action.

I will be signing off for now.

Kind regards!

Nick

 

[wannabeSE]

While it is counter intuitive and confusing, I believe that ASCE 7-05 considers a building that has no openigs in one wall and is completely open on the three other sides to be classified as an enclosed building. It is not classified as an open building because one wall is not 80% open. It is not classified as partially enclosed because the total open area in one wall is not more than 110% of of all the openings in the building. If it is not open or partially enclosed, by definition it would be classified as an enclosed building. See the commentary for the wind load definitions where it states: "All other buildings are considered to be enclosed by definition, although there may be large openings in two or more walls. An example of this is a parking garage through which wind can pass."

 

[swm25]

It seems to me that the trusses would be designed with a component and cladding wind load, while the uplifts taken down to a foundation would generally be from Main Wind Force Resisting System Loading. These loads are different with the components and cladding being higher to take into account stronger isolated gusts that may occur over a smaller area.

 

[PianoGuy]

Dear swm25:

Yes, so it would seem.

But whether on not wind gusts would or should be resisted by extra-high component strength at the truss / beam connections, the governing concept of resisting uplift forces resides in main wind-frame and foundation analysis, and is independent of "components and cladding" considerations (important as these are).

Until I can be convinced otherwise, I believe that the official at the BO is simply wrong in her analysis and instructions. I and my brother (structural engineer who works for a multi-award-winning structural engineering firm) are working on this problem.

Thanks for your input.

Nick