Anyone verify the WWPA Spreadsheets?

[chessiebear]

Greetings all-

I have been using the WWPA spreadsheet design tools for a few years. I did a cursory check a long time ago and I trusted the results.

Now I am dealing with a smart alec truss manufacturer causing trouble who claims my 2x6 wall studs are under-designed. So before I open my mouth, I want to double check everything. Problem is I can not duplicate the spreadsheet numbers exactly. The spreadsheets were authored by Forum Engineers, I can't find them, their links in the spreadsheets are dead.

I have a calc page that I wrote based on Bryer beam column example (7.18) that I use too. I can not corroborate the results in the spreadsheet. I am using Excel 2013, 32bit. I can't imagine any compatibility issues with the spreadsheet.

Manually computing the value for Fb under Adjusted (P+w) by multiplying the adjustment factors times Fb in the spreadsheet I get 866 psi compared to 871 psi.

The results of my calculations, my interaction equation results are 1.38 vs 0.94 in the spreadsheet.

Has anyone else tried verifying the calculator? Can anyone look at the examples and comment?

Details: 2x6 Doug Fir No 2, 10' stud height (124.5" bearing elevation), single bottom, double top, 2400 pounds Dead + Snow, 20 psf C&C load for wind load.

Attached are the spreadsheet results and my verification. The attachment link seems to dld the wrong version I tried to post, the file link is correct.

Your file's link is:

http://files.engineering.com/getfil...-6923f36e9b87&file=2x6_20psf_Verification.pdf

Thanks in advance

 

[jayrod12]

I put a quick run on this. I get that with studs at 16" o/c this should work by a long shot. Granted that's using Canadian codes, but it should be pretty close (it usually is).

For 67% as snow and 33% as dead of that 2400lbs I can get up to a 3" eccentricity and it still works.

This does assume that the walls are sheathed.

 

[JAE]

I got similar to what jayrod12 got -

Unity = 0.99 with 1/3 of that 2400 as DL and 2/3 as SL, each with 2.85 inches of eccentricity additive to the 25 psf C&C wind with
studs at 16" o.c. and a total "design" height of stud at 10.375 ft.

My calcs do not consider composite action with any sheathing but does assume the sheathing braces the stud along their plane.

Normally you wouldn't have 1.85" of eccentricity - at about 1" I get a unity = 0.59.

This is using Fb from the tables for DF No. 2 = 900 psi and Fc = 1350 psi (older NDS 2005).



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[chessiebear]

Thanks folks.

The design aids I am referencing from WWPA is here-

http://wwpa.org/TECHGUIDE/DesignSoftware/tabid/859/Default.aspx

I am using 2005 values too. I get it to work no problem too. I did find a couple of flipped CD factors in my worksheet, but I am still trying to reconcile it against a known good example calc. Like I said, I stared with Bryer example years ago and it suited me fine. Any other published examples that anyone can recommend as accurate or good in their approach?

Best regards

Dennis

 

[KootK]

Any chance this is a situation where all of the load is coming to the wall through floor trusses spaced at 24" o/c?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Koot, are you thinking plate failure is the truss guys concern?

 

[KootK]

Not quite. Thinking that, if the axial load comes in at 24" o/c, then the tighter spacing of the studs is irrelevant with respect to that component of the load. But yeah, trying to guess where the truss guy's concern might be coming from.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Even at 24" o/c the studs would work for me under normal eccentricity. Based on the numbers I was getting at least.

 

[chessiebear]

KootK et al-

These are single span monoslope roof trusses (1/4" slope) so they are very much like floor trusses. Loading is 20-25-0-5. The 25 is snow load. And by single span, I mean a single 35' span. Studs and trusses at 24" o.c. The plate double top plate checks out for bearing. There is no load pass-through at the trusses. Is there something fundamental that I am missing?

Truss fabricator has made some significant input errors that I flagged on their shop drawings - wrong thermal factor, designing for unbalanced snow in flat roof. These would be appropriate to the local if it was shingled gabled roof building, but this is an insulated, fully adhered membrane flat roof. These input errors effectively add 40-50% more load to the truss. They pretty refuse to correct their error because they claim that they have consulted with their engineer/software company and based on what they have told the PE, he says they have done it 'right', just like they have been doing it for the last 15 years. And no, they have not submitted stamped and sealed calcs yet. Beginning to sound familiar? I think that we all have dealt with "this" guy before right? We have not approved the submittals yet either, I am not sure I will 'approve' them either.

Anyhow, his response to this is a I'm rubber/you're glue attitude. He claims that my studs are under-designed. So my nature is to double and triple check everything before I say that they are ok as designed/specified. Hence the overly c5ritical reflection on my part.

The anomalies that I have between my calc and the WWPA spreadsheet I have tracked down to are:
[ul]
[li]le gets figured with 1.84lu instead of the equation for le/d>7.[/li]
[li]They seem to reduce the wind load bending stress by a factor of 1.3/2, or 0.65. I can not figure out why this is appropriate. Bending stress is bending stress[/li]
[li]Duration factors used do not seem consisted to load cases. By that I mean that for combined loads cases, the code allows you to use the modification factor for the shortest duration. So for DL+SL, use 1.15 for snow loads. But for DL+SL+WL, use 1.6 for wind is appropriate instead of 1.15 for snow. The WWPA spreadsheet appears to use the lowest CD factor for any combined case instead of the highest. The result is effectual reducing the stud capacity when in reality its higher. [/li]
[/ul]

Can anyone comment on these three anomalies?

Best regards,
Dennis

 

[JAE]

WWPA is wrong on the duration of load factor application.

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[chessiebear]

JAE-

Did you look at the spreadsheet or are you responding to my interpretation? I would feel better if someone corroborated my conclusion.

Any idea on the 1.3/2 factor?

Dennis

 

[JAE]

I was responding to your statement that WWPA's spreadsheet used a lower duration factor rather than the highest one among the individual cases within the combination.

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