Eave deflection calculation for post frame construction

TroyD · Feb 12, 2019

I was reviewing the 'Simplified Lateral Design of Post-Frame Buildings' guide (2012) prepared by Don Bender, Ph.D.

Available here:

http://wwwcdn.constructionmagnet.com/wp-content/uploads/1112-FN-RT-final.pdf

I have attached a small portion of the guide regarding the eave deflection calculation. I have several questions on what values are typically used for the variables in the deflection equation. Is there a typical value used for Ga, apparent shear wall stiffness (k/in)? The NFBA manual Chapter 7 provides shear wall test assembly results for various cladding products. NFBA Table 7-2 provides Effective In-Plane Stiffness, c, values ranging from 7,400 - 19,000 lb/in, and Effective Shear Modulus, G, values ranging from 5,500 - 14,000 lb/in. Is there a correlation between these engineering properties C, G, and Ga? From my online research I found computed values for Ga ranging from 10,000 to 11,500 lb/in.

Lastly, am I interpreting the value of x (the distance from chord splice to nearest support) correctly? In the third term of the deflection equation, this value 'x' must be multiplied by the number of chord splices. Would that be the number of splices along the length of the building? (The deflection due to chord slip is not significant, but I want to make sure I'm understanding the calculation correctly).

Any input is appreciated.

KootK · Feb 12, 2019

I'll leave the shear stiffness stuff for a better man as I'm unable to speak authoritatively on that. The various values do sound like the same property however.

OP said:
Lastly, am I interpreting the value of x (the distance from chord splice to nearest support) correctly

This I can help with although it will be a mouthful. That term is is doing this:

1) Evaluating the curvature discontinuity caused by each splice.

2) Adding up the effect that each curvature discontinuity has on max deflection, accounting for the fact that such discontinuities have a greater affect the closer they are to mid-span (longer "x" values over which the resultant slope is amplified.

So you're doing a summation of a bunch of terms, one for each splice. And, at each term, you're multiplying the term by the distance from that splice to the nearest point of lateral resistance (shear wall etc).

See? Mouthful.

wannabeSE · Feb 12, 2019

Hard to tell what Ga to use when we don’t know the diaphragm material or detailing. If it is a wood diaphragm similar to conventional wood framing, you might want to use the values in the the American Wood Council’s Special Design Provisions for Wind and Seismic

https://www.awc.org/pdf/codes-standards/publications/sdpws/AWC-SDPWS2015-ViewOnly-1508.pdf

dauwerda · Feb 14, 2019

My interpretation is that the property "Ga", apparent shear wall stiffness, given in the equation in question is there to take into account the deflection at the top of the shear wall that is receiving the load from the diaphragm. You would need to use whatever stiffness corresponds to your actual shear wall. Table 7.2 of the NFBA manual presents engineering information on different shear wall construction methods typically found in post-frame construction (light gauge metal cladding over wood framing), including stiffness of tested shear wall panels. To use the stiffness, "c" given in the table in the equation above, first the test panel would need to correlate to your actual shear wall construction and then the stiffness value would need to be adjusted to match the actual aspect ratio (height) of your shear wall. Section 7.3.4 of NFBA shows the process for doing this and Section 7.7 has an example problem.

You are probably aware, but NFBA also provides a program (dafi)for determining eave deflections that is covered in the design manual as well, it can be downloaded here:

http://www.nfba.org/resources/technical

They also have a few recorded (and live) webinars that go through post frame design, including diaphragm design and stiffness determination:

http://www.nfba.org/index.php/resources/education/online-university

TroyD · Feb 14, 2019

Thanks for the good responses. I am familiar with the AWC SDPWS design guide. I'm using typical 29 gauge metal cladding for wall and roof skin. The stiffness values for plywood/OSB are significantly larger, and that makes sense, given the material thickness, etc. I understand that the value for 'Ga' is dependent upon a number of variables on a specific building (height, steel gauge & rib spacing, fastener spacing, column and girt lumber grade, etc). I am signed up to attend a NFBA webinar training event next week that will discuss diaphragm design using DAFI software. I plan to attend more in the near future. I installed DAFI a while ago but have not explored it yet. I'll go thru the example problem you mentioned in Section 7.
Thanks.

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Eave deflection calculation for post frame construction

TroyD

Structural

KootK

Structural

wannabeSE

Civil/Environmental

dauwerda

Structural

TroyD

Structural

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