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# Discrepancy in formula for ring doubler in shear web?

## Discrepancy in formula for ring doubler in shear web?

(OP)

A colleague is working on a presentation that our company is putting together for basic stress analysis topics for our customer.

We are going through the presentation that was originally created by a previous engineer (who is no longer with the company) and discovered some discrepancies between different references.

For a ring doubler in a shear web (Nui only does this for non-buckling webs), there is a slight difference in methods, and seemingly in results, between Nui (1st Ed. Section 6). and Flabel.

The total stress in the ring doubler is calculated by Nui as bending stress plus tension stress:
fb = 2q(0.25R)R1/[tW^2/6] Where R is the hole, and R1 is the distance to the center of the doubler. W and t are the thickness and width of the doubler.
ft = 2qR/Wt <-- this is basic P/A
While Flabel goes about it slightly differently, the fb is the same. However, his ft is double what Nui has, namely:
ft = 4qR/Wt

I’m working through to see if there’s much in the way of how this is derived that causes the seeming discrepancy, or if there’s an actual error (I’m thinking the error is probably in my understanding, these books and procedures have been around for decades!) or typo in the text.

Anyone familiar enough with this to speak to the seeming discrepancy?

TIA.

### RE: Discrepancy in formula for ring doubler in shear web?

To which Niu book are you referring?

LBK

### RE: Discrepancy in formula for ring doubler in shear web?

(OP)
I always forget that there are more than one, but we were actually referencing both Airframe Structural Design (1st edition) and Stress Analysis for Airframe Structures (2nd edition). I eventually just sat down and drew all the freebodies, and it turns out that the way Nui and Flabel use P is slightly different. When the freebody cut is taken at one end of the curved beam analysis, the load is divided by 2, which solves the difference in the equations.

So I followed my own advice that I give to young engineers: Go back to the basics. :)

Cheers.

### RE: Discrepancy in formula for ring doubler in shear web?

I have to confess that I have my Flabel book at home, and my Niu book at work, so every time I noticed your question I only had 1/2 of what I needed to answer you.

STF

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