That was the problem. The flexibility of the fastener for double shear is, as you wrote, based on a single-spring assumption (per Fig. 4 of Huth paper). In my spreadsheet, I model double shear with two springs and the flexibility I was using per Eqn. 1 of the Huth paper requires a factor of two...
Regarding the Huth Fastener Flexibility equation (1) in the ASTM STP 927 paper, has anyone been able to reproduce the results shown in Fig. 15? Specifically, the top plot labelled 'A' which applies to the double shear specimen. I'm using Eqn. 1 for doubler shear (n = 2) with the specimen...
