Braced Cuts (Pecks Approximation) and Multi-Layered

I am studying for the Civil PE an I cam accross a question that says to use Peck's Approximation (P=1.1 * Tribrary Area)  to solve the load in each strut for a Braced Cut ( 3-Strut System). I know how to solve this using the hinge method (Sum Moments/Sum Forces at the bottom hinge point), but Pecks method seems faster and I can't find a good source to use/read up on it.  Also, does any one have a source for solving a braced cut problem for a multi-soil layered cut?

Thanks. Happy Studying!

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