A. John Bologna, P.E., of Coastal Engineering, in Orleans, Mass., responds: It’s not that simple, because the two materials have widely different mechanical properties. Simply stated, the stiffer member will always carry the greater percentage of the load. Given both the high modular value of steel over wood and the engineering effort required to analyze and detail a properly connected composite section, I typically wouldn’t attribute any value to the wood section and would count on the steel alone to do the work.
To determine the share of load carried by each member, you have to take the relative stiffness of each into account. “Stiffness” is defined as the ratio of a given material’s properties in the form of EI/L, where “E” is the elastic modulus, “I” is the member’s moment of inertia, and “L” is the span length. To carry a load in unison, sufficient...
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