Design Guide 11 defines Dg as Ig/Lj, where Lj is the span of the joists on either side of the girder. Note that if the joists on either side of the girder have different spans, Lj should be the average of the two lengths. Also note that the trib width of the girder is one half of the joist span on each side of the girder, for a total of one joist span; therefore Avg Lb = trib width = Lj. So FloorVibe calculates Dg by taking Ig / Average joist span, which is consistent with the req't of the Design Guide.
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