This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The width (b') of the flange is usually considered to be equal to the width between adjacent beams, or that it extends from the middle of one panel to the middle of the next. The chief danger in such an assumption lies in the fact that if the beams are very far apart, they must have corresponding strength to carry such a floor load, and the shearing stresses between the rib and the slab will be very great. The method of calculating such shear will be given later. It sometimes happens (as illustrated in Article 296), that the width of slab on each side of the rib is almost indefinite. In such a case we must arbitrarily assume some limit, and say that the compression in the slab which is due to the T-beam is confined to a strip which is (say) fifteen or twenty times the thickness of the slab. If the compression is computed for two cases, both of which have the same size of rib, same steel, same thickness of slab, but different slab widths, it is found, as might be expected, that for the narrower slab width the unit-compression is greater, the neutral axis is very slightly lower, and even the unit-tension in the steel is slightly greater. No demonstration has ever been made to determine any limitation of width of slab beyond which no compression would be developed by the transverse stress in a T-beam rib under it. It is probably safe to assume that it extends for seven to ten times the thickness of the slab on each side of the rib. If the beam as a whole is safe on this basis, then it is still safer for any additional width to which the compression may extend.