Structural Design..

The flexural strength of an I beam is Z*F_y where Z is the plastic section modulus and F_y is the yield strength of the material. Z is listed in the steel handbook for various sections, but is approximately A_f(d-t_f)/2 + A_w(d-2*t_f)/4 where A_f and A_w are the area of the flange and web respectively, d is the overall depth and t_f is the flange thickness. From this relationship, it is easily seen that the flange has much more influence on bending strength than the web.

The shear strength is 0.55t_wdFy, so the web carries all of the shear and the flange carries none.

The dimensions of an I beam vary to afford more choice to designers in various applications.

thanks for your answer.. your information is very much helpful to me. i want to design a I beam. where can i refer such a formula like what you mentioned. can you provide any links to view the formula's and step by step procedure to design a I beam safely.. thanks in advance

Your geographic location will determine which code you use. They should all be similar, but there will be some variations. Coming from Canada, I use CAN CSA S16-01. We use Limit States Design (LSD). Years ago, we used Allowable Strength Design (ASD) but it is no longer an accepted design method in Canada.

The US permits both types of design, but they refer to LSD as LRFD (Load and Resistance Factor Design)

The following provide several links to steel beam design. Here are some links to LRFD. The following links compare ASD to LRFD.

If you have never designed in structural steel before, it is dangerous to attempt it on your own as there are many factors which must be taken into account. For example, if the beam is laterally braced, it will carry significantly more load than a beam which is unbraced. You cannot expect to take a crash course in steel design on a forum such as this and proceed to design steel structures.