Nouman Mukhtar

I have only done one Class 4/ Slender section (it was an angle) design. My line of work has more standard sections. Class 4/ Slender Sections fail locally before yield moment can be obtained thus greatly reducing section capacity. If your line of work has a lot of Slender Sections, I would advise you to get a book and use the examples in the book to understand the design of these sections. These sections are common in pre-engineered building industry and if you deal with them on daily basis, get a reference book. That would help you.
Thanks.

What is the material of the section, its strength grade?
Decrease depth of web and increase the thickness, section like W250x73 will pass on this loading.

Strength would't effect much as I explained above. Also your member web is Class 4 or Slender Sections (I believe thats what they call them in US). I would never do a non-compact section design on a software. Use software for analysis only and design it manually. 
Thanks.

SAP2000 is calculating the moment resistance of beam considering all design limit states like lateral torsional buckling, maximum unrestrained length etc. What you are referencing is Mr= Mf/Sx which is not the only governing parameter. Strength of beam depends upon its unbraced length.
Thanks.
SAP2K Output.pdf

I am working on a project where we are looking to build a 7 bay shed with 30m Span each bay and length of 100m. Now the feedback I got from some chinese engineers was that the standard design in china is either 24m or 27m or 30m, with columns at every 9 meter. So I am looking for some help in choosing between either of the 3 spans. I will be running 2 20 Tons capacity double girder cranes in each bay.  In the future we will double the length of each bay so does that also play a part in span selection ?.
Also the steel grades we can easily source are either SS400, a36, or s355 or q345. Does the grade of the material help in weight reduction and if so how much percentage, so we can compare.
Few details I have regarding the crane is 
if 24m span, the total weight of the crane is 29 tons (including all components)
if 27m span the total weight of the crane is 33 tons
if 30m span the total weight of the crane is 38 tons
There will be lot more inputs you may need from my side so you can post here and I will try to answer quickly.
The lightest shed design and model will be awarded with a cash reward and a job offer based in Karachi. We are not a designing firm, we are the end user of this shed.
 

Welcome aboard Nouman!
Thanks.

Hi @ll,
I am Hiep from Vietnam . I have been working in C&S structural engineer for over 05 years . Now I am working for Belgium consultance company , so I usually use Eurocode and British standard . Hope we would learn together a lot in this forum .
Kindest regards,