First - please don't call me "Sir phamENG" - I'm flattered, but there hasn't been a knight in my family for over 300 years.
You still haven't clarified the use of this stair. Is it leading from an exit route used for life safety planning? If so, then your "normal live load" needs to be 100psf. If not, then you can use something lower, like 40psf or 50psf.
Based on the clarification statement issued by OSHA that you posted above, the 5x sounds like a safety factor. It specifically says it is for ultimate design and should not be used with AISC's ASD specification (the letter is a bit dated). I would still use the AISC design equations, but be sure to ignore Ω and Φ. Instead, compare your 5xNormal Live Load directly to the result of Mn, Vn, etc. It's very important that you understand the codes, how and why they are written the way they are, and what all of the adjustments, factors, etc. mean. If you don't, get with a senior engineer to go over them and help you through this. Swapping out the wrong thing can mess everything up really easily.
The 1000 lbs is applied everywhere, but not all at once. You need to put it in the middle of the tread (not spread over the tread) to check bending in the tread. You need to put it on one stringer, in once place at a time, and develop and enveloped bending moment diagram and shear diagram (and torsion, depending on your design). But you don't have to put them all over the stair all at once. It's to test individual components and ensure their strength and resiliency (uniform loads work well for the overall structure, but don't always capture the little stuff sufficiently).
The guard exemption in ASCE 7 doesn't get you out of the OSHA requirements. ASCE 7 has the 50plf requirement AND the 200# requirement, whereas OSHA only has the 200# point load requirement.
Where is this project located? Looks like you're in the Philippines. I know you guys use several American building codes, but do you all use our OSHA standards, too? Or are you designing a US project?
Hope this helps.
