What code are you designing to? I've heard the 300 pound business too but am not aware of a code requirement. I've always thought it to be a stand alone load meant to simulate a person of substance standing in an unfavorable location. That location may well be at the end of a mid span or end tread depending on the load effect being analyzed. I find that these types of stairs are often governed by vibration. The new AISC design guide #11 has some good information.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

In the US, the 100 psf requirement applies.

For a typical 44 inch wide tread having an 11-inch effective depth, the load per tread is 336 lbs.

A 200 pound person stepping downstairs in a slight rush can easily put 300 pounds on their boot print. I'd put it at the worst place and analyze that. Code is one thing, comfort and the perception of strength and safety is another.

Just to clarify,

My interpretation of ASCE 7 for the minimum design of stairs is 100psf OR 300lb point load. They do not need to occur simultaneously. I always check the uniform load and then I check a 300lb point load (4in^2) at the worst location and make sure my design is satisfactory. Hopefully others can confirm.

EIT

IFRS:

I wouldn't apply the 300# concurrent with the 100 psf... You might apply the load at the edge of the tread for max effect.

Dik

As Deadblow indicates, the requirement from ASCE 7-10 Table 4-1 or IBC 2012 Table 1607.1 is to design stairs for the worst effect from a uniform load equal to 100 psf OR a concentrated load equal to 300 pounds. The concentrated load is to be applied to an area equal to 4 square inches. The uniform and concentrated loads need not act concurrently per the footnotes to the respective tables.

Ditto KootK's comment, plus deflections - especially at the end of a cantilevered tread. People are very nervous about floors (stair treads) the vibrate, or flex, more that they think they should. The ideal design is to minimize these to no perceptible effect, if possible.

Thaidavid

I've done a few of these. For the loading, it depends on the use; 100 psf for commercial 40 psf for residential. At a minimum you should analyze the entire system (stringer+treads) for the uniform loading, which will probably control for the stringer design, and the treads for the 300 lb point load, located at the very end of the tread, this load case will control for the treads. Be very wary of deflection, if it is too much the system will be bouncy or shaky and not perform well, and the owner won't be very happy. You have 3 things you need to look at; deflection of the tread, rotation of the tread/stringer connection, and rotation of the stringer. Don't be afraid to use a thick tread (I would recommend 3/4" minimum, possibly thicker depending on the length of the tread) and a thick walled stringer, and you might want to beef up the tread/stringer connections with angles top and bottom to prevent rotation of the joint.

Perhaps 300 lb is your requirement. Know that current OSHA requires 1000 lb. If OSHA must be met on your project, then check out OSHA 1910.

OSHA governs workplace safety, 1910.24 (if that is what you are referring to) is for fixed industrial stairs. The OP has not clarified if this thread is for a commercial, residential, industrial or special use application.

By coincidence, I (actually one of my new engineers) checked some aluminum stairs last week. Turns out there's a big guy (he either weighs 450 lb. or 540 lb.), that is at a site we're working on. And he's pretty nimble, considering his size, so we needed to check some stairs. I thought, "No problem, stairs are designed for 100 psf, these are 4'-0" wide...," easy. But if you put 300 lb. on two bearing bars (four square inches, 2 inch by 2 inch), they barely work. I can't say that a 450 lb. worker taking stairs two at a time (probably not, but maybe) ain't going to happen and it's safe.
My point is that the concentrated load case controls for grating style stairs and don't ignore it. For concrete, I'm sure it's not a problem.
Also, maybe the 300 lb. requirement needs to be changed. We all know guys (and some gals) that push 350 lb. Look at the offensive line for an NFL team. Or a weight watchers meeting. And these people will take the stairs two at a time.

To reiterate on my previous post, it is an either or condition, 300 lb or the required uniform load. The treads should be analyzed for the point load as it will likely control, the stringer should be analyzed for the uniform load, as it will likely control. You should also probably look at some possible combinations of point loads for the stringer design, such as 2 or 3 300 lb point loads on treads that are in close proximity for the stringer design to see if it governs over the uniform load.

I agree with Deadblow's post about 100 psf OR 300-lb point load, whichever governs.

But to further dnlv's point, the same Table 4-1 of ASCE 7-10 permits uniform load of 40 psf for 1-or-2-family dwellings instead of typical 100 psf. The 300-lb point load still applies.

A gray area might be a part of an apartment/condo complex where the staircase leads to just a small portion of an upstairs area that is only utilized (in general) by 1 or 2 families. I would stick with the 100 psf even for this scenario and avoid the need to have that argument later.