Most codes I'm aware of exclude the requirement for concurrency.

Dik

For the design of the slab, I do not consider the guardrail post reactions simultaneously with the uniform floor load. Logically, it makes sense that you should. I mean, how can you have a load on the handrail or guardrail without the people (i.e. live load) to produce the floor load. I guess I consider the guardrail/handrail loads to be applied locally as opposed to a uniform live load that is applied globally. Also consider that the guardrail/handrail loads usually occur at the perimeter of an area or slab where the moment from the uniform load is usually the smallest.

In the AASHTO Bridge Design spec. I cannot find an exclusion of concurrent pedestrian live load and reactions due to the loads on the railing. So, there appears to be at least one design spec. that requires application of dead load, live load, and railing loads simultaneously. As Motorcity points out, it makes sense that you can't have a load on the railing without a load on the floor adjacent to the railing.

Upon closer reading, the bridge spec. also requires the 200lb point load in any direction concurrent with 50lb/ft uniform loads BOTH HORIZONTAL AND VERTICAL for the railing and the posts. Granted, you could throw a load 10 times higher at a bridge deck cantilever and it would pass easily, because we must also consider the case where the sidewalk is removed and traffic rail is on the cantilever, subject to truck wheel loads and impact loads.

This guardrail is at the free cantilever edge of a thin precast concrete balcony. The free edge is 3.5" and increases at 1% toward the building. This edge projects 3.5' beyond the outer support with an 8' back span to the inner support. The design is per 2012 IBC and live loads are 60psf.
In the logic camp for concurrent load is the argument that you need the people to produce the handrail load.
In the logic camp for non-concurrent load is the argument that the gaurd rail load is from a person falling against the rail and is it not likely that that this rairly occuring insident would occur when you have the deck loaed to 60psf.

Having seen many news articles and reports about balcony and handrail failures in the last decade I am firmly in the applied concurrent camp.

If I was to rationalize another way if the balcony is loaded to the full live load then the standing room between people is about nil and you'd have the edge pedestrians pressed against the railings.

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering