huangyhg

beam with kink
aimmee,
if your beam is carrying relatively small loads, as a stair does and has a relatively short span say less than 8-10m and is a relatively unimportant part of the structure, then i would say that you could design as if it were straight. you must allow for the additional wself weight due to the slope or, more conservatively, use the actual length as the "span".
if the beam is a significant part of the structure or a significant proportion of the structure, i would recommend using at least a simple frame analysis progam to investigate its behaviour under load.
don't forget that your kinked beam will have a tendency to act as an arch, and will introduce horizontal reactions at the ends (unless you provide some sort of expansion bearing at one end, or the supports have sufficient flexibility to move slightly and eliminate the thrust reactions)
be careful about how you detail your reinforcing in the kinked beam as well.
its difficult to describe - but i'll try - suppose you have a beam that is sloping from the left - upward to the right to a peak in the middle. and the right half of the span is sloping downward to the right. the bottom reinforcing should not be placed simply along the bottom of the beam and kinked over the peak to follow the two slopes. what would happen is that, as the bottom bars are put in tension, they would try to straighten out - think about a cable laying on the floor in a kinked arrangement and you pull...the cable will try to straighten. with your rebar like this, the bottom bar would spall out of the beam....stirrups would help, though.
the bars should be left straight and developed over the kink and past it. assume a crack at the midspan kink and develop past this crack for two sets of bars coming up each slope.

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