huangyhg

reinforced opening in concrete wall
i am designing the reinforcement for a six (6) foot diameter opening in a 17" thick submerged concrete wall. i have based prior designs on the simple rule of replacing each cut rebar with two (2) 45-degree diagonal bars about the opening. using this premise in this case , i've got twelve (12) diagonals in each quadrant around the opening, which appears excessive! the opening crosses the midspan of the wall where higher bending moments have been observed. any suggestions?
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model your wall with the opening in fem. this will give you indication on if really you need your stated amount of reinforcement.
the replacement rule is somewhat empirical and as you say for a large opening will give you a lot of diagonal bars. you have to ask yourself whether all that cut rebar was working at full capacity or was it there for distribution. with a wall you can get a range of load paths. normally the diagonal bars around holes are to prevent cracks at points of stress concentration. for loads in the plane of the wall you could look at a possible load path made up of struts and ties which finds its way round the holes. for loads perpendicular to the wall you could look at load strips. then when you are satisfied that the rebar is adequate, you can put a couple of diagonal bars around the hole to prevent cracking.
carl bauer
i'd use whatever +ve moment reinforcing is required for the top of the openingl (i would treat it as a fixed end beam with a span equal to the size of the opening unless there's another opening in close proximity) and use the same steel for the sides of the hole, the bottom of the hole, and the same for the 45 degree bars at the top right and left of the hole. the bottom 45 degree bars at the bottom left and right are optional and you might want to add a couple of smaller bars. i would usually extend the bars past the point of crossing a distance equal to the tension development length.
if you have a lot of holes, then you might want to take a closer look and maybe save some reinforcing.

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