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Determining Steel Area When Beam Dimensions Are Predetermined

The variation of beam sizes for a particular floor system should be minimized, minimizing the variation of beam dimension will produce a floor system with good appearance, reducing beam size will save an appreciable amount of money by simplifying the formwork. as a result, More uniform and attractive appearance of the floor system will be achieved by reducing the variation of beam dimensions. experienced engineers will tend to reduce the dimension variation for beams in the floor system, but if a new engineer designed the same floor system, a major difference would be in beam dimension. an experienced engineer will begin by designing the beam with the largest bending moment, he will try to reduce the size as much as possible, this beam will require a big amount of reinforcement, then the engineer will use this dimension to design beams with smaller bending moment, the reinforcement will be less because the dimension is big.

in some cases, the dimensions of beams are predetermined due to some factors, such as selecting one dimension to simplify the formwork, or to comply with a minimum thickness for controlling of beam deflection or for architectural reasons. if the dimensions of beams are predetermined then we can use three methods to design the beams.

The first method(design tables)
This considered the quickest method in designing beams, tables contain the value for Mu/(Ф*b*d^2) and ρ. this table for tensely reinforced rectangular members, therefore, Ф value should be checked. 

The second method(The use of ρ formula)


The third method(Trial and error method)

in this method value for a is assumed, then we calculate As using the assumed value of a, then we calculate the value of an again and so on until we obtain a good value of a. this can be used to design any section with tensile reinforcing. 


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