RC Design question
RC Design question
(OP)
I was thinking, The Strength design is governed by the servicibility and the material strength. Then Why do we allow/design beams to crack at the tension fiber? (because if you can check, when we design using USD method the stress at the tension fiber exceeds the Modulus of Rupture of concrete).
Since the tension fiber will crack even if it has not reached the design moment, then in terms of serviciblity the beam has already failed? because it would be an eyesore for the occupants (if there is no ceiling finish to cover it). Please enlighten me....
Since the tension fiber will crack even if it has not reached the design moment, then in terms of serviciblity the beam has already failed? because it would be an eyesore for the occupants (if there is no ceiling finish to cover it). Please enlighten me....






RE: RC Design question
RE: RC Design question
RE: RC Design question
Cracks in R.C. are unavoidable due to low concrete tensile strength and the way that reinforcement is calculated.
RE: RC Design question
And when we design with the notion that the concrete will crack, the cracks will be very small, until the steel yields. Current design practice limits strain in the reinforcement to 0.003 in./in. at ultimate loads, which are approximately 40% above service. This translates to a crack width at maximum service loads of 0.026", or less than 1/32 inch per foot.
That said, grade 40 or 60 steel reinforcement sized for strength will typically restrain crack widths very well at service loads. Grade 75/80 also provides good crack control. Flexural members reinforced with grades above will begin to have excessive crack widths, primarily because of the very low reinforcement ratios required to reach strength. Crack width is a function of the stress in a given area of steel crossing a crack, i.e., dL=P*L/(As*E). In a well-developed steel reinforcing bar the "L" length is short, and even shorter in welded wire mesh.