When designing a concrete beam in the ultimate limit state, a free body diagram is drawn where concrete compression forces are represented as a stress block and reinforcement tensile stresses as a point load.
Example picture can be found on this website:
In ULS, we normally assume that all concrete below the neutral axis (in tension) is cracked and not accounted for in the calculation. Only reaction inside this cracked zone comes from the rebar in tension.
But: if the concrete is cracked, can the rebar be considered bonded with the concrete? If not, does it matter in the calculation?
We are normally calculating the strain in the rebar from equation:
e_rebar = e_cu * (d - x) / x
where d is the distance of rebar from concrete top, x the distance to neutral axis and e_cu the ultimate strain of concrete.
Does this calculation work if the rebar is no longer bonded with the concrete? When using the formula, we are assuming that the strain in rebar follows the strain in concrete, but these strains are no longer the same, as cracking implies different strains in the rebar and tensile concrete.
Example picture can be found on this website:
In ULS, we normally assume that all concrete below the neutral axis (in tension) is cracked and not accounted for in the calculation. Only reaction inside this cracked zone comes from the rebar in tension.
But: if the concrete is cracked, can the rebar be considered bonded with the concrete? If not, does it matter in the calculation?
We are normally calculating the strain in the rebar from equation:
e_rebar = e_cu * (d - x) / x
where d is the distance of rebar from concrete top, x the distance to neutral axis and e_cu the ultimate strain of concrete.
Does this calculation work if the rebar is no longer bonded with the concrete? When using the formula, we are assuming that the strain in rebar follows the strain in concrete, but these strains are no longer the same, as cracking implies different strains in the rebar and tensile concrete.
