The section of a double tee (DT) prestress concrete joist is shown in Figure. The joists are simply supported on a span of 7.5m and are pre-tensioned with total initial force of 1100 kN from low-relaxation strands. The joist supports a total dead load of 2.3 kPa (including beam weight) and live load of 6.2 kPa. There is a loss of prestress of 20% at service loads. Unit weight of concrete is 24 kN/m^3 a. Calculate the stress at the top fibers of the DT at end span due to initial pre-stressing force only. b. Calculate the compressive stress at the bottom fibers of the DT at midspan due to initial pre-stressing force only. c. Calculate the additional load in kPa such that the stress at the bottom fibers of the DT at midspan (due to service loads and prestress force) will be zero.
The section of a double tee (DT) prestress concrete joist is shown in Figure. The joists are simply supported on a span of 7.5m and are pre-tensioned with total initial force of 1100 kN from low-relaxation strands. The joist supports a total dead load of 2.3 kPa (including beam weight) and live load of 6.2 kPa. There is a loss of prestress of 20% at service loads. Unit weight of concrete is 24 kN/m^3
a. Calculate the stress at the top fibers of the DT at end span due to initial pre-stressing force only.
b. Calculate the compressive stress at the bottom fibers of the DT at midspan due to initial pre-stressing force only.
c. Calculate the additional load in kPa such that the stress at the bottom fibers of the DT at midspan (due to service loads and prestress force) will be zero.
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