initial prestress
Class 3 is partial prestressing
Class 4: fully reinforced
Class 1= 1 MPa, Class 2=fctm(t)
which is limited to 0.6fck(t)
I = moment of
inertia of
transform section
Class 1= 0MPa, Class 2=fctm
limited to 0.45fck- 0.6fck
Examples of Design
for
Serviceability Limit
States (Class 1 & 2)
Similarly, Figure below illustrates the balancing of a uniformly distributed load by means of a
parabolic cable whose upward component wb (N/M).
Again if wb exactly balances applied uniform load w, the beam is not subjected to any transverse
load (e.g. w being the weight of the beam).
At the ends, the vertical component of prestress Fsinθ is transmitted directly into the supports,
while the horizontal component Fcosθ creates a uniform compression along the entire beam.
Balancing of a
uniform load.
Mn is unbalance
moment can be
designed for later by
adding steel rebar
Example
The net downward (unbalanced) load
on the concrete beam is (45 ‐ 35.3) =
9.7 kN/m, and the moment at mid‐span
due to that load is:
Example : For the beam shown in Figure below, determine the prestress force required to balance
a total applied load of 20 kN/m. Determine, also, the stress distribution at midspan.
The uniform lateral load from a parabolic tendon is given by
Example: Determine, also, the stress distribution at midspan.