Structural Concrete - Hassoun

928 Chapter 22 Prestressed Concrete Bridge Design
WORKING STRESS DESIGN
Allowable Stress (AASHTO Table 5.9.4.1.2-1, Table 5.9.4.2.1-1, and Table 5.9.4.2.2-1)
1. Temporary tensile stress limit in prestressed concrete before losses, fully prestressed
components
In area with bonded reinforcement sufficient to resist √ the tensile force in the concrete computed
assuming an uncracked section shall be 0.24 f ′ Cl (ksi).
√
√
f ti_a = 0.24 f ci_BT ksi = 0.24 6 = 0.588 ksi
2. Compressive stress limits in prestressed concrete at service limit state after losses, fully prestressed
components
f ci_a = 0.6f ′ = 3.6 ksi
ci_BT
f c1_a = 0.6f ′ = 0.6(8) =4.8
c_BT
f c_deck_a = 0.6f ′ = 0.6(4) =2.4 ksi
c_deck
f c2_a = 0.45f ′ = 0.45(8) =3.6 ksi
c_BT
f c3_a = 0.4f ′ = 0.4(8) =3.2 ksi
c_BT
Temporary compressive sress limit before
losses, fully prestressed components
Final compressive stress limit due to effective
prestress, permanent loads, transient loads
during shipping and handling
Final compressive stress limit due to deck slab,
effective prestress, permanent, transient
loads during shipiing and handling
Final compressive limit stress at service limit
state after losses due to effective prestress
and permanent loads
Final compression − live load + one-half the
sum of effective prestress and permanent
loads
3. Tensile stress limits in prestressed concrete at service limit state after lossess, fully prestressed
components
For components with bonded prestressing tendons that are subjected to not worse than moderate
corrosion conditions shall be
√
f tf_a = 0.19
√f ′ c_BT ksi = 0.19 8 = 0.537 ksi
The minimum value of prestress force F f to ensure that the tension in the bottom fiber of the
girder at midspan does not exceed the allowable stress of f tf (ksi) in the composite section
under final service condition can be expressed as follows:
where
f bg = −F f
A g
− F f e m
S bg
+ M DC1 + M DC2
S gb
+ M DW + 0.8M LL
S bc
≤ f tf_a
e m = 27.59 in.
M DC1 = 2045.6 kip ⋅ ft
Distance from center of gravity of girder to
centroid of pretensioned strands
Moment due to self-weight of girder