Structural Concrete - Hassoun

914 Chapter 22 Prestressed Concrete Bridge Design
where
Δf pT = total losses (ksi)
Δf pF = losses due to friction (ksi)
Δf pA = losses due to anchorage set (ksi)
Δf pES = sum of all losses or gains due to elastic shortening or extension at time of application of
prestress and/or external loads (ksi)
Δf pLT = losses due to long-term shrinkage and creep of concrete and relaxation of the steel (ksi)
22.8.2 Instantaneous Losses (AASHTO 5.9.5.2)
22.8.2.1 Anchorage Set (AASHTO 5.9.5.2.1). Anchorage set loss is caused by the movement
of the tendon prior to seating of the wedges or the anchorage gripping device. The magnitude
of the anchorage set depends on the prestressing system used and shall be the greater of that
required to control the stress in the prestressing steel at transfer or that recommended by the
manufacturer of the anchorage. A common value for anchor set is 0.375 in. and can be as low
as 0.0625 in.
22.8.2.2 Friction (AASHTO 5.9.5.2.2). Losses due to hold-down devices for draping or harping
tendons in pretensioned members should be considered.
Elastic Shortening (AASHTO 5.9.5.2.3). The loss due to elastic shortening in pretensioned members
shall be taken as
Δf pES = E p
f
E cgp (22.36)
ct
where
f cgp = concrete stress at center of gravity of prestressing tendons due to prestressing force
immediately after transfer and self-weight of member at section of maximum moment (ksi)
E p = modulus of elasticity of prestressing steel (ksi)
E ct = modulus of elasticity of concrete at transfer or time of load application (ksi)
22.8.3 Approximate Estimate of Time-Dependent Losses (AASHTO 5.9.5.3)
The long-term prestress loss due to creep and shrinkage of concrete and relaxation of steel may be
estimated using
Δf pLT = 10.0 f piA ps
γ
A h γ st + 12.0γ h γ st + Δf pR (22.37)
g
in which
γ h = 1.7 − 0.01H (22.38)
γ st =
5
1 + f ′ ci
(22.39)