Diagonal Tension and Direct Shear of Prestressed Elements

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EQUATION:
(ppwfps/f'dc) + (dfdy/dpf'dc)(pw-p'w)
< /= 0.36K1
(150)
where ppw, pw, p'w are the reinforcement ratios for flanged sections
computed as for pp, p and p', respectively, except that b shall be the
width of the web and the reinforcement area will be that required to develop
the compressive strength of the web only.
(2) Diagonal Tension and Direct Shear of Prestressed Elements.
Under conventional service loads, prestressed elements remain almost
entirely in compression, and hence are permitted a higher concrete shear
stress than non-prestressed elements.  However, at ultimate loads the effect
of prestress is lost and thus no increase in shear capacity is permitted.
The shear capacity of a precast beam may be calculated using the equations
of paragraphs 3.d and 3.e of Section 3.  The loss of the effect of prestress
also means that d is the actual distance to the prestressing tendon and is
not limited to 0.8h as it is in the ACI code.  It is obvious then that at
the supports of an element with draped tendons, d and thus the shear
capacity are greatly reduced.  Draped tendons also make it difficult to
properly anchor shear reinforcement at the supports, exactly where it is
needed most.  Thus, it is recommended that only straight tendons be used for
blast design.
e.  Dynamic Analysis.  The dynamic analysis of precast elements uses the
procedures described in Chapters 5 and 6 of NAVFAC P-397 and Section 1 of
this manual.
(1) Since precast elements are simply supported, the
resistance-deflection curve is a one-step function (see Figure 34).  The
Table 5. As precast structures are subject to low blast pressures, the dead
load of the structures become significant, and must be taken into account.
(2) The elastic stiffness of simply supported beams with various
loading conditions is given in Table 7.  In determining the stiffness, the
effect of cracking is taken into account by using an average moment of
inertia, Ia, as given by Equation (76):
Ia = (Ig + Ic)/2
For non-prestressed elements, the cracked moment of inertia can be
determined from Section 5-8 of NAVFAC P-397.  For prestressed elements the
moment of inertia of the cracked section may be approximated by:
EQUATION:
Ic = nApsd2p[1-(pp)1/2]
(151)
where n is the ratio of the modulus of elasticity of steel to concrete.
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