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Structural Concrete - Hassoun

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22.9 Deflections (AASHTO 5.7.3.6) 929<br />

M DC2 = 255.628 kip ⋅ ft<br />

M DW = 349.956 kip ⋅ ft<br />

M LL = 2389.7 kip ⋅ ft<br />

Moment due to dead load of wet concrete:slab<br />

+ haunch<br />

Moment due to added dead load after girder<br />

hardens<br />

Moment due to live load + dynamic load<br />

allowance (Service III)<br />

0.537 ksi − (M DC1 + M DC2 )12 in.∕ft<br />

S bg<br />

− (MDW + 0.8MLL )12 in.∕ft<br />

S bc<br />

F f =<br />

−1∕A g − e m ∕S bg<br />

[ ]<br />

0.537 − (2045.6 + 255.6) (12)∕17409 −[350 + 0.8(2389.7)](12)<br />

=<br />

= 896.6 kip⋅ ft<br />

−1∕713 − 27.59∕12229<br />

Stress in strands after all losses. Typically assume ΔfpT = 35.0 ksi, and this value will be<br />

refined.<br />

Δf pT = 35 ksi<br />

f e = f pe − Δf pT = 194.4 − 35 = 159.4 ksi<br />

A ps = F f<br />

= 896.6<br />

f e 159.4 = 5.625 in.2 Total area of strands required<br />

( ) Aps ( ) 5.625<br />

N Ps = ceil<br />

= ceil = 37; use N<br />

A ps_single 0.153<br />

ps = 38<br />

Total number of 0.5 in. diameter stands required. Even number of strands may be used as shown<br />

in Figure 22.15 under strand pattern.<br />

4. Strand Pattern<br />

At Midspan<br />

At End Section<br />

N y_m = 172 in.<br />

N y_e = 682 in.<br />

y m = N y_m<br />

= 172<br />

N ps 38 = 4.526 in. y end = N y_e<br />

N ps<br />

= 682 = 17.947 in.<br />

38<br />

e m = y bg − y m = 32.12 − 4.526 = 27.59 in. e end = y bg − y end = 32.12 − 17.947 = 14.17 in.<br />

5. Prestress Losses<br />

a. Instantaneous Losses, Inst_Loss (AASHTO 5.9.5.2)<br />

Instantaneous losses are losses due to anchorage set, friction, and elastic shortening.<br />

Friction<br />

The only friction loss possible in a pretensioned member is at holddown devices for<br />

draping or harpening tendons. The LRFD specifications specify the consideration of<br />

these losses.<br />

Elastic Shortening, Δf ES (AASHTO 5.9.5.2.3)<br />

E p = 28500 ksi<br />

E ci = E ci_girder = 4.696 × 10 3 ksi<br />

M g = M DC1_NC = 928.385 kip ⋅ ft<br />

Modulus of elasticity of prestressing steel<br />

Modulus of elasticity of concrete at transfer<br />

Midspan moment due to member self-weight

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