20 Aug 92
C-3. Thrust Blocks for Horizontal Bends. Refer to paragraph C-7
for sketches. The size of thrust block is calculated by;
ATB $ FS[T -- f(WS + WTB)]/(SZRKP
ATB = area of thrust block,
= thrust force in pipe,
coefficient for concrete cast on soil; varies
with type of soil; refer to Table C-1,
WTB = weight of thrust block,
(S = unit weight of soil,
Z = embedment depth to bottom of thrust block,
R = reduction factor applied to Kp (reference 4),
FS = factor of safety,
KP = coefficient of passive pressure for undisturbed soil of
trench sidewall, preferably determined using the log-spiral
method (reference 4).
In the absence of information regarding maximum allowable
displacement of pipe and pipe joints, use Ko instead of RKP. The
above formula is then;
ATB $ FS[T - f(WS + WTB]/(S Z K0
Ko = coefficient of at-rest pressure.
For a conservative approach, the term f(Ws + WTB) may be neglected
resulting in either ATB $ FST/(S Z R KP or; ATB $ FST/(S Z K0.
C-4. Thrust Blocks for Vertical Bends. Refer to paragraph C-B
C-4.l. The size of thrust block for upward directed thrust is
VTB $ FS(Ty -- WW - WJ -WS)/(C
VTB = volume of thrust block,
Ty = vertical component of thrust force=2PA Sin 0/2 Cos 0,
WW = weight of water in joint,
WJ = weight of pipe joint,
WS = weight of soil or backfill over thrust block,
(c = unit weight of concrete, and
FS = Factor of Safety.
For a conservative approach, the terms WW, WJ, and WS, may be
neglected resulting in VTB $ FSTy/(C.