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Strength Design Method

Concrete hydraulic structures are defined as structures containing sludge, fluids or gasses, including

cover slabs and beams, and structural sections potentially subject to groundwater pressures. The

concrete design of hydraulic structures should conform to ACI 318-05 and ACI 350-06, Appendix C,

except as modified below. ACI 318, Appendix C, should be used for below grade structures not subject to

groundwater pressures. If a continuous structure is partially hydraulic structure and partially below grade

and not subject to groundwater pressure, ACI 350, Appendix C, should be used for the entire design.

MWH normally uses Appendix C for hydraulic and below grade structures because the load factors in

ASCE 7 for building structures unduly penalize walls subject to backfill loads. While the load factor for

backfill pressures was reduced from 1.7 to 1.6, the strength reduction factor for concrete shear was also

reduced from 0.85 to 0.75. Also, ASCE 7 2.3.3, Equation 7, 0.9D+1.0E+1.6H, in effect, does not permit a

stress increase for seismic backfill pressures.

ACI 350 RC.1.1, designs using the provisions of Appendix C satisfy the code, and are equally

acceptable. While ACI 318 RC.1.1 simply states that the load and strength reduction factors in Appendix

C are considered to be reliable for concrete construction, it should be noted that no exceptions to the

use of Appendix C are made in IBC 2006 Section 1908. Before completing the design criteria during the

project preliminary design phase, acceptance of the use of Appendix C should be verified with the clientand the building department.

The calculated area of steel reinforcement required for flexure, shear and torsion and direct tension must

be increased by a coefficient of durability factor (Cd) to limit the allowable crack width in the concrete. The

Cd factor varies with the exposure and the type of loading (flexure, shear and direct tension) as shown

below. Do not mix Appendix C U-equations with IBC 2006/ASCE 7-05 equations, as the phi factors

differ for the two methods.

Load Factors

Load factors for non-hydraulic structures (U) should be in accordance with ACI 318/350, Appendix C.

For environmental engineering structures, for dead load (D), live load (L), soil backfill load (H), liquid

load (F), external groundwater load (G), wind (W), earthquake (E), snow (S) and load combinationsas outlined below:

U=1.4D+1.7(L+F+G+H+S), or Mu=1.4Md+1.7Ml+1.7Mf+1.7Mg+1.7Mh+ 1.7Ms

U = 0.75 [1.4 (D) + 1.7 (L + F+ G + H + S)] + 1.6W or 1.0E

U = 0.9 D + 1.6W or 1.0E

It is permitted to use 1.7 D for static, except uplift and overturning, load cases.

Phi factors are given in ACI 318/350 C9.3.2.

Durability factors, Cd, for reinforcing design per ACI 350, C.9.2.9, are as follows:

1.3U Flexural design

1.3U Shear design

1.65U Direct Tension

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Note that it is the intent of ACI 350 that durability factors be applied to the calculation of As for

bending, shear and direct tension reinforcing for static load cases and not be applied to the

calculation of U. Application of durability factors to the calculation of U results in an overestimation of

the applied Vu.

For the seismic and wind load cases, durability factors need not be applied to design of reinforcing

steel.

Serviceability requirements should be in accordance with the provisions of the ACI 318 and the

requirements for structures with normal sanitary exposure and severe environmental exposure in

accordance with ACI 350 C10.6.

The final selected As must meet the requirements for minimum shrinkage and temperature

reinforcement of ACI 350 7.12.2.1.

For shear reinforcing

12"spacingswhere,)/()( ==

+=

dfsCdVA

VVV

ysv

scu

(and d/2)

For direct tension bars:

)/( yus fCdPA =

Basins not protected by a fail-safe overflow

In general, structures should be protected by a fail-safe overflow. In some cases, a fail-safe overflow

may be impractical, or the hydraulic profile may show that the structure is protected by a free flowing

weir or top of wall, upstream from the structure.

If a fluid holding structure is not protected by a fail-safe overflow, the structure should be checked for

safety for the maximum hydrostatic elevation that could occur due to system failure or operator error.

Coordinate with the process engineer for the hydraulic profile for such an event. Normally, the

loading for an emergency event should be checked against the completed structure with backfill, if

any, in place, and without seismic loading or other unusual temporary loading considerations. In

conformance with ASCE 7, a load factor of 1.4 may be used for static fluid loads, F, and durability

factors need not be applied to reinforcing steel calculations.