ML19329E023

App 5F of Rancho Seco PSAR, Justification for Field Reduction Factors. Includes Revisions 1-4

ML19329E023
Person / Time
Site:	Rancho Seco
Issue date:	10/31/1967
From:	SACRAMENTO MUNICIPAL UTILITY DISTRICT
To:
References
NUDOCS 8004090638
Download: ML19329E023 (3)

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Og APPENDIX 5F V

JUSTIFICATION FOR YIELD REDUCTION FACTORS (0 - FACTORS) USED IN DETERMINING YIELD STRENGTH OF CONTAINMENT STRUCTURE The 0 factors are provided to allow for variations in materials and work-manship.

In the ACI Code 318-63, 0 varies with the type of stress or member considered; that is, with flexure, bond or shear stress, or compression.

The 0 factor 'is multiplied into the basic strength equation, or, for shear, into the basic permissible unit shear, to obtain the dependable strength.

The basic. strength equation gives the " ideal" strength, assuming materials are as strong as specified, sizes are as shown on the drawings, the work-manship is excellent, and the strength equation itself is theoratically correct. The practical, dependable strength may be something less, since all these factors vary.

The ACI Code provides for these variables by using these 0 factors:

0 = 0.90 for concrete in flexure 0 = 0.85 for diagonal tension, bond, and anchorage

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0 = 0.75 for spirally-reinforced, concrete compression members V

0 = 0.70 for tied compression members 0 is larger for flexure because the variability of steel is less than that of concrete and the concrete in compression has a fail-safe mode of behavior; that is, material understrength without failure. The 0 values for columns are lower (favoring the toughness of spiral columns over tied columns) because columns fail in compression where concrete strength is critical. Also, it is possible that the analysis might not combine the worst combination of axial load and moment, and since the member is critical in the gross collapse of the structure, a lower value is used.

The additional 0 values used represent the engineer's best judgment of how much understrength should be assigned to each material and condition not covered directly by the ACI Code.

The additional 0 factors have been selected based on material quality in relation to the existing 0 factors.

Conventional concrete design of beams requires that the design be controlled by yielding of the tensile reinforcing steel. This steel is generally spliced by lapping in an area of reduced tension.

For members in flexure, ACI uses 0 = 0.90.

The same reasoning has been applied in assigning a value of 0 =

0.90 to reinforcing steel in tension, which now includes axial tension.

How-ever, the code recognizes the possibility of reduced bond of bars at the laps by specifying a 0 of 0.85.

Mechanical and welded splices will develop at least

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.125% of the yelid strength of the reinforceing steel and a value of 0 = 0.90 is

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recommended.

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' The only significantly new value introduced is. 0 = 0.95 for prestressed

. tendons in direct tensions.

A highsr @ value than for conventional rein-

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forcing has been allowed because (1) during installation the tendons are each jacked to about 94 percent of their yield strength, so in effect, each tendon has been proof tested, and (2) the method of manufacturing prestressing steel (cold drawing and stress relieving) ensures a higher quality product j

.than conventional reinfor2ing steel, i

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