ML20008D773
| ML20008D773 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 01/13/1969 |
| From: | CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | |
| References | |
| NUDOCS 8007300667 | |
| Download: ML20008D773 (3) | |
Text
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O APPENDIX SE I
V YTUTD RIDUCTICN FACTCRS The % factors are provided to allov for variatiens in =aterials and verk=an-ship.
In the ACI Code 318-63, % varies with the type of stress or =e=ber considered; that is, with flexure, bond or shear stress, er cc=pression.
The p factor is =ultiplied into the basic strength equation cr, for shear, into the basic per=issible unit shear, to obtain the dependable strength.
The basic strength equatica gives the " ideal" strength, assu=ing =sterials are as strong as specified, sines are as shewn en the drawings, the verk=an-ship is excellent, and the strength equation itself is theoretically correct.
The practical, dependable strength =ay be sc=ething less, since all these factors vary.
The ACI Code provides for these variables by using these % factors:
% = 0 90 for concrete in flexure.
p 0.85 for diagonal tension, bcnd, and anchorage.
% = 0 75 for spirally reinforced, concrete ec=pression =e=bers.
0 70 for tied ec=pression =e=ters.
O p is larger for flexure because the variability of steel is less than that of concrete and the concrete in ec=pression has a fail-safe = ode of behavicr; that is, =aterial understrength without failure. The p values for colu=ns are lover (favoring the toughness offspiral colu=ns over tied colu=ns) be-cause colu=ns fail in ec.pression where concrete strength is critical. Also, it is possible that the analysis =1ght not ec=bine the worst ce=bination of axial load.and =c=ent ccd, since the =e=ber is critical in the gross collapse of the structure, a lever value is used.
'The additional % values used represent 3echtel's best judg=ent of hov =uch understrength should be assigned to each =aterial and condition not covered directly by the ACI Code. The additional % factors have been select ?d.
based en =aterial quality 1, relation tc ' the existing % factors.
Conventional concrete design of bea=s requires that the design be centrolled by yielding of the tensile reinforcing steel. This steel is generally spliced by lapping in an area of reduced tension. For =e=bers in flexure, ACI uses
.p 0 90. The sa=e reasoning has been applied in assigning a value of
% = 0 90 to reinforcing steel-in tension, which new includes axial tension.
However, the code recognizes the possibility of reduced bend of bars at
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the laps by specifying a % of 0.85 Mechanical and velded splices will develop at least 125 percent of the yield strength of the reinforcing steel.
Therefore, p. 0 90.is recc== ended for the =echanical' type of splice and Q
0.85 is'recc== ended for the lap type.
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1 The only significantly new value introduced is 6 = 0.95 for prestressed pg tendons in direct tension. A higher 9 value than for conventional reinforc-V ing has been allowed because (1) during installation the tendons are each 29l jacked to from 90 to 100 percent of their yield strength, so in effect, each tendon has been proof tested, and (2) the method of =anufacturing prestress-ing steel (cold drawing and stress relieving) insures a higher quality pro-duct than conventional reinforcing steel.
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