huangyhg

bottom longitudinal reinforcement at supports for simply sup
hi to every one.
i'm an italian engineer.
i'm studying the aci 318 code and at the moment i'm interested to shear desgin. reading the code i didn't find any provision about bottom longitudinal reinforcement to place at supports and capable to withstand a tensile force equal to shear value v.
i saw some shop drawings of precast beam (reverse t and double tee) and there's no similar reinforcement ...
can someone explain me why the aci code doesn't prescribe this reinforcement at supports? is it a difference of theories used to study the shear failure? it seems that this kind of reinforcement is needed for deep beams and not for "normal beams".
thanks to every one wants to help me to understand better.
sorry for my english.
best regards
the shear in concrete beams is resisted by vertical stirrups, not by longitudinal reinforcement. the idea is that shear in concrete beams will cause diagonal tensile failure of the concrete and these vertical stirrups will cross those diagonal sections and provide the necessary reinforcement.
many (most?) codes have a requirement for the anchorage of positive moment reinforcement at simple supports to ensure adequate shear capacity at the support. i believe that is what alemanda is asking about.
i don't use aci318 and therefore can't comment definitively, but i would be suprised if it does not have support anchorage requirements.
aci 318 does prescribe requirements for bottom bars extending into the supports. if you read through chapter 12, there is a traditional requiremnt that bottom bars extend at least 6 inches into the spports.
there are also requirements for bottom bars to extend further and either be hooked or lap spliced. these are in chapter 7 under "structural integrity".
there is also another equation for bottom bars in simply supported beams. i am away from my desk - but i'll look it up on monday. it has, i think, an m/v ratio in it but i'm not sure.
alemanda,
i haven't used aci318 in a number of years, but ever since aci318-673, there have been some provisions for anchorage of positive moment reinforcement at simple supports. am sure jae will point you in the right direction.
the australian code as3600 requires development of a tensile force of 1.5 design shear force at face of support, where the shear force is taken at distance d from support. there are a couple of other provisions, but you are correct that the extension of the bottom bars past the potential diagonal crack is required.
prestressed beams are more complex, as the prestressing forces themselves change the shear stress and also increase the shear load capacity.
the requirement that jae mentions using mn/vu is in chapter 12 under development of positive reinforcement.
hi to every one,
thanks for your replies and contributes.
i read teh chapter 7 and chapter 12 of aci code, but i didn't found the provision about the minimum area of bottom reinforcement to place at the supports for a simply supported beam ... better, there's a minimum area but it is very different than that one prescribed by ec2. in fact ec2 prescribes that at the supports an area of bottom reinforcement equal to vu/fy (vu is the design value of shear and fyd is the design value of the steel tensile strength). example: simply supported beam; span = 10.00m, depth of beam = 70cm; dl (including self weight) = 5000 dan/m, ll = 5000 dan/m. the design value of shear at support is vu=(1.2*dl+1.6*ll)*10/2=140000/2=70000 dan. according to ec2 i have to place at least 70000/3826 = 18.29 cm2 of well anchored reinforcement. how many cm2 of reinforcemen should be place according to aci 318 code?
about theories ... yes, i agree with ucfse ... maybe the shear strength is derived from different theoretical approaches. i've read about morsch strut and tie model (ec2) and mcft (modified compression field theory) ...
regards
alessandro mandalà
in practice, the longitudinal main (positive and negative) reinforcement shall be cut-off at a distance (development length by code) beyond the point at which the bars are no longer required according to the moment curveture, and the number of the remaining bars shall be no less than two. these longitudinatal bars can be counted when designing the shear reinforcement (stirrups), however, the effect is usually small, and thus ignored.
almait,
previous posts by structuraleit and kslee1000 seem to discount your concern. if they mean the anchorage of bottom bars at simple supports is unimportant, i beg to differ. sorry i can't give you guidance in studying the aci code, but the provisions must be there somewhere, probably with the requirements for flexural reinforcement rather than the shear (diagonal tension) provisions.
here are the relevent sections from the aci code (318-02) that address bottom bars extending into supports:
7.13 requirements for structural integrity
specifically sections 7.13.2.1 through 7.13.2.4
12.10.3 this section indicates that reinforcing should extend beyond the point where it is no longer required to resist flexure - by an amount of 12 x bar dia. or d.
12.11.1 development of positive moment reinforcement
requires 1/3 of as bottom bars of simple spans to extend at least 6 inches into the support for beams (1/4 of as in continuous
thanks to each contribute.
back to my example but considering the length of the beam equal to 5.00 m.
according to ec2 as at supports is equal to 9.14cm2
according to aci 318-02 a flexural reinforcement at midspan is required and it's equal to about 27.85cm2. one third of this area is equal to 9.28cm2. similar results. ok.
now my question is.
are the provisions you quoted referred also to prestressed

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