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Re: Subject: ACI 318 -Construction Joint

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Ladies, Gentlemen and others

Just to mess up the logic, the best place to put the joint is NOT at a point of lowest shear, which is a high moment zone and a critical plastic hinge area requiring ductility where you want your best concrete, but at the point of highest shear capacity with relatively low applied shear and very little effect from induced curvatures from redistribution from plastic hinges, which is at the point of contra-flexure. At this point, according to ACI eqn 11.5, the shear capacity is greatest (because a flexure-shear crack needs flexural tension at the surface to begin the crack). Also, as the applied moment is lowest, flexural crack control will be best and ductility is not a problem.

It is also the best point to place it if the first poured slab is to be stripped before the second is poured (eg prestressed slabs) or stripped and back-propped (eg RC slabs) as the cantilever will be self-supporting (until you get a builder who pours the long part first and then strips it completely as the drawings did not specify back-propping and he missed the pour sequence note).

So the joint should be at about .15 - .2 x span length from the support.

At 10:27 PM 18/06/01 +0300, you wrote:
Gentlemen

Sure it is stipulated by ACI 318: CLAUSE-6.4.4, that the construction joints in floor shall be located within the middle third of spans of slabs, beams and girders. But one should focus on the technical reason why middle-third? Actually, if you read thru the commentary, just beside the main code para, you will understand the background. It says,"construction joint should be located where they will cause the least weakness in the structure. And, from the pointof view of strength of the structure, it is desirable to position C.J at points of minimum shear. Now, every body knows shear is maximum at the support, and, depending on loading conditions, changes sign at mid span, where it is zero. Therefore, for slabs and beams it is usual to have C.J at mid span or in the middle third of the span. Coming back to the incumbents problem, if the C.J is not at the middle third it is not a problem really. Important thing to note is where is it then located now? If it is close to the mid span, its no problem really either, infact it is desirable. And, if it is towards the support (from the middle third), the important thing then to note is where & how far from the support. The critical location of shear is at a distance "d" from the support. If the C.J is away from this critical location (towards mid span) then there is no problem as well. But if it is too close to this critical location then some analytical check has to be done. Like, shear capacity of the section should be determined & if the capacity is far above the induced shear, then nothing to worry. You should only worry if the member is heavily loaded (implying high shear value) & by mistake the C.J is too close to critical location, that is too close to distance "d" from the support. In case of otherwise, my advise to you is to sit back & relax.
Hope this explains. Regards,

SYED FAIZ AHMAD; MENGG, Mem ASCE
SENIOR STRUCTURAL ENGINEER
SAUDI OGER LTD
RIYADH, SAUDI ARABIA.

P.S: By the way, its my personal observation (should not be generalised)
    some of the Europeans (like French for example, who are too many in
    the company i work for) prefer to locate C.J  at mid span rather
    than at middle third. For me its difficult time always to convince
    them to locate at the middle third, which i have noticed is against
    their usual or cultural practice. I couldn't figure out until today
    WHY?

From: "Mark Geoghegan" <mgeoghegan(--nospam--at)structural-tech.com>
Reply-To: seaint(--nospam--at)seaint.org
To: "Seaint" <seaint(--nospam--at)seaint.org>
Subject: Subject: ACI 318 -Construction Joint
Date: Mon, 18 Jun 2001 22:27:40 +1000

regarding construction joints to ACI-318....

i do not think it is an MANDATORY requirement - you can "engineer" a CJ to
work at almost any location. sure - there are engineering preferences for
the locations, but it is common to "engineer" a joint to assist certain
construction staging/sequencing etc.

also, it is common for CJ's in post-tensioned concrete construction to be
located outside the "middle third" - does that mean they do not comply with
ACI-318? i do not think so! also, think about precast joints etc etc....

if the joint was located "by the contractor without approval from the
engineer" and you need to check/verify its integrity/acceptance you will
need to know the actual construction details (shear key size/depth, T & B
rebar, etc), then from your structural analysis you can determine the
maximum shear and moment actions (including lateral effect if applicable) at
the actual joint location, from these actions you then calculate the joint
capacity.

Para-phrasing from T.Y. LIN...do not "...blindly follow the
codes-of-practice, but seek to apply the laws of nature...".

what are you going to do if the joint is rejected - demolish the concrete?
PLEASE DON'T - sure contractor's make errors (and engineers goof-up too, on
occasions)- but jointly (engineer and contractor) (pun intended) can
re-establish the joint capacity, if it is indeed calculated to be
"defective".


Regards,

Mark Geoghegan BE (Hons.-Structural)

S T R U C T U R A L   T E C H
AUSTRALIA  -  GUAM  -  HAWAII




I don't think you have to "evaluate the integrity" if it is, as you say, =
a
"mandatory requirement." If the contractor locates the construction joint=
 in
the wrong place, then he goofed, as simple as that.
William L. Polhemus, Jr., P.E.
Polhemus Engineering Company
Katy, Texas
Phone 281-492-2251
Fax 281-492-8203


  -----Original Message-----
  From: =B1=E8=BE=E7=BC=F6 [mailto:kimslove(--nospam--at)kopec.co.kr]
  Sent: Sunday, June 17, 2001 9:58 PM
  To: seaint(--nospam--at)seaint.org
  Subject: ACI 318 -Construction Joint


  Hello everyone

  ACI 318-99 Code 6.4.4 described for construction joint as follows;

  " Construction joints in floors shall be located within the middle thir=
d
of spans
    of slabs, beams, amd girders."

  1. Code dose not provide any exception for this requirements?
     ("shall be" - mandatory requirements)

  2. If construction joint  was not located within the middile third of
spans
    of slabs, beams, and girders, how can I evaluate the structural
    integrity and acceptance of construction joint location ?


  Thanks in advance,
  Yangsoo Kim


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