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 &
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
From: "Mark Geoghegan" <mgeoghegan(--nospam--at)structural-tech.com>
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
the actual joint location, from these actions you then calculate the joint
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
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, =
"mandatory requirement." If the contractor locates the construction joint=
the wrong place, then he goofed, as simple as that.
William L. Polhemus, Jr., P.E.
Polhemus Engineering Company
From: =B1=E8=BE=E7=BC=F6 [mailto:kimslove(--nospam--at)kopec.co.kr]
Sent: Sunday, June 17, 2001 9:58 PM
Subject: ACI 318 -Construction Joint
ACI 318-99 Code 6.4.4 described for construction joint as follows;
" Construction joints in floors shall be located within the middle thir=
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
of slabs, beams, and girders, how can I evaluate the structural
integrity and acceptance of construction joint location ?
Thanks in advance,
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