TP 020 - Re-examining Conventional Wisdom in Rebars Detailing
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Transcript of TP 020 - Re-examining Conventional Wisdom in Rebars Detailing
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Conventional Wisdom in Reinforcing Steel
Re‐examining Conventional Wisdom
By Amadeus L. Magpile,
President and CEO of Barlines Reinforcing Steel Co., Inc. (Phils)
Founder and President of Barlines Rebar Estimating & Detailing, Inc. (U.S.A.)
Change is not easy in a large organization. In an industry as a whole change may not be possible. There
is, predictably a strong reliance in any organization on conventional wisdom that had worked numerous
times before. But innovations make paradigm shifts. What was accepted wisdom years ago are one by
one falling apart as a misconception. The concrete reinforcing steel industry is no exception. What was
once a robust cut and bend practice at the jobsite in the Philippines is slowly giving way to offsite pre‐
fabricated supply and demand market. It cannot be helped. The economy is getting stronger , the
playing field is now global and climate change demands frugal use of resources. While I am not
advocating a complete change, I strongly urge the purveyor of rebar to re‐examine at least a few
conventional wisdom, that I find prevalent only in the Philippines.
Single Stock Length Vs. Multi Stock Lengths
While other countries revel on a one stock length; 60 feet in the US and 15 meter in Canada, Australia
and most of Europe ‐ the Philippines has several lengths of stock, 6.00m, 7.50m, 9.00m, 10.50m, and
12.00m. During the height of the cut and bend practice at the jobsite, multiple stock lengths was a
bright and useful innovation. The steel mill did construction a big favor by providing them. In so doing,
however, it seemed that they also benefited very handsomely on its drawback.
It was obvious, that the idea on multiple stock lengths stemmed from the absence of reinforcing steel
(commonly
called
rebar)
fabricators
with
equipment
to
properly
shear
and
bend
rebar
to
required
lengths as designed by structural engineers. The availability of multiple stock lengths, allowed
contractors to order rebar lengths as closed to what they need, to eliminate shearing at the jobsite and
save on materials. For example, when 5.50m long bars are required, they will order 6.00m long stocks.
Trimming it to length becomes a wasted labor. All 6.00m long bars will be placed in concrete and the
contractor feels delighted that he did not waste any material. Whatever he bought he used in its
entirety. Never mind that the required length needed is only 5.50m long.
In countries where one stock length prevails, this same contractor can buy exactly 5.50m long and pay
only for the exact length. However, the difference is that he is buying from a fabricator instead of the
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Conventional Wisdom in Reinforcing Steel
steel mill. Obviously the fabricator added a certain premium for cutting the bars to length, which made
the cost a bit higher.
But wait. When a steel mill produces rebar, they produce them in one length only. After their
production and the subsequent shearing to different stock lengths, an added process, would not they
add a premium too? In fact, in this system – it seems to me the premium is there whether you buy the
post cut 6.00m or the production 12.00m length.
This is in essence is the simple advantage of buying rebar for use in construction from an offsite rebar
fabricator rather than a mill. The end user buys and pays only for the exact quantity, length and shape
that he needs from the rebar fabricator – no more and no less. While from the steel mill the end user
buys and pays for what he needs and then some – the remnants and scraps. Scraps and remnants are
best
left
in
the
steel
mill
–
after
all
they
are
the
ones
that
need
them
badly.
Rebars
are
produce
directly
from scrap metal.
The Philippines is not totally out of the cut and bend industry. It may be years before they totally
embrace offsite reinforcing steel fabrication. The multiple stock lengths, therefore still has a place in the
Philippines. What I want to see is more and more rebar fabricators to properly bend rebar against
hairline fractures, reduce waste and help make a jobsite a safe place to work. Exactly what Malaysia
and Singapore are aiming to do.
Shorter Class A Laps Saves Material than Longer Class B Laps
No one I know can argue this statement at face value. But of course – shorter laps are less material than
longer laps. This is inedibly stamped as a mindset among project engineers. Because the science of
rebar detailing was slow to come to the Philippines – this mindset was never held up close to scrutiny.
Sadly, the statement does not hold true in the design of seismic columns.
If you look at the construction skyline of the Philippines, you will see column vertical bars alternating
long and short, as if the bars are racing towards the sky. Traveling abroad, you will never find a similar
construction skyline. In San Francisco, the city that practically invented earthquakes, you see vertical
bars all cut at about the same height and column ties are all the way to the top of the vertical bars.
Same is true In Vancouver, Sydney, Dubai, Malaysia and Singapore.
Two factors made this happen in the Philippines. First ‐ the use of bundled bars, due again to the
absence of a rebar fabricator with shearing and bending equipment to handle large vertical bars.
Second ‐ the mindset of shorter laps against longer laps. The American Concrete Institute (ACI) code
requires that shorter laps and laps of bundled bars be staggered. Yet the same code allows longer laps
in single bars not staggered.
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Conventional Wisdom in Reinforcing Steel
Because of the staggered laps, columns in the Philippines are prevented from being assembled into a
cage. That is why we do not see column ties in our skyline. What we see instead is our version of Cirque
Soleil ‐ironworkers
gingerly
climbing
swaying
vertical
bars
to
drop
multiple
ties
from
the
top,
while
other ironworkers are holding bars from swaying and relatives at the street level praying. It seems to
me – Corporate Social Responsibility (CSR) gone awry.
While bundling of bars is an acceptable practice, the diminished use will only come when more and
more rebar fabricators are around. In the case of the shorter class‐A lap versus the longer class B lap
mindset – re‐examining this wisdom is necessary.
Looking at the skyline of the Millenium Tower in San Francisco.
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Conventional Wisdom in Reinforcing Steel
This may be a shock to construction engineers, just like the first time I showed them coiled rebar stocks
in our fabricating shop.
THERE IS ONLY ONE WAY TO SAVE ON REBAR MATERIALS – AND THAT IS OPTIMUM SHEARING.
This is not a trade secret. Everyone in the rebar industry outside the Philippines knows this. Optimum
Shearing is sorting all the required lengths and combining them to fit a stock length to reduce waste. If
an engineer is given the task of scheduling the shearing of rebar from a long list of bars of different
lengths such that the outcome will produce the least amount of scrap and remnants – he may come
close but not against a computer. Why? There is no known formula. His calculations will require him to
iterate over and over again. You need to do a number of what ifs before you can come close to an
optimum result. Then even if he succeed, he will not find anyone that can maintain and follow his
schedule.
The
shearman
will
always
grab
the
most
convenient
stock
to
shear
against
his
schedule.
Meanwhile, a properly tooled rebar fabricating shop has a software that can provide an optimized
shearing schedule in nano‐seconds and an automatic shearline that can process such a schedule. Not a
portable shearing equipment but a long shearline like as shown below.
A fully automatic shearline capable of processing optimum shearing schedule. Loaded and sorted at one
end and segregated by length at the other end after shearing.
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Conventional Wisdom in Reinforcing Steel
Now, let me show you a typical seismic column in the Philippines. Both are reinforced with 32M vertical
bars. One column lap spliced with a shorter class‐A lap staggered and the other lap spliced with a longer
class‐B
lap
but
not
staggered.
Typically
vertical
bars
such
as
these
columns
are
fabricated
from
a 9.00m
stock.
The final results after fabricating from a 9.00m stock are as follows:
The class‐A lap generated a vertical bar 8.38m long and a 0.62m scrap (remnants shorter than 10% of
the stock length is considered scrap), while the class B lap generated a vertical bar 8.79m long and a
0.21m scrap. At 8.79m long, I will not even bother to cut the 9.00m stock to save labor. Even If you do
not add the advantage of cage assembly and lifting in place by the tower crane, thus avoiding
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Conventional Wisdom in Reinforcing Steel
ironworkers climbing bars – Class‐B lap is the only logical choice. Anyone would conclude that since
they will throw away scraps, wouldn’t it make sense to use them instead?
ACI overwhelmingly recognized this that in their Detailing Manual they did not even mention class A in
drawing a Typical seismic‐resistant details of columns. See Fig. 6, from ACI below.
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Conventional Wisdom in Reinforcing Steel
Closed Stirrups vs Open/Capped Stirrups
The ACI Fig 6 brings me to the next subtle difference in construction in the Philippines ‐ that is puzzling
to me. Open stirrups are seldom found in any jobsite in the Philippines. Even though the majority of
floor framing designs are beams and slabs variety. Very few are flat plate designs, which are very
common in most high‐rise building projects in the world.
Construction does not seem to allow open and capped stirrups in assembling girders and beams. If
someone decided that open and capped stirrups require more materials than closed stirrups –again I
will say this is a misplaced decision. Once again, to save materials you need optimum shearing, period.
When beams and girders are crossing each other, the only way to build them with closed stirrups is to
temporarily assemble them above floor, and slowly lower them after they are all tied together. Beams
framing into girders will require that the girder be raised also. Girders framing into columns will require
that a number of ties be displaced. I see ironworkers having an easier time on single span beams and
girders but a horrendous time on multiple spans.
Consider Fig. 6 from ACI Detailing Manual. The ties that need to be out of the way for the girders
assembly are the critical ties within the Joints and above the joints. These critical bars are usually
multiple and are closely spaced. How are they displaced? They are gathered within the girder assembly
and slowly put in place as the girder is lowered. This is, of course, easier said than done – when you are
dealing with not one but multiple ties within the columns, spaced too close to each other. What really
happens when
ties,
stirrups
and
girder
bars
are
in
each
other
way
and
the
inspectors
are
not
around?
The ironworkers bring out their only tool in getting out of tight spot – an acetylene torch. Either the ties
or stirrups get torched or worse the girder or column bars get torched. There is a brand new building in
Las Vegas – the Harmon Hotel, scheduled for demolition by the owners, whose structural engineers are
claiming a lot of rebar are missing in the column and beam joints.
We suspected that the detailing was outsourced out of the United States to a group who is unfamiliar
with standard practice in the United States. In the installed rebar market in the United States all stirrups
are detailed open and capped per ACI code, notwithstanding how they are shown in the contract
drawings.
Once
again,
the
American
Concrete
Institute
overwhelmingly
acknowledged
this
practice,
by
showing alternate open capped stirrups in their detailing manual. See Fig. 5 on the next page.
When stirrups are open and capped, bars can be dropped and tied in place inside the forms. Main bars
are threaded into girders and columns and no ties are displaced whatsoever. It makes for a cleaner
assembly. Inspectors are able to see inside the beam cage before cap ties are tied in place. Leaving a
few cap ties open allow for concrete hose and vibrators to reach critical joints. And lastly it speeds up
the work tenfold. Ever wonder how construction in the U.S. can rise every three or four day cycle?
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Conventional Wisdom in Reinforcing Steel
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Conventional Wisdom in Reinforcing Steel
Open and
capped
stirrups
simplify
placement
of
girders
and
beams,
without
displacing
other
bars.
Finally, if change is necessary, where does the first line of change need to occur? I suspect, maybe, at
the drafting department of a structural engineers office. Could it be that it is high time to sort through
old boiler‐plate typical details and get rid of some that has seen its usefulness? Rebar detailers, such as I
are sometimes, without thinking, detail exactly according to what is shown on contract drawings – true
to what we learned from rebar detailing 101.