Is there a difference? Yes, of course, there is. The following are my opinions based on 40 years of experience in the pile and pile driving industry. I have been selling both for approximately the last 40 years and this article will point out general information and my opinions for the pile driving industry.
Hot-rolled steel sheet pile has been around for a century, being used successfully all over the world, in every kind of project and no one can deny its acceptance and viability. On the other hand, cold-formed steel
sheet pile has less history, and less undisputed acceptance. In this report, I will try to illustrate the differences, and, if I seem to defend cold formed, it is only because cold formed has been questioned more and is subject to much more skepticism because of its recent development and shorter historic record, so I will be trying to address these issues.
History
sheet pile manufacturers has been around for over a hundred years. A lesser-known fact is that cold-formed sheet pile in the heavier sections that we are familiar with has been around since the 70s, or approximately 45 years. Both play a very important role in the pile driving industry. Steel sheet pile was originally a flat-sheet design used for cellular cofferdams and structures. This design is as important today as the early 20th century, and in some ways, with the advent of a patented process of open-cell bulkheads and cofferdams will ensure its importance for many years to come. Hot-rolled has evolved from flat sheets to U-shaped sheets, to the Z-shaped sheets we install today and will remain a viable product for many years to come. The first cold-formed I heard about was a mill designed and owned by Krupp Steel. This mill was sold in the late 70s to the Canadian company
Casteel and moved from Germany to Canada and later to the United States. The first cold-formed steel sheet pile sold in America was in the mid 70s by an agent for Krupp Steel. Actually, there was over a half-mile sold to McDermott Corp. for bulkheads in New Orleans and Amelia, LA, and Gulfport, MS on the industrial canals. All these bulkheads are still in use today, over 40 years later. There will always be a debate over the merits of hot-rolled vs. cold-formed just as there will always be a debate over what pickup truck a contractor should purchase.
MANUFACTURING PROCESS
So what is the difference between hot-rolled and cold-formed? There are a couple of differences. Hot-rolled is manufactured in a single- or two-step process heating prerolled steel beam blanks to such a high
temperature that it allows it to be easily shaped and formed into sheet pile. Cold-formed is manufactured in two steps. First, hot-rolled steel coil is manufactured and later it is run through a mill, which forms the coil into the steel sheet pile sections. Although this combines two processes, the method, as I will point out later, has an advantage of economy and scheduling in the cold-formed process. Both sections of steel sheet pile (hot-rolled or cold-formed) have the same steel properties and are usually rolled to the ASTM 572 grade 50 specifications with identical properties. So what’s the big difference? The basic difference is the interlock.
INTERLOCKS
ALL cold-formed interlocks are basically similar. But hot-rolled are usually different and one manufacturer’s generally won’t interlock with another’s unless a transition piece is fabricated. Most cold-formed interlocks of similar size will interlock regardless of the manufacturer.
I describe cold-formed interlocks as a-hook-and-a-grip. The hook is captured by the grip and when pile is driven cannot be dislodged. In my opinion, it may be a better feature than the traditional ball-and-socket but you have to see the hook-and-grip up close to really appreciate
the technology. Generally, all pile can be driven out of the interlock, but because of the design, I believe it is much more difficult to drive cold-formed out of the interlock. Cold-formed interlocks are looser. Therefore, thread easier and drive easier, which is especially an advantage when driving and pulling temporary sheets. This feature can speed up the process in permanent installations. A major Southeastern pipeline contractor uses hundreds of tons of cold-formed sheeting exclusively for temporary trenching projects because, in his opinion, the interlocks perform much better in multiple driving and pulling.
On the other hand, many contractors feel it takes a more precision to drive the cold-formed piling plumb (vertically). It can be done but the installation crew has to be a bit more precise. How well any type of pile drives is directly proportional to the experience and techniques of the pile driving crew. As to water tightness, no interlock is water tight. All interlocks achieve a leak resistance by the pressure exerted on the wall by hydraulic pressure and then fines and materials being trapped
within the interlock.
FEATURE
The Larssen interlock appears to be tightest. My opinion is that compared to ball and socket the cold-formed interlock can hold its own. There is about the same interior surface area actually touching in both hot-rolled and cold-formed so generally both should be about equal under hydraulic pressure. In very wet or water conditions, hotrolled
may seal up faster where there isn’t as much load. And of course with either condition, most pile driving superintendents have their own favorite method for stopping leakers which range from horse manure
to sugar cane grindings. Plus, today there are sealants that can be installed in the interlock to create an almost perfect seal. Wadit and Adeka are two of these sealant brand names. An international distributor of both hot-rolled and cold-formed recently told me that for permanent installations, the cost of cold-formed plus the sealant is still less expensive than most hot-rolled products.
A WORLD VIEW OF COLD-FORMED VS. HOT-ROLLED
From Singapore to North America, cold-formed steel sheet pile may be set to overtake and out-produce hot-rolled steel sheet pile in harbors, shipyards, docks, water treatment plants. Since cold-formed steel sheet pile was introduced over 40 years ago, worldwide there are now over a dozen mills producing cold-formed steel sheet pile compared to a half-dozen hot-rolled mills. Over a million tons of permanent and
temporary projects have been supplied in North America alone for every type of soil condition including driving in extremely hard glacier tills, coral, cemented sands and heavy clays. In fact, cold-formed piling has never had a propensity to drive out of the interlocks in difficult driving conditions like many hot-rolled sheets. The following are some reasons cold-formed is proving to be superior and in many cases outselling hot-rolled:
LOWER PURCHASE COST
·Because of an economically efficient process, with faster setup and manufacturing, and less labor costs, cold-formed has been selling for approximately $200 to $400 a ton less than hot-rolled. This is up to 25 percent savings per ton which can save an owner thousands of dollars.
·Cold-formed sheet pile manufacturers offer, at no additional costs,
1) installation of handling holes and pair sheet pile
2) greater flexibility in selecting exact lengths; whereby most hot-rolled charges are mill specific.
In many projects this flexibility can produce 25 percent savings in material.
ORDER-TO-DELIVERY TIME SAVINGS
The shorter lead time to roll cold-formed orders can save money by allowing faster scheduling and less idle time while waiting for sheet
pile delivery. Cold-formed mills have the ability to economically change over to roll different sections and smaller orders with less lead time. Some mills can roll smaller minimum quantities for A690 and Grade 60 because of more availability of coil. Many sections of hot-rolled are
rolled infrequently and customers have to wait for the hot-rolled mills’
arbitrary rolling schedules due to the waiting period for an accumulation of orders.
INNOVATIONS
Cold-formed first created many innovations in the pile industry, such as, wider-, deeper-, and generally multiple-thickness sections within the same section. All of these services combined help to explain why there is a demand for cold-formed steel sheet pile.
ATTEMPTS TO DENIGRATE COLD-FORMED STEEL SHEET PILE
I would be remiss if I didn’t point out that there has been a concerted effort by a couple of hot-rolled mills and a major U.S. distributor to denigrate cold-formed pile and I have to admit they have had some relative success. How were they able to achieve this?
FEATURE
In 1992, they hired a well-known engineering firm to do some testing and write a report. I am looking at a copy of the 1992 report and in its conclusion states, “The primary cause of the difference in capacity is not the difference between hot rolling and cold rolling. It is considered likely that the shape of the CZ114 section (greater depth, larger width, to thickness ratios, etc.) is a major factor contributing to the lower capacity.” (Note: CZ114 has not been manufactured in a number of years now.) The test compared a section of pile .375 inches thick by dpproximately 12 inches deep by 18 feet wide to a section .335 inches thick by approximately 13.39 inches deep by 24 feet wide that claimed a similar section modulus.
Guess what! The thinner, wider, deeper section failed before the thicker, more compact section due to extreme loading to produce transverse stresses. They compared an apple to an orange. The test used a method of extreme loading in the laboratory which caused what are called transverse stresses which would be difficult to achieve in actual field installations. An engineer friend of mine pointed out that the test loading created in the laboratory could only be duplicated in a really-deep cofferdam such as 50 feet plus excavation with a limited number of water levels.
I believe the first report in 1992 was the one and only laboratory test.
Another report in 1997, which extrapolated data from the 1992 test, made the claim that it was a comparison between hot-rolled and cold-formed sections of similar published section modulus, which is not totally accurate. Actually, it was a comparison between two different geometric shapes with different widths and thickness claiming a similar section modulus. The reason we have never heard of a failure is that any engineer designing a structure that may be subject to these stresses would typically add more levels of bracing to distribute the loads on the
water and/or sheet pile and of course use a sheet pile thicker than .335 inches. Many government agency engineers, after being exposed to these reports, have chosen to specify hot-rolled; or, if cold-formed is used, it must have a section modulus of 120 percent of the hot-rolled sections. This is a very conservative mandate and historic data on performance of cold-formed in sheet pile walls does not justify this approach. Anyway, I sell both and must report to you, that in 40 years, the industry has never heard of a cold-formed sheet pile section fail due to traverse stress in the field.
I commend the engineering company for its research and bringing attention to the industry the condition of transverse stress, but I am sure, if they represented a distributor who sold cold-formed steel
sheet pile, they would have pointed out the potential problems in an enlightened but completely different format. In closing, expect to be using both hot-rolled and cold-formed successfully on projects that require steel sheet pile for many years to come.
