The following is an excerpt from an email from John R. Adams:

Pittsburgh-Des Moines Steel Company (PDM) fabricated the "forked" columns at the base of the World Trade Center. The initial articles and videos that I saw during the first week after the convoy made no mention of PDM as the fabricator. The steel plate was made and rolled by Lukens. The fabrication and welding of the plates (several inches thick) into their final form was done by the Pittsburgh-Des Moines Steel Company (PDM) at their Neville Island Plant on the Ohio River near Pittsburgh, PA. PDM was best known as the fabricator and erector of the St. Louis Arch. PDM as a corporation was sold in several parcels to various companies including Chicago Bridge & Iron (CBI) around 2002.
This is a picture from PDM's Heavy Assembly Shop (HAS) on Neville Island. The building is still there today, but it is under different ownership. PDM's fabrication plant at Neville Island was established in 1907. They never had a fabrication facility on Staten Island.

From the book Men of Steel - The Story of the Family that Built the World Trade Center, by Karl Koch III with Richard Firstman:

. . . the trees might look simple once they were delivered, but you couldn't tell that to the shop men at Pittsburgh Des Moines who had to assemble, blast clean, paint and then weld together 152 units that had a variety of widths and grades of carbon, alloy and high-strength steels - a $3.2 million contract."
"All of the shops were responsible for acquiring their own raw steel, with the Port Authority sending out inspectors to their plants to approve the material before it was fabricated."

Based on the wording in the Koch book, the contract for the forks would have been awarded to PDM and they would have been responsible for purchasing the steel from Lukens and others.

My first-hand knowledge of PDM's involvement is as follows: I graduated from high school in 1965 and spent the summers of 1965 - 1968 working in the drafting room of PDM. I saw the "forked shaped panels" in fabrication during some of those summers. They were shipped one to a railroad car to NYC. Starting in 1969, I worked for PDM as a graduate engineer for over 16 years. I have two copies of PDM's 100-year history "Towering over America" (1892 - 1992) that has a picture of a column on a flatbed car and other parts in fabrication.
I have spoken with several other engineers that worked for PDM at the time of the World
Trade Center fabrication. They are located in several states now. One of them said that he signed his name on the inside of some of the columns. They all agree that all of PDM's work was done at Neville Island (near Pittsburgh on the Ohio River). None of them could recall if PDM received the plate for the forked as full plates or precut into the forked shape. PDM certainly had the capability to cut the plates to the required shape. All of the former PDM employees agree that PDM (at Neville Island) did the all of the welding of the 56 foot long forked sections that were sent - one to a rail car - to NYC.
 
It is important that the accomplishments of PDM are not forgotten. My father worked at PDM for 38 years and retired in 1980 after having been Chief Engineer for many years. He was Chief Engineer during the completion of the World Trade Center Project and the St. Louis Arch. PDM had a reputation for building difficult structures such as nuclear containment vessels for nuclear power plants, elevated water tanks, cryogenic tanks and vacuum chambers and wind tunnels for the aerospace industry.
I do not have access to any specifics, but I believe that Lukens and PDM had a long supplier ¬purchaser relationship ... For .example: A553.1 Type I steel (9 % nickel) .was .used .for many of the LNG (liquefied natural gas @ -260 deg F) storage tanks that PDM built around the world. In addition, at the time that PDM fabricated and erected the St. Louis Arch, it was the largest tonnage of stainless steel ever used on a single project.
I look forward to visiting Coatesville at some time in the future and seeing the columns once again and the rest of your Museum.
It seems strange to say this - and you may agree, but after only 40 years it is not that easy to figure out who did what. It is almost like trying to figure out how the pyramids were built.