The Green Thumb
Let's Get More Familiar (Part Three)
Roll Out the ShapesAt the end of the steel making process for LGMF, large coils of "hot-rolled" steel-manufactured to the cold formers specifications for base metal thickness, type, strength and coating weight-pass through a molten bath of zinc. This is known as "hot-dip galvanizing" (more about this process later). Now, the rolls of galvanized steel are ready for transport to the cold former for their transformation into LGMF components, such as track, stud, joist, etc.
A "slitter" is used to reduce the large coils to a proper, precisely measured width for the intended production run. The steel's surface is then "pickled" in a bath of sulfuric or hydrochloric acid. Allowed to cool, the resulting strips or "ribbons" are then fed into a roll forming machine, whereby it passes through a series of dies (rollers) that bend the steel to the desired profile. This entire process is computerized and will hold a tolerance of within 1/8 inch for any predetermined length. Typically, 40 feet is the maximum length for LGMF members since a flatbed trailer is 40-feet long-but longer lengths are possible. The computer controls the number and length of members, and the dies in the roll forming machine are set to roll the size and shape desired.
Long, cut-to-length framing members have several advantages, particularly for floor joists. Lap splices can be avoided since floor joists can be ordered to the full width of the house typically. This eliminates the need to order only stock-lengths and reduces field cutting significantly. As the shapes emerge from the roll former, a stamp imprints an embedded code on the surface of the steel member that allows inspectors, tradesmen, etc., to identify the framing component according to:
• Base metal thickness (uncoated)
• Coating weight
• Minimum yield strength
For example, a code stamp on a stud might read: XYZ 0033 G40 33KSI. Decoded, this stamp tells the world that the XYZ manufacturing company fabricated this framing component with a 33 mil (20 gauge) base metal thickness, applied a G40 weight hot-dip galvanized coating and used 33KSI (33,000 kips/square inch) minimum yield strength steel to do so. Not all cold rollers provide such a stamp, but it is fast becoming standard practice-particularly among the larger manufacturers.
A Cut BelowFor the end-user, LGMF can be purchased directly from the cold former or more typically, from a distributor/supplier. For the former, a fabricator may significantly increase their price for lower production runs for custom lengths. Though field cutting LGMF to length is not yet as efficient as cutting wood, it still may be cost effective to consider avoiding a price premium for custom lengths by ordering stock lengths and field cutting them to their desired length.
Many of us in the walls and ceilings industry have long-term, established relationships with the "middle-men" of the construction industry (the distributor/supplier). These lumber yards and supply houses are typically "one-stop" shopping venues which are price competitive with manufacturers, easily accessible and nowadays stock an inventory of commonly used LGMF components such as stud, joist, track, angle, etc., in standard stock sizes and lengths. Typically, they do not cut-to-length as do roll-formers.
Setting the StandardThe American Iron and Steel Institute's "Prescriptive Method" designates ASTM A653 for hot-dip galvanizing of LGMF components. This standard for corrosion protection breaks down as follows:
LOCATION ASTM A653
*Harsh environment G90
Note: for the Prescriptive Method, minimum 33KSI (yield strength) steel is used for uniformity and simplification of the span tables
By weight, LGMF components are 3- to 5-percent zinc-depending on coating thickness, the result of the bath of molten zinc the bare steel is subjected to after being cleaned, pickled and fluxed. This ant-corrosion treatment known as hot-dipped galvanizing, effectively protects the steel from corrosive damage for its entire lifecycle and can itself-like the steel-be recycled at the end of the structure's lifecycle. Corrosion resistance is proportional to the zinc coating thickness. The weight (thickness) of the coatings-as set forth by ASTM A653-are defined as follows:
• G40 and G60: "For members located within building envelope and adequately shielded from direct contact with moisture from the ground or the outdoor climate."
• G90 and heavier (recommended for):
- Additional protection requirements (i.e. oceanfront buildings)
• As compared to G60:
- 50 percent thicker
- Should last 1.5 times longer
Making ContactPressure-treated lumber, used to protect against termites, fungal decay, insects etc., often comes in contact with steel framing components when used as sill and top plates, truss plates, metal connectors and fasteners. Of the three types of pressure treatments used to force preservatives into the cellular structure of wood-waterborne, creosote and oil-borne-waterborne is typically used for building materials. Chromated copper arsenic, which uses arsenic as the preservative, has been banned. As of Dec. 31, 2003, CCA ceased being produced for residential and consumer use after decades of use by the construction industry. Replacing CCA are a new generation of waterborne preservatives, most of which are copper based, such as:
• Alkaline copper quat
• Copper azole (CBA-A and CA-B)
• Ammoniacal copper zinc arsenate
Unfortunately, testing has demonstrated that all of these copper-based preservatives are much more corrosive to LGMF than CCA ever was. This has caused concern in the steel framing, wood fastener, connector and metal plate truss industries. Copper-based pressure treatment manufacturers recommend using coatings greater than G90 but this is not a practical, cost-effective alternative. Rather, there are three more preferable alternatives.
Typically, building codes do not require a wood sill plate beneath steel framing. This eliminates the need for a sill plate-the most common PT /LGMF interface. Also, a wood top plate is not required in LGMF. A load-bearing track carries the axial loads downward.
Use of closed cell foam, heavy plastic, paint or felt paper, prove useful in providing a barrier between PT wood and LGMF. In any event, the integrity of the barrier must be maintained. Regardless of the barrier, fasteners made of metal penetrate the barrier and seat in the PT wood. Self-tapping screws, typically used in LGMF, are subject to corrosion when in contact with PT wood unless properly protected. Therefore, care should be taken in selecting fasteners and manufacturers recommendations followed when this condition occurs. Stainless steel fasteners when in direct contact with galvanized metal, will accelerate corrosion due to the electrolytic action between the two dissimilar materials.
If avoidance and isolation are not practical, then use of sodium borate pressure-treated wood is recommended (check for availability). Tests have shown that SBX is less corrosive to galvanized steel than CCA. Since it is water soluble, it should not be used where it will be exposed to the elements and should be covered during transport and storage on site. As such, SBX PT wood is especially good for use as sill plates, if they are required.
Pressure treatments are often referred to by trade names and have many variations. Two useful industry Web sites are www.awpa.com and www.preservedwood.com.
In part four, we'll continue our familiarization process with LGMF by starting our discussion of the many pros and few cons of LGMF from an environmental perspective.