The structural steel and its installation at the job site is one of the major construction activities that have been ongoing in recent weeks. The steel pieces had been previously delivered to the job and were being stored in available open areas on the very space constrained site for the Visitor Center. Two groups of steel have been placed first. The vertical steel columns surrounding the high bay Gallery area and the horizontal beams that will support the raised floor slab in the same high bay Gallery area.
Wielding tons of steel into place in the field is not done as precisely as when it is drawn wielding electrons in the computer 3D model. It’s easy to get carried away with laying things out to within tenths of an inch in a computer drawing. But we have to be realistic and realize that assembly to that level of exactness in the field is another matter. We are talking about lifting vertical steel columns that weigh hundreds of pounds which are dangling from an outreached crane boom on a cable that is swinging as the crane operator works to place the unwieldy passenger onto a set of 4 bolts that have been cast in the concrete weeks earlier and they themselves may be off by some fraction of inches. Some of the columns weigh as much as 650 pounds and the larger beams can reach upward of 1000 pounds.
A degree of accuracy, however, is still required if the various pieces of steel are going to fit together to create a superstructure that is true and plumb. There are acceptable tolerances in the field that allow the contractor to some extent room for the challenges presented in putting together a full scale model of the building in what are sometimes challenging field conditions.
The structural steel is specified in FSB’s Section 051200 Structural Steel Framing. There are a number of industry standards referenced that are already established for the steel industry so that professionals don’t have to reinvented them from scratch every project. The granddaddy standard is AISC (American Institute of Steel Construction) and their bible is the Code of Standard Practice for Steel Buildings and Bridges. The code contains standards for definitions, shop drawings, materials, shop fabrication and delivery, erection, quality assurance and Architectural Exposed Structural Steel (AESS). As with all of these standards, you can find them on the internet. One my favorite tabs on their site is “e-pubs and free pubs.” Unfortunately, it did not lead me to any local watering holes of interest.
Within the AISC code there are some tolerances of interest:
- Anchor bolts (which are cast into the concrete in our case) within groups shall be within 1/8” center to center of the dimensions on the embedment drawings and the center of the group shall be not more than ¼” off the column grid line.
- For an individual column shipping piece, the angular variation of the working line from a plumb line shall be equal to or less than 1/500 of the distance between working points (i.e. the top and the bottom). Our tallest columns are in the Gallery area and are around 25 feet in height. Do the math to see just how out of plumb is acceptable.
Other standards referenced are AWS, RCSC, and ASTM (you can research them on the web). There is a significant amount of exposed structural steel especially in the Gallery area and at the exterior soffits. We want the exposed steel to look particularly nice so we reference the AESS (Architectural Exposed Structural Steel) standards in the AISC code which defines the level of pretty.
There is also a LEED requirement specified in that there shall be an average recycled content of steel products so that the post-consumer recycled content plus one-half of pre-consumer recycled content is not less than 25 percent. I will have to check on how many points towards LEED Gold that is getting us.
The base plates for our project come fastened to the columns when delivered from the fabrication plant. The typical Visitor Center column base plate has 4 holes to line up with 4 anchor bolts that have been cast into the concrete. The columns with their base plates are held above the concrete by nuts that are placed above and below the plate at each anchor bolt. These sets of nuts allow the contractor to adjust the plumbness of the columns to get them spot on vertically. Once the columns are plumb, non-shrink grout is packed into the space between the base plate and concrete. The grout provides a full contact area for the column base to bear upon and thus transfer of its load evenly into the concrete.
Next we’ll look at the placement of the beams over the mechanical crawl space that creates the elevated slab of the Gallery area floor.