The Use of Rectangular Steel Box Members in the U.S. Building Construction Industry

This report summarizes the findings of a study on the use of hollow rectangular steel members in the U.S. building construction industry. Hollow rectangular structural steel sections have been used in structures since the advent of steel construction at the end of the nineteenth century. The first sections were built up by rivetting together plates and angles to form a box. Current practice is to weld together four or more plates, or to weld plates onto a rollwed wide flange (W) section in order to form the built up box section. After World War II, rectangular sections started being manufactured from a single plate by a cold forming process. These cold formed sections are commonly known in the United States as structural tubes. At present both welded built up box sections and cold formed structural tubes are used in U.S. practice.

Built up box sections can be made to almost any dimension. Size and availability are limited by cost, the number of fabricators with the expertise to make these sections, and transportation and erection limitations. Built up boxes used in buildings are usually several feet on a side, with plate thicknesses on the order of 1 to 5 inches, and are typically of all welded construction.

Structural tubes manufactured in the United States generally conform to the ASTM 500 Grade B specificaiton and are produced in standard sizes. Compared to W sections, structural tubes are only manufactured in a relatively limited number of light sizes. Most tubes produced have a weight of 100 pounds per linear foot or less, a maximum perimeter of 64 inches, and a maximum wall thickness of 5/8 of an inch. Eight domestic manufacturers account for 80 to 85% of the tubes used in U.S. building construction.

Hollow rectangular sections, both built up boxes and structural tubes, offer several advantages in building construction. These members are structurally efficient for compression, bi-axial bending, torsion, and long unbraced uniaxial bending. Of these conditions, compression is by far the most common. Box sections also generally occupy less space than W sections. Tubing is often small enough to be placed within a wall or partition, thus eliminating the need for protrusions. The hollow interior of boxes can be used to hide utilities, or can be filled with concrete for greater strength. Finally, when left exposed, tubing has a clean, sleek and pleasing appearance.

The material, fabrication and erection costs of boxes are generally higher than for rolled sections. On a tonnage basis, rolled sections cost anywhere from 50% to 75% less than boxes in the U.S.. In addition, connections to box sections, especially structural tubes, are more difficult and costly than connection to W sections.

Hollow rectangular sections at present comprise less then 10% of the total structural steel used in buildings. The primary applications of built up boxes are columns, often only at corners, in mid to high rise buildings. Structural tubing is primarily used as columns in one story retail, commercial, and institutional structures, and as supports for one story canopy structures. Structural tubing is also sometimes used as columns in two story buildings, braces and as ornamentation.

Connections are a key element that affects both the safety and cost of a structure. Connections to boxes generally require some welding, and sometime require bolts, if only for erection. Connections to built up boxes are often similar to connections to W sections, with the exception of internal stiffeners. Many engineers lack familiarity with structural tubes connections, and consider them difficult and costly. However, many proven connections do exist and qualitative examples are presented in this report.

The availability of heavier jumbo steel wide flange sections, high strength steels, and the increasing use of high strength concrete may cause a decrease in use of the built up box member. However it is anticipated that manufactured tubes will continue to be used in one story buildings, and in exposed architectural applications. Promotion, publicity and the development of design literature will contribute to a gradual increase in structural tubing used in the U.S. building construction industry.