Steel Solutions Center

12.5. Structural Members and Connections

12.5.1. How does high-rate loading, as produced by blast loads, affect steel properties?

The yield stress of low-carbon structural steel subjected to dynamic loads tends to increase. The ultimate strength is less affected. Elastic modulus remains the same. Steels with higher-static yield stresses achieve a lower percentage in yield-stress increase under dynamic loading, as do weaker steels.

For example, an experiment on structural steel members consisting of mild steel (static yield stress of Fy = 37 ksi) associated with time to yield, showed dynamic-yield stresses in the range of 45 ksi and 50 ksi (an increase in the range of 22 percent and 35 percent). In this series, the time to yield ranged from approximately 1 s to 1 ms, and the fundamental period of the respective structural members was approximately 100 ms. For structural members with fundamental periods of less than 100 ms, test results indicated a dynamic-yield stress of more than 50 ksi.

[1] Air Force Design Manual, Principles and Practices for Design of Hardened Structures. Research Directorate, Air Force Special Weapons Center, Air Force Systems Command, Kirtland Air Force Base, New Mexico, December 1962

[2] Crawford, R. E., Higgins, C. J., Bultmann, E. H. The Air Force Manual for Design and Analysis of Hardened Structures. Air Force Weapons Laboratory, Kirtland Air Force Base, New Mexico, October 1974

12.5.2. What are the common ranges for steel-deck gages and concrete-slab thickness in floors designed for blast resistance? Can lightweight concrete be used in blast-resistant design?

The traditional reinforced-concrete slab on top of steel deck, composite and non-composite, is an efficient blast-resistant floor system. Concrete-slab thickness depends on the magnitude of design-blast pressure and the span between supporting beams. Two layers of reinforcement are usually required to sustain upward and downward loads. Steel deck can effectively prevent concrete fragmentation. Steel-deck type and gage are selected to support construction loads during concrete placement. Lightweight concrete is less effective in resisting blast loads than normal-weight concrete.

12.5.3. In blast-design applications, what can be done to ensure that concrete floor slabs do not separate from structural steel beams when subjected to uplift blast pressures?

To ensure that concrete floor slabs do not separate from structural steel beams, one approach is to weld slab reinforcement to connector studs (in composite floors) or directly to steel support beams. Another option is to design and cast the beams integrally with the slabs.

12.5.4. What structural shape is the optimal choice for beams in blast-resistant floors?

The choice of structural members supporting a slab depends on the load magnitude and where it is expected to act. If the blast load is expected only on the top of the slab, such as a slab over a basement, then either a W-shape or hollow structural section (HSS) is likely to be effective. If the maximum blast load is as likely to act on top of the floor slab as on its lower surface, then both shapes are likely to be effective. When the underside is loaded, the support beams will be loaded both on the bottom and on their sides. The net direct load on the webs of W-shapes is likely to be minimal. Where significant torsion effects are likely, HSS are preferred for their superior torsion resistance.

12.5.5. What types of column sections are preferred in blast-resistant design?

Military manuals for blast-resistant design base procedures on material properties increased by approximately 10% to account for strain-rate effects. Columns designed to resist high blast loads usually have sufficiently small slenderness ratios, and buckling occurs plastically rather than elastically. Also, because dynamic-impulse load tends to suppress the occurrence of buckling, it is conservative to adapt static formulas to the dynamic case. The choice of structural shape will depend on a number of factors, like whether the column is subjected to an axial load, or to flexural and axial load. Since in the latter case the load can come from any direction, it is useful to use a shape that has equal flexural strength in all directions, such as a round or square HSS.