In past 3 decades, there has been a huge development in construction of steel structures related to design concepts, materials, section profiles, manufacture, and erection methods, code provisions, etc. Evolution of Pre-Engineered Buildings (PEB) is one such revolution, which has gained importance and mostly replaced the conventional steel buildings for many applications. PEB uses systems approach by combining elements such as roof panels, wall panels, purlins, girts, main frames, wind bracings etc to act integrally, also rely on standard designs with standard configurations, with each component uniquely profiled to suit design requirements. By using sophisticated manufacturing facilities and engineering software, the PEBs are custom designed to meet the specific customer needs. PEBs are always manufactured in factories and shipped to job sites for assembly assuring a 100% quality structure from single source of supply with on time deliveries.
In recent developments, the concept of PEB is extended to heavy industrial structures and high-rise buildings where mezzanine floors with heavy loads, large and unequal column spacings is required. Composite construction can be adopted in two major elements: floor beams and columns.
Composite Floor Beams
Steel-Concrete Composite floors constitute of steel beam grids. Profiled metal deck sheets are laid over steel beams. Shear connectors are welded to the steel beams through metal deck sheets as per design requirement. In steel-concrete composite beams, the concrete is assumed to take most or all the compression while the steel takes all the tension.
Figure 1: Steel- Concrete Composite Floor
Shear connectors are usually provided to transfer the horizontal shear force across the interface, prevent vertical separation and slip between them is eliminated. Full composite action is achieved when the shear connectors can take the full shear, assuming that either the steel beam is fully plastified or the effective concrete slab is stressed to its maximum capacity in compression, whichever is less. Thus, steel beam and concrete slab act as a composite beam.
Composite floor induces diaphragm action, adding stability to the overall structure resisting severe seismic forces in both mutual directions and provides an economical solution.
Composite Concrete Filled Steel Tube Columns (CFST)
Figure 2: CFST (a) Behaviour of Infill on Envelop (b) Behaviour of Envelop on Infill
Concrete filled steel tubes (CFT) uses the advantage of both steel and concrete. Infill concrete in steel tube delays local buckling, steel tube reinforces the concrete to resist tensile forces. This result into triaxial state of compression in the core and pure hoop tension in the envelope which increases the overall strength and ductility of section.
Figure 3: Typical Concrete Filled Column Sections
Strength of the concrete filled steel tubular columns against direct compression and combined bending and compression is assessed using the international codes CIDECT, ANSI/AISC360-16, AIJ, JSCE, EC4, IS 11384 etc.
Figure 4: Behaviour of Concrete Filled Column Under Axial Compression
Composite High Density Polyurethane Foam filled Aluminium and Light Gauge Steel Tubes
Aluminium and GI Light gauge steel having better properties like, light weight, high strength–to-weight ratio, anti-corrosive, recyclable, and high ductile characteristics, proved a good choice for tube envelop materials. Polyurethane foam is proved to have merits like higher quality, higher strength-to-weight ratio, and improved bond characteristics with envelop and without micro cracking problems.
Experimental investigation on empty and composite high-density (150kg/m3) PU foam filled circular aluminium (D/t of 30.15, 40.20) and square steel tubes (B/t of 83.33, 100,125) is carried out. Improved performance of composite high-density PU foam filled tubes is assessed in axial compression and bending.
Figure 5: (a) Mode Shapes of Empty Vs PU FF CAL Tubes, (b) cut specimen
Study on bond strength between foam and tube wall is also done. An attempt is made to understand their behaviour in tension and torsion. The results are validated through numerical investigation using ABAQUS software. They are also compared with analytical models proposed by codes and relevant past research, wherever applicable. This study revealed that foam filling in tube can enhance the overall strength and behaviour of aluminium and steel tubes, and the interaction between tube wall and foam provide composite behaviour to the filled tube, with requisite bond at the interface.
Figure 6: Failure Modes of Empty and EXP Vs FEM of PU FF Square Steel Tubes
The various material combinations of composite filled tubes are compared for their Strength-To-Weight Ratios in compression, tension, and flexure, to suggest the newly evolving building materials. High strength Steel and Aluminium tube envelops filled with high -density PU Foam proved to have highest strength-to-weight ratio. There is scope for further research in the areas of torsion and combined behaviour especially in joint regions of structural members. The resultant Product can be produced as a PEB component to suit the following applications.
Figure 7: Total foam filled composite frame and wall modular housing unit.
In many fields like modular housing Units in remote areas, a combination of foam filled composite tube column beam framing with foam filled composite wall and roof panel can form an efficient modular housing unit, Bridge Truss members, Transmission Line Tower members, Automobile components, Structural members in High – Rise buildings, Seismic resistant structural bracing system in high rise buildings and also as energy absorbing medium, Structural members for Solar Panel supporting systems, Highway Crash Barriers, Light weight shelters, Wherever weight reduction without compromising the strength is applicable, Wherever transportability and maintenance for corrosion is an issue, these members can be easily adopted.