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Due to the softening at the heat-affected zone (HAZ) in ultra-high strength steels (UHSS), the mechanical properties can deteriorate depending on the steels’ grade, welding heat input, and its associated cooling rate. Hence, a design based on the properties of the base material (BM) may not be appropriate, and there is a need to retain the lost properties of the weldment. This study presents an effort to strengthen butt-welded UHSSs using ultra-high modulus (UHM) carbon fiber reinforced polymer (CFRP) plates. Although the rehabilitation and strengthening of steel structures using CFRPs have been available during the last two decades, they have not been used to strengthen higher grades of steel such as UHSSs. For this aim, a series of tensile tests were carried out on butt-welded UHSSs strengthened with adhesively bonded UHM CFRP plates on both sides of the weldment. Two welding processes, i.e. the gas metal arc welding (GMAW) and laser welding, were implemented separately to join the plates. Microhardness measurements and tensile tests were carried out for the experimental part, as well as finite element (FE) analysis of the joints. The results were also compared with other available methods, i.e. high-frequency mechanical impact (HFMI) treatment and TIG-dressing (tungsten inert gas) of the weld toe that are commonly used to increase the fatigue life of UHSS joints. It was found that CFRP strengthening can not only retain the lost properties of the weldment but can also increase the joint strength up to 32% compared to the BM, and a considerable 50% higher stiffness. In contrast, the other methods (HFMI and TIG-dressing) had no impact on the tensile characteristics of the weldments. Post-print / Final draft

This paper reports the properties of fly ash based geopolymer mortars made with dune sand. The geopolymer mortars of different cation type, namely sodium based (Na), potassium based (K) and a mixed Na/K, were prepared with dune sand (DS) and river sand (RS). The corresponding geopolymer pastes were also prepared. A series of tests including compressive strength, modulus of elasticity, splitting tensile strength, microanalysis (using scanning electron microscopy), porosity (using mercury intrusion porosimetry), sorptivity and air void (using section analysis method) were carried out. The results showed a strong correlation between strength and porosity of geopolymeric materials. The addition of DS had influences on the chemical compositions and physical properties of geopolymer mortars. These influences were dependent on the type of cation. Based on the results of mechanical properties, DS can be utilised as the fine aggregate for the production of geopolymer based construction material. Keywords: Microstructure, Compressive strength, Elastic moduli, Aggregate, Alkali activated cement

This paper examines crack growth associated with carbon fibre reinforced plastic (CFRP) repairs to cracked bridge steels and boron epoxy composite and fibre metal patch repairs to cracked aluminium alloy structures. It is first shown that the da/dN versus ΔK curves associated with bridge steels is very similar to that seen in the high strength aerospace steel D6ac. The importance of 1st ply failure, which was first observed on a boron epoxy repair to the F-111 D6ac steel wing pivot fitting, and how to alleviate this failure mechanism is then discussed as is the common design approach whereby after patching the repair is designed to have a ΔK beneath the ASTM long crack threshold ΔKth. It is shown that crack growth in bridge steels repaired with CFRP patches and in aluminium alloy structures repaired with either boron epoxy or glare patches exhibit a near linear relationship between the log of the crack length and the number of cycles. We then show that crack growth in these repairs can be represented by the same simple master curve relationship that has been found to hold for cracks growing in both operational aircraft and full scale fatigue tests. These findings are important since they suggest that the methodology used by the Royal Australian Air Force to certify structural modifications to operational aircraft may also be applicable to composite repairs/modifications to steel bridges, which are generally experience significantly lower stresses.

Publisher Summary The chapter discusses analytical bond models between steel and normal modulus carbon-fiber-reinforced polymer (CFRP). The first model is an empirical one based on the experimental results. The second one is built upon the model by Hart-Smith for adhesive bonded joints; it is called the modified Hart-Smith model. Two types of joints are discussed in the chapter. One is the double-strap joint between steel plates and CFRP. The other is between steel circular tubes and CFRP. Both models gave reasonable predictions of the effective bond length and the ultimate load-carrying capacities. Adhesively bonded joints have been widely used as structural members or machine elements in engineering industry for many years. In recent years, with the utilization and the growth of advanced polymer composite materials, such joints have been of more importance because thin-sheet materials can be joined by adhesive efficiently.

Advanced composite materials offer remarkable potential in the upgrade of civil engineering structures. The evolution of CFRP (carbon fibre reinforced polymer) technologies and their versatility for applications in civil constructions require comprehensive and reliable codes of practice. Guidelines are available on the rehabilitation and retrofit of concrete structures with advanced composite materials. However, there is a need to develop appropriate design guidelines for CFRP strengthened steel structures. It is important to understand the bond characteristics between CFRP and steel plates. This paper describes a series of double strap shear tests loaded in tension to investigate the bond between CFRP sheets and steel plates. Both normal modulus (240 GPa) and high modulus (640 GPa) CFRPs were used in the test program. Strain gauges were mounted to capture the strain distribution along the CFRP length. Different failure modes were observed for joints with normal modulus CFRP and those with high modulus CFRP. The strain distribution along the CFRP length is similar for the two cases. A shorter effective bond length was obtained for joints with high modulus CFRP whereas larger ultimate load carrying capacity can be achieved for joints with normal modulus CFRP when the bond length is long enough.

This chapter discusses the bending of rectangular hollow structures (RHS). For the RHS in bending, the distribution of strain across the section is assumed linear according to engineering bending theory regardless of the stress state, and the value of the strain at the extreme fibers is proportional to the curvature. At larger curvatures and strains, yielding spreads inwards toward the neutral axis. For the elastic–plastic–strain hardening material, the section yields almost completely and is fully plastic at high values of curvature. Also, the shape of the bending moment diagram can significantly affect the lateral buckling of a beam. The elastic buckling moment is derived for a beam experiencing uniform bending moment, which is the most critical condition for lateral buckling. The chapter explains the concept of lateral buckling.

This chapter presents some recent developments in the design of cold-formed tubular members and connections. The issues discussed are limiting width-to-thickness ratios for concrete-filled tubes and for those subjected to large-deformation cyclic loading, fatigue design using the hot spot stress method, bolted moment end plate connections and plastic design of portal frames. It is well known that in general concrete-filling increases the limiting width-to thickness ratio, whereas the large-deformation cyclic loading decreases the limiting ratio. In this chapter, the limits for unfilled tubes under static loading are compared with the limits for concrete-filled tubes under static loading, limits for unfilled tubes under cyclic loading and those for concrete-filled tubes under cyclic loading. The chapter also illustrates the effect of large-deformation cyclic loading on the limiting width-to-thickness ratios for unfilled tubes. The chapter concludes that concrete-filling increases the limiting width-to-thickness ratios by approximately 50% and large-deformation cyclic loading reduces the limiting width-to-thickness ratios by approximately 50%.

This chapter discusses the welded connections subjected to fatigue loading. The fatigue phenomenon is characterized by a progressive degradation of strength under time variant stresses. The fatigue life of a structure includes a crack initiation phase and a crack propagation phase. The fatigue life of welded connections depends upon the connection detail and applied cyclic loading spectrum. Several methods can be used to determine the fatigue resistance of welded connections. This chapter covers the classification model, which include the classification method, punching shear method, failure criterion method, static strength method, hot spot stress method and fracture mechanics method. The chapter covers the fatigue design of hollow sections and simple connections. Finally, the classification method is based on structural details for different types of connections, which are classified into various detail categories. Each detail category corresponds to a nominal stress range under which a connection fails, with a given probability, after 2 million cycles.

This chapter describes the manufacturing process of cold-formed tubular sections. Cold-formed tubular sections are manufactured in accordance with different standards, for example, the Australian Standard AS 1163 in Australia, EN 10219 (ENV 1992). The chapter discusses the manufacturing process in 5 tables: uncoiling and joining coils; forming; welding; sizing and shaping; and cutting and bundling. The painting offers protection for steel tubes during transport, handling and fabrication. The in-line galvanizing enhance the strength of the steel tubes. The painting step is between the shaping and cutting operations. The in-line galvanizing step occurs before sizing and shaping. Moreover, facture toughness is an important design parameter for cold-formed tubes under dynamic loading. There is an increasing awareness of material notch ductility after the Northridge and Kobe earthquakes. Fracture toughness can be measured using the Notch impact test. Furthermore, limit states design is a design method, in which the performance of a structure is checked against various limiting conditions at appropriate load levels. Finally, the limiting conditions to be checked in structural steel design are ultimate limit state and serviceability limit state.

This chapter deals with the member subjected to concentrated forces. Cold-formed RHS members may be subjected to concentrated forces when used in a Vierendeel truss or in a floor system. The chapter also illustrates that, when the force is applied through a welded brace, the major design concern is web buckling and chord flange yielding. It also explains that, when the force is applied through a bearing plate, the major design concern is the web bearing capacity and the influence of bending moment in the RHS beam. An RHS member subjected to a concentrated force applied through a welded brace is illustrated, where various dimensions are defined. Tests on cold-formed RHS subjected to beating loads found that the external comer radii of RHS introduce load eccentrically to the webs, which is not the case for hot-rolled RHS. Finally, the load carrying capacity of RHS under end-bearing force has found to be less on average than half of that of RHS under interior beating force.

This chapter deals with the tension members and welds in thin cold-formed tubes. Tension members are those structural elements, which are subjected to direct axial loads that tend to elongate the members. They occur as components of trusses, hangers and cables for floors or roofs, in bracing systems, as tie rods, and similar members. The design of tension members is relatively simpler than the design of members in bending or compression. In limit states design, the basic requirement is simply that enough cross-sectional area be provided in order that the factored resistance of the member is equal to or greater than the factored load in the member. The chapter discusses welded connections. This chapter summarizes that most tubular tension members contain connections, which are welded so that the connection strength may limit the tension member strength.

Publisher Summary The chapter presents a nonlinear finite element analysis of steel tubular beams filled with self-compacting concrete (SCC). Constitutive models for steel, compressive concrete, and tensile concrete are carefully chosen to depict material nonlinear properties. In the initial elastic and final plastic stages, the middle span deflections calculated by finite element (FE) method are in good agreement with experiment results. While in the inelastic stage, simulated curves are higher than the experimental ones. The FE analysis gives reasonable prediction of the failure mode, compressive and tensile strains at the tube extreme fibers, and the moment capacity. Based on the study, it is found that the nonlinear finite element method can reasonably predict moment capacities, failure mode, extreme fiber strains, and deformation of the steel tubular beams filled with SCC.

Described are a series of compression tests carried out on concrete filled double skin tubes (CFDST) that consist of two concentric steel cylinders or boxes with the annulus between them filled with concrete. There are four possible combinations in constructing CFDST: 1) square hollow section (SHSS) as both outer and inner skins; 2) circular hollow section (CHS) as both inner and outer skins; 3) CHS as the outer skin and SHS as the inner skin; and 4) SHS as the outer skin and CHS as the inner skin. The strength, ductility and energy absorption of stub columns with the first three combinations were studied. This paper describes a series of tests on stub columns with SHS as the outer skin and CHS as the inner skin. Discussed are the conclusions and observations made based on the tests described in the paper.

Publisher Summary Very high strength (VHS) tubes offer superior strength and enormous reduction in weight. Test carried out at Monash University showed that the average yield stress of the VHS tubes is around 1,350 MPa with an ultimate tensile strength of 1,500 MPa. There is no information on the behavior of longitudinal fillet welded connections in VHS tubes. Existing research on this topic is limited to square and rectangular hollow sections (SHS and RHS) with a yield stress up to 450 MPa. Existing design rules may not be appropriate for VHS circular tubes. This chapter describes a series of tests on longitudinal fillet welds in VHS steel circular tubes. The thickness of VHS tubes varies from 1.6mm to 2.0mm. The weld length (Lyv) of longitudinal fillet welds varies from 15mm to 65mm. Two failure modes are observed, namely failure along the weld and tube failure. The test results are compared with the formulae developed for C450 SHS. The effect of strength reduction in the heat-affected-zone (HAZ) on design is included in the proposed design formulae. Critical weld length formula is also derived in this chapter, which can be used to predict the failure mode.

Publisher Summary Welded thin-walled T-joints made up of square hollow section (SHS) chords and circular hollow (CHS) section braces are tested under static in-plane bending load. The hollow sections are cold formed and have thicknesses less than 4 mm. The SHS–CHS T-joints are used in building the undercarriages and structural supports of equipment and structural systems used in the road transport and agricultural industries. Failure in the SHS–CHS T-joints is observed to occur as a result of chordface yielding. Chord cracking is also observed after large deformations, resulting in a peak load being attained in these joints. In this chapter, load versus chord flange indentation graphs for the SHSCHS T-joints are used to determine the deformation limit that can be used in defining the ultimate strength of the joints. The deformed shape of the chord observed from experimental tests is used to create a yield line model. A formula is derived for the ultimate strength of the SHS chord and CHS brace vierendeel connections based on a plastic mechanism analysis using yield line theory. The ultimate strength determined through the use of the deformation limit criteria is compared to the ultimate strength calculated using the formula obtained from yield line theory.

Static tests of welded thin walled square hollow sections (SHS) and circular hollow sections (CHS) T-joints, under in-plane bending, have been used to produce load-deformation curves from which the maximum linear response moment of connection has been determined. The loads corresponding to the maximum linear response moment and below have been applied during cyclic loading and found to produce a high-cycle fatigue response in the SHS-CHS joints. A formula has been derived for predicting the ratio between maximum linear response moment and static strength as a function of the nondimensionaL parameters. The fatigue data of the welded thin-walled SHS-CHS T-joints was found to lie within the same scatter band as the S-N data of thin-walled SHS-CHS T-joints. An analysis of the S-N data of the thin-walled SHS-CHS T-joints shows that there is a minimal difference in the design curve obtained when the inherent scatter in fatigue data is taken into account.

Publisher Summary Tube-to-plate nodal joints under cyclic bending are widely used in the road transport and agricultural industry. The Square Hollow Sections (SHS) used in these constructions are thin-walled and cold formed, and they have thicknesses of less than 4 mm. The only design curves available for tube-to-plate connections under cyclic bending are given in the Canadian Standard. However the data used for determining these curves are from three plates, which have been welded together, to form the I shape. The category, which has been assigned to tube-to-plate connections under cyclic bending stress, is the lowest S-N curve determined from I-shaped welded plates. Fatigue tests are carried out and the results compared with the existing S-N curve for base plate connections from the Canadian Standard to verify the applicability of the existing S-N curve to tube-to-plate connections. Prior to fatigue tests, static tests were carried out to determine the maximum load below which the connection behaves elastically, thereby determining the loads, which can be applied to determine high-cycle fatigue strength of the tube-to-plate joints. The nodal joints tested consist of a square hollow section, 50 x 50 x 3 SHS, fillet welded to a 10 mm thick plate, subjected to cyclic bending stress.

Publisher Summary This chapter describes a series of static and cyclic axial compression tests on empty and concrete-filled columns made from cold-formed C450 Rectangular Hollow Section (RHS) profiles. It also demonstrates that concrete filling increases the postpeak-load residual strength and reduces the rate of residual strength loss per cycle for thinner RHS columns subjected to overload cyclic situations. The tensile coupon tests were performed according to the Australian Standard AS1391 (SAA (1991)). The 0.2% proof stress was adopted as the yield stress for the cold-formed steel tubes. The average compression strength was found to be 44.4 MPa. A 1000 kN capacity Instron Performance Reckoner actuator was used together with an Instron 8500 controller to apply the cyclic axial loading. Two spring pots were used to measure the axial deformation and the mid-span lateral deformation. Inward folding mechanism with cracks was observed for the empty RHS under cyclic loading. The peak load for one RHS increased by about 58% due to the concrete filing. The increase in peak load for another RHS was about 23%.

Publisher Summary This chapter investigates the dynamic performance of steel lightweight floors. Cold-formed square hollow sections and rectangular hollow sections are used as piers, bearers, and joists. The dynamic performance of a floor at three different stages of construction is determined. The three stages are a standalone floor; a floor with wall frames and ceiling; and a floor with wall frames, ceiling, and roof loading. Three types of tests are performed at each stage: dynamic tests to determine natural frequencies, damping, and vibration modes; static tests to measure maximum deflection of the floor; and subjective tests to obtain human response to vibration. The tests demonstrate the change in the dynamic parameters, the maximum deflection, and the human response at different stages. The static deflection test shows no change in the deflection under 1 kN concentrated load. The damping for the first mode increases from 3.2% to 6.6%, and a similar increase for other modes is observed, as well. This could be explained by the absorption of some of the vibration energy by the wall and ceilings. The roof loading changes the first natural frequency and damping, and correspondingly there is change in the mean of human acceptability.

Publisher Summary Monash University has recently conducted elevated temperature research on the feasibility of using high strength concrete as an infill in steel tubes, without external fire protection. Several columns were eccentrically loaded in compression while subjected nonuniformly to the ISO834 fire curve. The columns tested were all 3600 mm long and consisted of both circular and square sections with various eccentricities, thicknesses, and widths. The thermocouples were positioned in the steel tube prior to concreting. Holes were drilled at various places throughout the cross-section along the tube. Type K fiberglass sheathed thermocouples were strung together, and inserted in the holes at a predetermined depth from the steel tube surface. Two room temperature tests were conducted on the strong floor at Monash University to ensure acceptable performance of the test rig. The load in the CT-Stress bar was slowly increased in stages until failure of the test specimens occurred. The tubes started to deflect as soon as the furnace had been started. The rate of deflection remained at a constant level and then rapidly increased as the column began to fail. The residual deflection was measured once the column had cooled down.

Publisher Summary This chapter describes welded connection tests, which were performed on C450L0 Rectangular Hollow Sections (RHS). The thickness of the RHS sections varied from 1.6 mm to 3.0 mm. Butt welds, transverse fillet welds and longitudinal fillet welds were tested, and the test results are compared with existing Australian and American design formulae. The test results are also compared with the proposed design rules for welds in RHS members derived from the previous research project on C350L0 RHS members. The reliability analysis method is used to calibrate the existing and proposed design rules. Tests are performed on welds in C350L0 RHS members and proposed design rules are given for butt welds, transverse fillet welds and longitudinal fillet welds. The cold-formed Square Hollow Sections (SHS) and RHS sections used in the tests were supplied by Tube makers of Australia Limited. These sections were manufactured by a unique cold forming process, including in-line galvanizing so that the complete penetration butt weld of C350L0 tubes may be designed in accordance with the tension member rule in AISI- 1991 (AISI, 1991) with a capacity (resistance) factor of 0.95 or the tension member rule in AS4100-1990 with a capacity (resistance) factor of 0.90.