Short-term elongation variation of post-tensioned tendons

Abstract: When tension is applied to steel tendons they elongate in proportion to the tensile force, as predicted by Hooke’s law. This elongation is used by the South African standard on concrete structural works (SANS 2001-CC1) and South African standard specification for road and bridgeworks (COLTO) to determine the adequacy of the force applied in a tendon. The standards prescribe an elongation variation limit of ±6% and an average elongation variation limit of ±3%. According to these standards, if the elongation variation of the tendon falls outside these prescribed limits it must be brought to the attention of the engineer. The scatter of tendon elongation results is often greater than the range prescribed by these standards. This usually requires the contractor to re-tension the tendons at huge financial costs. In most cases the results obtained after re-tensioning are the same. This paper analyses tensioning data obtained from a variety of projects that have been completed in South Africa in recent years. The aim of this investigation is to determine the causes of variation in elongation and suggest adjustments, if any, to the current elongation variation

Column buckling tests of hot-rolled concrete filled square hollow sections of mild to high strength steel

Abstract: This paper examines the behaviour of 29 square concrete-filled steel tubes (CFST) under concentric axial compression. The cross-section of the hot-rolled steel tubes ranges from 60x60x3.0mm to 150x150x4.5mm, and are grouped in three series (Series 1, 2 and 3). For the sections tested, the slenderness ratio (L/b) for Series 1, 2 and 3 ranges from 6.67 to 24.61, 27.00 to 45.00 and 18.00 to 27.0, respectively; and the depth-to-wall thickness (b/t) ranges from 33.87 to 42.86, 13.33 to 33.33 and 22.22 to 50.00, respectively. The slenderness ratio provides a range of columns, from relatively short to slender composite columns. Particular attention is paid to the existence of circumferential strain in the square composite columns. The compressive resistance of the composite columns are compared with the design strengths predicted by the South African standard (SANS 10162-1), as well as the European design standard (EN 1994-1-1). SANS10162-1 is based on the Canadian steel code (CAN/CSA-S160-01). A two-stage equation is proposed to model the results of short columns (Equation 4) and intermediate to slender columns (Equation 5)

Compression tests of Mi Panels

A number of compression tests of Mi Panels are presented. The use of lightweight Mi Panels as building system allows the reduction of construction time and waste in the building environment. The compression tests are performed to simulate the behaviour of the wall, when subjected to vertical loads from the roof. Two LVDTs were placed at mid-height of the panels during the compression tests, to record the lateral displacement. Mi Panels failed by fracturing into two parts at mid-height. No cracks in the panel were observed before overall flexural buckling failure. The tests showed that the panels are capable of carrying the required loads, and can be used in place of brick walls

Eaves connections of double-bay portal frames with staggered single channel cold-formed rafters

Abstract: Experimental tests were performed to evaluate the structural performance of the eaves region of double-bay portal frames with staggered cold-formed rafters. This region represents the distance from the point of maximum moment at the eaves joint to the point of zero moment (contraflexure) in the rafters and columns respectively. The column and two rafter members are formed from single cold-formed channel sections, which are bolted back-to-back at the eaves joint. In order to simplify the connections the rafters are connected directly to the back of the web of the column at different levels. Particular attention was focused on the failure modes of the structures, the strength (resistance to moment) of the connection, the momentcurvature performance and the connection’s ability to form plastic hinges. In all the tested structures, the column failed by local buckling in the compression flange and web, between the upper and lower rafters. The moment-curvature graphs proved that plasticity could not be achieved in these connections

Gusseted rafter-to-column connections of double-bay single channel portal frames

Abstract: Double-bay portal frames have several advantages over two single portal frames positioned adjacent to each other, however the connecting system can be a challenge, especially if the portal frame is formed from single cold-formed channels. The portal frames described in this paper consists of a column and two rafter members, formed from single cold-formed channels sections, and bolted back-to-back at the eaves joint through a gusset plate. Tests were performed to evaluate the structural performance of the eaves region of double-bay portal frames. The eaves region represents the distance from the point of maximum moment (eaves joint) to the point of zero moment (contraflexure) in the rafters and columns respectively. The structures failed by local buckling in the compression flange and web of the rafters, outside the joints. The moment- curvature graphs proved that plasticity could not be achieved in these connections

Angle Cleat Base Connections

Tests, performed on base connections fabricated from cold-formed channels and hot-rolled angle cleats, are presented in this paper. This research is part of an on- going research to develop portal frames made out of cold-formed steel. The base connections are subjected to an axial load and moment. Hot-rolled angle cleats are used to prevent premature failing of the base connections. Several loading configurations are considered and these are dependent on the eccentricity of the load. In all the tests the cold-formed channels failed by local buckling. A significant amount of bearing distortion was observed in the heavily loaded flange. The use of bolted angle cleats a llows for a simple connection to be developed, which can result in significant cost savings within the steel construction industry

Strength and ductility of simple supported r/c beams retrofitted with steel plates of different width-to-thickness ratios

Abstract: The concept of strengthening reinforced concrete beams using epoxy-bonded steel plates (EBSP) is a well-known solution in structural engineering, however, there is little information about the effect of the width-to-thickness ratio of steel plates on the behaviour of steel-concrete composite beams. This paper presents an experimental study of the flexural behaviour of under-reinforced concrete beams, strengthened in flexure by externally-bonded steel plates (EBSP) of varying widthto- thickness ratios. A total of 23 reinforced concrete beams were tested; 6 beams in Series 1 tests and 17 beams in Series 2 tests. One beam in Series 1 tests and two beams in Series 2 tests were regarded as control specimens, whilst the remaining beams were strengthened with steel plates of different width-to-thickness ratios. In each group, the width of the bonded steel plate varied from 75 mm to 175 mm, in increments of 25 mm. The beams were tested as simply supported, under two-point static loadings until failure. From the experimental results, it was observed that the externally bonded steel plates led to substantial increase in flexural stiffness, which resulted in an increase in the capacity and cracking load of the strengthened beams and a decrease in vertical deflections and crack-widths, compared to the control beams. It was also found that the width-to-thickness ratio of steel plates as low as 12.5 can promote flexural yielding and extensive ductility in strengthened beams

Internal eaves connections of double-bay cold-formed steel portal frames

Abstract: Double-bay or multi-bay portal frames of hot-rolled steel have been in existence since the development of the plastic analysis theory, and the behaviour of the eaves connections of these frames are well established. With the increasing use of cold-formed steel in portal frames, it is necessary to develop connecting structural systems that are favourable to these elements, and to understand the behaviour of these connections. The purpose of this investigation is to develop internal eaves connections of double-bay portal frames of cold-formed steel channels, with the main frames members connected back-to-back. In double-bay portal frames, the columns of two single-bay portal frames, positioned adjacent to each other, are replaced by one internal column. Two internal eaves connections were developed and investigated, and are referred in this paper as Eaves Connection Type 1 (ECT-1) and Eaves Connection Type 2 (ECT-2). Tests were then performed to determine the capacity and failure mechanism of these connections, including the members forming these connections. Although the failure mechanisms included local buckling of the compression zone of the web and flange of the channels, and bolt-bearing deformations, all frames finally failed by local buckling of the web and flange of the channels. The moment-curvature graphs show that enough plasticity could not be achieved in both connections

Flexural tests of Mi Panels

Two series of flexural tests on 2700x600x75 mm Mi Panels are presented. The inner core of these lightweight panels is made from mixing polystyrene beads called Neopor, high strength cement, and water. A chemical is used as binding agent to bond the inner core to the external fibre cement sheets of 4.5 mm thickness. The experimental programme consists of two series of tests with different loading conditions; the first series has line loads at one-third point from each end support and the second series has line loads, at quarter and three quarter span of the panels. All flexural tests are performed to simulate the behaviour of the wall, when subjected to wind loads. Although the panels failed by fracturing, the tests showed that the panels are capable of carrying the required loads

Use of angle cleats to restrain cold-formed channels against lateral torsional instability

Abstract: It is common practice in the steel construction industry to restrain members that largely in flexure and torsion using a combination of angle cleats, connected at the top flange, and fly-bracings. This system is complicated and expensive, especially when used to restrain channels in bending. This paper investigates experimentally the use of angle cleats, connected to the webs of both the purlin and the channels, as a restraining system. Pairs of channels were subjected to a two point loading system in order to simulate a distributed load. Variable in the tests include the unbraced length between the two-point loads and the size of the channels. Failure of the channels occurred by lateral torsional buckling and catastrophic distortional buckling of the intermediate unbraced length. Tests showed that the purlin-cleat restraining system is able to resist lateral torsional buckling of the channels, and that this system can be used without any fly bracing. Distortional buckling was the final failure mode, and it occurred at moments less than the predicted lateral-torsional buckling moment of resistance. Distortional buckling is more critical in frames with shorter unbraced lengths and thicker channels