Full-Scale Load Testing of Steel Strutting System. For. Yongnam Holding Limited

Transcription

1 Report on Full-Scale Load Testing of Steel Strutting System For Yongnam Holding Limited Prepared by Dr Richard Liew PhD, MIStrutE, CEng, PE(S pore) Department of Civil Engineering National University of Singapore 08 December 006 (This Report Contains 48 Pages)

2 1 CONTENTS Page 1 Modular Strutting System 3 Test Objectives 3 3 Test setup 4 4 Instrumentation 4.1 Displacement measurements 4. Stress measurements Test Procedure 7 6 Mechanical Properties of Used Steel 7 7 Test Results 7.1 Axial load-displacement relationship 7. Applied load- vertical displacement relationship 7.3 Applied load- lateral displacement relationship Stresses in the main struts Force distribution in the ties and lacing members 7.6 Test Observations and failure modes Comparison with Code s Design Capacity 11 9 Conclusions 1 References 13 Table 1: Loading intervals and observation 14 Table : Table : Initial out-of-straightness deflection (downwards deflection) of struts before loading 14 Table 3: Locations, alignment and direction of sensors 15 Table 4: Axial load capacity of laced strut -comparison of predicted results with test 17 result Figure 1: Test and instrument layout 18 Figure : Load application using hydraulic actuators and high-strength strands 19 Figure 3: S Strut with the loading tendons in position 0 Figure 4: Base support allowing horizontal movement 1 Figure 5: Base support preventing lateral translations. 1 Figure 6: Transducer D3 measuring axial displacement at the real end of the strut. Figure 7: Vertical displacement transducers D5 and D6 at the splice joint of strut. 3 Figure 8: Details of coupon specimens and extracted location 4

3 Figure 9: Applied load versus axial displacements 5 Figure 10: Applied load versus vertical deflection 6 Figure 11: Vertical deflection profile of the strut 7 Figure1: Load versus lateral displacement at mid-length 7 Figure 13: Stresses on top and bottom flanges of the strut sections at mid-length 8 Figure 14: Axial forces in the Channel Sections 8 Figure 15: Applied load versus axial forces in the lacing members 9 Figure 16: Buckling of the main struts and lacing members after collapse 30 Figure 17: Buckling of the mid-length strut section 31 Figure 18: Another view showing the buckling of the laced strut 3 Figure 19: Applied force versus lacing forces comparison with EC3 approach 33 Appendix A: Certificate of Calibration for Digital Pressure Gauge 34 Appendix B: Coupon Test Results 35 Appendix C: Design Approaches for Calculating Buckling Resistance of Build-up Members 37 C.1 BS5950:Part1:000 Approach C. BS EN :005 Approach C.3 EC 3 approach to evaluate the shear force of the axially loaded laced struts C.4 Example

4 3 Full-Scale Load Testing of Steel Strutting System 1 Modular Strutting System Yongnam has developed a proprietary Modular System of components that may be assembled to provide a structural strutting system appropriate for the majority of excavation support requirements. The system comprises of: Laced universal beams of various cross-sections in modular lengths. Single and double waler beams in various lengths, intermediate supporting beams, king posts, bracing and waler support brackets. Various strut to waler joints have been produced to suit to suit the site conditions as required. The Yongnam strutting system has been used in many basement construction and civil engineering excavation works including high-rise and mass rapid transit projects in Singapore, Hong Kong and the Middle East. There are, however, a number of questions asked with regard to the performance of the modular strutting system, such as the performance of the splice joint, strength of the reusable materials, force distributions in the lacings and channels between the main struts. In response to these questions, a full scale load test of used modular strutting system spanning about 0m was carried out in the premise of Yongnam Holdings Limited, located at 51 Tuas South Street 5, Singapore , on 8 November 006. This report provides the test results and their interpretation with regard to the performance of the strutting system in comparison with the code s predicted results. Test Objectives The objectives of carrying out full scale test on the laced strut system are: To determine the maximum load capacity of the strut system and compare against the design capacity To investigate the performance and to identify the possible failure mode of the strut system To ascertain the maximum induced forces on lacing members and compare them with results predicted by the design equations given in BS5950:000:Part 1 and Eurocode 3 To investigate the force distributions on the lacing members along the length of the strut

5 4 To study the load-displacement behaviour of the strut system To determine the strength of re-usable strut materials and their implication to structural safety. To accomplish the objectives, the strutting system was load tested to failure to establish its buckling capacity and failure mode. The failure load is compared with the code s predicted load to gain insight to its ultimate strength behaviour. The load-displacement relationship and internal force distribution in the main struts and lacing members were monitored on-line during the test. 3 Test Setup The test specimen consists of two universal beams, UB 610x34x195 kg/m, inter-connected by diagonal laces of equal angle section 80x80x9.66kg/m, and ties of channel sections C54x76 and C54x90 as shown in Figure 1. The length of the strut is 19.6m consisting of three modular segments of lengths 3.8m, 1m and 3.8m. The three strut segments were connected by using 8 number of M4 Grade 8.8 bolts via flush end plate connections. The two end-strut 3.8m segments was about 4 years old and they were used in KPE strutting works for project C-41 and circle line MRT project C-853. The central strut segment of 1m length was more than 6 years old and had been used in Hong Kong MRT project and subsequently deployed to LTA projects C-41 and C-851 (Note: This information was provided by Yongnam Holdings on requested by the author). The laced strut specimen was arranged in a horizontal position and connected to waler beams at both ends. Loads were applied horizontally from the ends of the struts through the waler beams using three hydraulic jacks of maximum capacity of 800 tons each (Figure ). The loading system utilizes high tensile strands running along the sides of the strut and mounted onto the walers at the ends applying a compressive loading on the struts (see Figure 3). A third party contractor, VSL Singapore Pte Ltd, was engaged to supply and operate the hydraulic jacks. The jacks are linked to share equal hydraulic pressure during load application using a hydraulic pump. Each hydraulic jack was connected to 31 numbers of 0.6" diameter super low relaxation strands with the following properties: Nominal Diameter of the strand: 15.7mm Nominal Area: 150mm Yield Strength: 1500 MPa Min. Breaking Load: 65 kn

6 5 The load was controlled by using a digital pressure gauge which was calibrated to ISO The certificate of calibration is attached in Appendix A. Total applied load was manually communicated to the data acquisition system at predetermined loading intervals listed in Table 1. The two ends of the strutting system were bolted to waler beams which were supported by short columns. The column bases were welded to end-plates. One end of the column base was seated on a smooth concrete pad allowing free translation in the longitudinal direction of the strut as shown in Figure 4. The other end of the column base (where the hydraulic jacks were mounted) was bolted down to the concrete pad preventing any lateral movement (see Figure 5). The bolt connection to the concrete pad offered very little resisting against overturning moment. The two end boundary conditions of the strutting system simulate a pinned support and a roller support condition. Before loading, the initial vertical deflection of the struts was measured and the results are shown in Table. The strut has initial out-of-straightness with maximum vertical deflection of about 19 mm at the spliced joint. This is slightly less than the maximum out-of-straightness tolerance of span length/1000 (or 19.6mm) for column design as in BS5950:Part1:000 [Ref. 1]. The initial out-of-straightness in the lateral direction of the strut was found to be very small. This was due to the present of lateral bracing members which controlled the straightness of the two struts in the lateral direction. 4 Instrumentation The test specimen was instrumented with strain gauges and displacement transducers to determine the stresses in the strut and the lacing members and the lateral and vertical displacements. The instrumentation layout plan is shown in Figure 1. A summary of the sensor locations and their sensing direction is given in Table Displacement measurements Displacements were measured using spring mounted strain gauge based displacement transducers. The transducers have the following maximum travel distances (see Figure 1):

7 6 Displacement Transducers D1 and D Maximum Travel Distance 50 mm D3, D4, D5 and D6 00 mm D7 and D8 100 mm The measurement accuracy of the displacement transducers is ±0.0mm. The instruments were connected electrically to a data logger of resolution 1 micro-strain and with measurement accuracy of ±0.05% of reading. The axial displacement was measured by taking the average readings from the displacement transducers mounted at mid-height of the strut at positions D1, D, D3, and D4 (see Figure 6). The vertical deflections at the mid-span of the strut were measured using strain gauge based displacement transducers mounted at positions D7, and D8. Additional vertical deflections were measured at the splice joints of the laced strut at 3.8m from both ends of the test specimen as shown in Fig. 7. Lateral displacements were measured at the mid-length of the strut. The transducers were mounted at positions D11, D1, D13, and D14 in Figure 1 measuring the lateral displacements of the top and bottom flanges of the struts at mid-length. 4. Stress measurements Strain gauges were attached at the mid-length section of the lacing members along one-half of the strut. They are indicated as L1 to L16, comprising 8 top and 8 bottom laces. Each lace was instrumented with two strain gauges mounted longitudinally on each leg of the angle lace. The first 4 laces nearest to the end were instrumented with 4 strain gauges with two gauges at each leg, labelled as -1 to -4, to monitor their bending stresses. Strain gauges were attached to the mid-length of the strut to measure the compressive and bending stresses at the top and bottom flanges of the strut. These comprise two strain gauges

8 7 mounted on the top and bottom flanges at the mid-length section of each strut member, labelled as S1 to S4. Strains gauges were attached to the channel tie sections located at the front end of the strut and at the splice joints location, indicated as C1 to C6. Each channel section was instrumented with three strain gauges to measure the average stresses acting on the channel section. Data collection was triggered manually when the applied load reached the predetermined values. The scanning rate of the data logger is approximately 0.08 second per channels. 5 Test Procedure The load procedure was divided in 3 stages: 1 Preload cycling from 0 to 500 tons Repeated loading up to 500 tons and unloading without recording data. The bolts at the splice joints were tighten at preload of 500 tons. This was necessary to remove any lack of fit in the test specimen. Load up to service load of 700 tons The service load of the test specimen is about 700 tons (design load capacity is about 1000 tons with factor of safety of 1.4). The service load indicated in Yongnam technical brochure for this laced strut is about 750 tons. A quasi-static load was applied at an incremental load of 100 tons up to 700 tons. At every load intervals, deflection and strain gauge readings were taken. 3 Loading from 700 tons to failure Without unloading, the load was monotonically increased at intervals of 100 tons up to 1000 tons and thereafter at 50 tons increment till failure. At each load increment, deflection and strain gauge readings were taken. Observations were made of the strut behaviour and failure mode. 6 Mechanical Properties of Used Steel Mechanical properties of the steel strut components were determined from coupon specimens extracted from the used steel similar those in the test specimens. Details of the coupons and

9 8 sampling location are shown in Figure 8. Coupons were cut longitudinal to the axis of the member and machined to dimensions. In total, 8 coupons were extracted from the twin struts, 4 coupons from the angle laces and coupons from the channel tie sections. The coupons were axially loaded in tension using universal testing machine with load cell measurement accuracy of ±1%. Strains in the coupon were measured using extensometer of measurement accuracy ±1%. Coupon test results are shown in Appendix B, Table B.1. The yield strengths of the steel components are summarised in the table shown below: Strut Section UB 610x34x195 kg 1m strut segment (6-year old) 3.8m strut segment (4-year old) Min. yield strength from tests 397 N/mm 389 N/mm Design strength for S355 steel in BS5950 Lacing Members 345 N/mm Angle 80x80x9.66kg/m Min. yield strength from tests 31 N/mm 341 N/mm Design strength for S75 steel in BS N/mm Channel tie members 50x90x5.5kg/m 54x76x8.9 kg/m Min. yield strength from tests 341 N/mm 358 N/mm Design strength for S75 steel in BS N/mm The yield strengths of the steel components are not affected by the age, and their actual yield strengths are higher than the nominal strength specified in the BS standards. The dimensions of the structural sections were measured and compared with the nominal values specified in the section table as reported in Table B in Appendix B. There is no evidence to suggest that the cross-section areas of the re-used sections were reduced due to repeated use of the steel strutting system.

10 9 7 Test Results 7.1 Axial load-displacement relationship The axial load displacement relationship obtained from the test data is almost linear up to about 1000 tons as shown in Figure 9. The axial load displacement at service load of 700 tons is about 13 mm. Thereafter, axial displacement of the left strut increased faster than the right strut with an average axial displacement of about 35mm at 1400 tons. Thereafter, axial deflection increased rapidly and the strut failed at 1438 tons. Failure was characterised by the buckling of the main strut members bending about their major axis (x-x axis) causing large deflection in the downward direction. 7. Applied load- vertical displacement relationship Figure 10 shows the applied load versus the vertical displacement curves taken at the mid-length section and at the splice joint positions of the strut. The mid-length vertical deflection is about 14mm at the service load of 700 tons. When the applied load exceeded 1000 tons, the lateral deflection increased in a nonlinear manner; the maximum measured vertical deflection is about 78 mm at applied load of 1400 tons occurred at the mid-length of the strut. The vertical deflection increased rapidly when the load approached the maximum capacity of 1438 tons. The strut buckled in the downward direction until the mid-length sections of the strut touched the ground. The deflection profile of the strut was measured at various load stages and the deflected curves are plotted in Figure 11. There is no slope discontinuity due to the present of splice joints along the strut length. No opening up of splice joints was observed up to the load of 1400 tons. 7.3 Applied load- lateral displacement relationship Figure 1 shows the applied load versus the lateral displacements measured at the mid-length section of the strut. The lateral deflection of the strut was very small at the service load level (less than mm). This increases to about 8.4 mm just before failure at 1400 tons applied load. The bottom flange of the Universal beam section defected more than the top flange under the increased load. The maximum difference in lateral deflection is about 4mm at 1400 tons of applied. The lateral deflection is considered to be very small as compared to the vertical displacement, indicating that the lacing members were effective in preventing buckling in the lateral direction (i.e, y-y axis).

11 Stresses in the main struts Figure 13 shows the applied load versus the average stresses taken at the top and bottom flanges of the main strut sections at the mid-length. The compressive stresses at the top flanges are higher than those at the bottom flanges because of the combined axial and bending stresses at the mid-length of the strut. It is noted that at 1400 tons of applied load, the strut sections at the mid length are fully in compression, indicating that the moment was not large enough to induce tensile stress in the strut. In other words, the strut remained in compression up to 1400 tons of applied load. Slight yielding was observed at the top flange fibres at 1400 tons of applied load. Significant yielding is expected beyond 1400 tons and up to failure load of 1438 tons since large displacement occurred suddenly and cross section distortion occurred as shown in Figs. 17 and Force distribution in the ties and lacing members The horizontal ties (channel members) experienced very small axial force of about 5 tons at the applied load of 1400 tons as shown in Figure 14. Larger axial force was observed at the top channel member near the supported end of the strut than those at the splice joints. Figure 15 shows the axial force distribution in the lacing members along the half length of the laced strut. Again the axial forces in the lacing members are very small. When the applied load is 700 tons (service load), the maximum lacing force is 4.4 tons which is about 0.63% the applied strut load. When the applied load is 1400 tons, the maximum lacing force is about 7.5 tons, which is about 0.54 % the applied strut load. The axial forces in the ties and lacing members are considered to be small as compared to the requirement in BS5950:Part1:000 of.5% of the axial force in the member, divided amongst the transverse lacing systems in parallel planes. Detailed comparison with code s requirement is discussed in Section Test Observations and failure modes Figures 16 to 18 show the deformed modes of the laced strut after collapse. The maximum load capacity of the laced strut is 1438 tons. The failure is due to the buckling of the two main struts buckled about their major axis (x-x direction). The large deflection caused yielding and distortion of the universal beam section nears the mid-length of the strut as shown in Figures All the

12 11 bolted connections (in the splice joints, the ties and the laces) remained intact without any sign of failure. The lacing members and their connections were adequate and effective in preventing lateral buckling of the struts (i.e, y-y axis direction). 8 Comparison with Codes Design Capacity The design of axially loaded laced struts, compared against that of conventional axially loaded struts with web plates, should includes strength, stiffness and overall stability verifications, and furthermore, the verification of local stability of single component should also be carried out and the design check for all bracings (lacings) is necessary. The design of laced strut is provided in BS5950:Part1:000 Clause [Ref. 1] and in Eurocode 3-1-1:005 Clause 6.4 for built up compression member [Ref. ]. The capacity of the laced strut is controlled by (1) global buckling of the two main struts about the major axis bending (X-X global), () global buckling about the y-y axis of the compound strut (Y-Y global), and (3) local overall buckling of I-beam between the two laced points, and (4) buckling of lacing or failure of connection. The comparison of axial capacity based on codes predicted values and test result are shown in Table 4. Detailed calculations of buckling capacity of laced strut using BS5950:Part1:000 and EC3 (005) are given in Appendix C. The maximum load predicted by EC3-1-1:005 is 984 tons, 1147 tons and 1314 tons assuming effective length of 1.0L, 0.85L and 0.7L, respectively. The capacity is controlled by global buckling about the X-X axis. This is consistent with the predicted failure mode and the predicted buckling capacity is conservative compared to the test failure load of 1438 tons. The maximum load predicted by BS5950:Part1:000 is 995 tons, 1168 tons and 130 tons assuming effective length of 1.0L, 0.85L and 0.7L, respectively. If a shear force of.5% axial load is assumed, the capacity of the strut is limited by the buckling capacity of lacing member which gives a value of 1131 tons. However, failure of lacing member was not observed in the test before buckling of the main struts. The BS5950:Part1:000 approach is conservative as compared to the actual failure load of the strutting system. Clause (i) of BS5950 Part 1:000 states that The lacings and their connections should be designed to carry the forces induced by a transverse shear at any point in the length of the member equal to.5% of the axial force in the member, divided equally amongst all the transverse lacing systems in parallel planes. At the applied load of 1400 tons,.5% of this load would indicate 18 tons of shear force. However, the measured maximum axial forces in the channel and angle section

13 1 are only 5 and 7.5 tons, respectively. Therefore BS5950 recommendation is found to be too conservative when compared to the measured forces in the lacing members in the test. Eurocode 3-1-1:005, on the other hand, provides a more reasonable interpretation of the transverse shear force acting on the lacing members. The shear force is depending on the maximum bending moment (i.e, axial force and lateral deflection) at the mid-length and the length of the strut. Appendix C.3 provides the derivation of the design shear force formula in Eurocode 3-1-1:005 based on second-order analysis of built-up compression. The predicted test result is compared with those obtained from tests in Figure 19. EC3:005 approach predicts a maximum axial force in the lacing member as 8.5 tons compared to the test result of 7.5 tons. The comparison is found to be reasonable. In summary, the strut capacity predicted by EC3-1-1 and BS5950:Part1 are conservative compared to the test result because: (1) Boundary conditions may be partial restrained against rotation rather than pin-ended as assumed in the design calculation. However, it should be noted that the bolts connecting to the column base to the concrete pad offered very little resistance against overturning moment. It is therefore reasonable to assume pin-ended boundary condition. () The actual measured yield strength of grade S355 steel strut section is about 400 MPa which is greater than the nominal yield strength of 345MPa in BS5950 (16mm<t<40mm) and 355MPa in EC3 (t<40mm); (3) The lacing members are assumed to resist the total shear force in design (bending about Y-Y axis). Actually, part of shear force was resisted by the I-beam sections. Therefore, the lacing force is much smaller than that predicted by the codes. The actual lateral deflection of the strut was very small and therefore the induced second-order moment and the corresponding shear forces are smaller than those predicted by the codes. 9 Conclusions The following conclusions may be derived from the full-scale testing of the laced strut system: 1) The predicted failure load of the strut based on BS5950:Part1:000 is 995 tons. Based on the design safety factor of 1.4, the working load is 710 tons. The actual collapse load of the test specimen is 1438 tons. The factor of safety against the design working load is about.0. The load capacity predicted by the codes is found to be on the conservative side. ) The ultimate load was not affected by the age of the strutting modules. Coupon tests show that old and reused struts do not diminish in strength over the years (it means old struts can

14 13 continue to be reused, if their thicknesses are not eroded due to sand blasting and re-painting). 3) All the connections were robust and adequate as the failure was due to the overall buckling of the main strut about the major axis (X-X axis) with plastic hinge formed at the mid-length of the members. The load-carrying capacity and the load-displacement relationship of the modular strutting system was not affected by the splice joint details. 4) Maximum axial force in the lacing members was approximately 0.54% of the applied strut load. The shear force of.5% of axial force assumed in BS5950:Part1:000 is too high. Eurocode 3 provides a better estimation of the shear forces for designing the lacing members. The laced members and their connections to the main struts were found to be adequate. Failure was due to the buckling of the main struts and was not governed by the buckling of the lacing member. References 1. BS5950:Part 1 (000), Structural use of steelwork in building, Part1: Code of practice for design rolled and welded sections, British Standards Institute.. Eurocode 3 Part 1-1 (005), Design of steel structures: Part 1-1 General rules and rules for building, British Standards Institute.

15 14 Table 1: Loading intervals and observations Total Applied Loads (ton) Observations Maximum loading for preloading cycles Design load of struts (750ton), no major deformation noted Visible deflection at mid-span Adjustment of hydraulic actuators, unitisation of actuator stroke 1300 observed reduction in strains and displacements Sudden buckling and collapse of strut, end of test Table : Initial out-of-straightness deflection (downwards deflection) of struts before loading Distance from the front end 3.8 m 9.8 m 15.8 m Top flange deflection mm 9 mm 1 mm Bottom flange deflection Average deflection 4 mm 8 mm 16 mm 3 mm 8.5 mm 19 mm

16 15 Table 3: Locations, alignment and direction of sensors Measuring Element Location Position Direction Ref in Ref in Data file Fig. Axial Front Waler Right of strut Centre line of load pts Outwards, +ve=comp strut D1 0 // Left of strut Centre line of load pts Outwards, +ve=comp strut D 1 Rear Waler Right of strut Centre line of load pts Outwards, +ve=comp strut D3 // Left of strut Centre line of load pts Outwards, +ve=comp strut D4 3 Vertical Strut Right Joint nearer Top channel of side strut Downward, +ve=defl down D5 4 Front Strut Left // Top channel of side strut Downward, +ve=defl down D6 5 Strut Right Mid-span Top flange of main strut Downward, +ve=defl down D7 6 Strut Left // Top flange of main strut Downward, +ve=defl down D8 7 Strut Right Joint nearer Top channel of side strut Downward, +ve=defl down D9 8 Rear Strut Left // Top channel of side strut Downward, +ve=defl down D10 9 Lateral Strut Right Mid-span Top Flange, 100mm ext Towards left, +ve=sway left D11 10 // // Bottom Flange, 100mm ext Towards left, +ve=sway left D1 11 Strut Left Mid-span Top Flange, 100mm ext Towards right, +ve=sway right D13 1 // // Bottom Flange, 100mm ext Towards right, +ve=sway right D14 13 Strain Strut Right Mid-span Top Flange, top surface Middle of right outstand S // // Top Flange, top surface Middle of left outstand S1-15 // // Bottom Flange, bottom surface Middle of right outstand S-1 16 // // Bottom Flange, bottom surface Middle of left outstand S- 17 Strut Left Mid-span Top Flange, top surface Middle of right outstand S // // Top Flange, top surface Middle of left outstand S3-19 // // Bottom Flange, bottom surface Middle of right outstand S4-1 0 // // Bottom Flange, bottom surface Middle of left outstand S4-1 Strain Channel #1 Mid section Flange nearer front Top channel C1-1 // // Middle of web // C1-3 // // Flange nearer rear // C1-3 4 Channel # Mid section Flange nearer front Bottom channel C-1 5 // // Middle of web // C- 6 // // Flange nearer rear // C-3 7 Channel #3 Mid section Flange nearer front Top channel C3-1 8 // // Middle of web // C3-9 // // Flange nearer rear // C Channel #4 Mid section Flange nearer front Bottom channel C // // Middle of web // C4-3 // // Flange nearer rear // C Channel #5 Mid section Flange nearer front Top channel C // // Middle of web // C5-35 // // Flange nearer rear // C Channel #6 Mid section Flange nearer front Bottom channel C // // Middle of web // C6-38 // // Flange nearer rear // C6-3 39

17 16 Table 3 (continue): Locations, alignment and direction of sensors Measuring Element Location Position Direction Ref in Ref in Data file Fig. Strain Lacing #1 Bolted flange 1mm from edge Top lacing L // // 1mm from other flange // L1-41 // Vertical flange 1mm from other flange // L1-3 4 // // 1mm from edge // L Lacing # Bolted flange 1mm from edge Bottom lacing L-1 44 // // 1mm from other flange // L- 45 // Vertical flange 1mm from other flange // L-3 46 // // 1mm from edge // L-4 47 Lacing #3 Bolted flange 1mm from edge Top lacing L // // 1mm from other flange // L3-49 Strain Lacing #3 Vertical flange 1mm from other flange Top lacing L // // 1mm from edge // L Lacing #4 Bolted flange 1mm from edge Bottom lacing L4-1 5 // // 1mm from other flange // L4-53 // Vertical flange 1mm from other flange // L // // 1mm from edge // L Lacing #5 Bolted flange Middle of flange Top lacing L // Vertical flange // // L5-57 Lacing #6 Bolted flange Middle of flange Bottom lacing L // Vertical flange // // L6-59 Lacing #7 Bolted flange Middle of flange Top lacing L // Vertical flange // // L7-61 Lacing #8 Bolted flange Middle of flange Bottom lacing L8-1 6 // Vertical flange // // L8-63 Lacing #9 Bolted flange Middle of flange Top lacing L // Vertical flange // // L9-65 Lacing #10 Bolted flange Middle of flange Bottom lacing L // Vertical flange // // L10-67 Lacing #11 Bolted flange Middle of flange Top lacing L // Vertical flange // // L11-69 Lacing #1 Bolted flange Middle of flange Bottom lacing L // Vertical flange // // L1-71 Lacing #13 Bolted flange Middle of flange Top lacing L // Vertical flange // // L13-73 Lacing #14 Bolted flange Middle of flange Bottom lacing L // Vertical flange // // L14-75 Lacing #15 Bolted flange Middle of flange Top lacing L // Vertical flange // // L15-77 Lacing #16 Bolted flange Middle of flange Bottom lacing L // Vertical flange // // L16-79 Sign convention: Displacement transducers (D1 to D14) + is extension, -ve is retraction Strain Gauges (S1 to S4, C1 to C6, L1 to L16) +ve is tension, -ve is compression Note: Strain gauges on channels and lacing installed on inner surface, as shown in figure

18 17 Table 4: Axial load capacity of laced strut -comparison of predicted results with test result Laced Struts Failure A f y L I Max. Load (tons) mode (mm ) (MPa) (mm) (mm 4 ) EC3 BS 5950 Test X-X Global S E / 1147 / 995 / 1168 / 1314 * 130 * 1506 / /1569 Y-Y Global S E+10 /1656* /1685* I-beam y-y Local 4900 S E ** 176** x-x/y-y 130 S E+05 Lacing u-u 130 S E v-v 130 S E Notes: * Different values with different effective lengths assumed L EX =1.0L/0.85L/ 0.70L; ** Value obtained based on effective length L EY =1.0L.

19 18 6 m 3.8 m 19.6m total length consisting of 3.8m + 1m + 3.8m segments Figure 1: Test and instrument layout

20 19 Figure Load application using hydraulic jacks and high-strength strands

21 Figure 3: Strut with the loading strands in position 0

22 1 Figure 4 Base support allowing horizontal movement

23 Figure 5 Base support preventing lateral translations. Figure 6: Transducer D3 measuring axial displacement at the real end of the strut.

24 Figure 7: Vertical displacement transducers D5 and D6 at the splice joint of strut. 3

25 Figure 8: Details of coupon specimens and extracted location 4

26 Total Applied Axial Load (tons) Axial displacement at Right Strut Left Strut Axial Displacement (mm) Figure 9: Applied load versus axial displacements

27 Total Applied Axial Load (tons) Vertical deflection at 1st splice joint from front end mid-length 1st splice joint from rear end Vertical Deflection (mm) Figure 10: Applied load versus vertical deflection

28 7 0 Distance from Front to Rear (m) Vertical Deflection (mm) Total applied axial load 700 tons 1000 tons 100 tons 1400 tons Figure 11: Vertical deflection profile of the strut Total Applied Axial Load (tons) Lateral deflection at Top flange Right strut Bottom flange Right strut Top flange Left strut Bottom flange Left strut Lateral Displacement (mm) Figure1: Load versus lateral displacement at mid-length

29 Total Applied Axial Load (tons) Stresses measured at Top flange Right strut Bottom flange Right strut Top flange Left strut Bottom flange Left strut 0 +ve indicating compressive stresses Stress at mid-length section of Struts (N/mm ) Figure 13: Stresses on top and bottom flanges of the strut sections at mid-length Forces measured at Channel #1 Channel # Channel #3 Channel #4 Channel #5 Channel #6 +ve indicating compressive forces Total Applied Axial Load (ton) Axial Force on Channels (ton) Figure 14: Axial forces in the Channel Sections

30 Forces measured at Lacing #1 Lacing # Lacing #3 Lacing #4 Lacing #5 Lacing #6 Lacing #7 Lacing #8 Lacing #9 Lacing #10 Lacing #11 Lacing #1 Lacing #13 Lacing #14 Lacing #15 Lacing #16 Total Applied Axial Load (ton) ve indicating compressive forces Axial Force on Lacings (ton) Figure 15: Applied load versus axial forces in the lacing members

31 Figure 16: Buckling of the main struts and lacing members after collapse 30

32 Figure 17: Buckling of the mid-length strut section 31

33 Figure 18: Another view showing the bucking of the laced strut 3

34 Predicted by EC3 Lacing #1 Lacing # Lacing #3 Lacing #4 Lacing # Lacing #6 Lacing # Lacing #8 Lacing #9 Lacing #10 Lacing #11 Lacing #1 Lacing #13 Lacing #14 Lacing #15 Lacing # Figure 19 Applied force versus lacing forces comparison with EC3 approach

35 Appendix A: Certificate of Calibration for Digital Pressure Gauge 34

36 35 Appendix B Coupon Test Results Table B.1 Mechanical properties of steel sections from coupon tests Strut Section 1m strut segment (4 year-old) 3.8m strut segment (6 year-old) UB 610x34x195 kg Flange Web Flange Web Coupon Sample Reference 5F1 5F 5W3 5W4 10F1 10F 10W3 10W4 Measured Width (mm) Measured Thickness (mm) Cross-sectional Area (mm ) Yield Load (kn) Yield Strength (N/mm ) Maximum Load (kn) Max. Tensile Strength (N/mm) Elastic Modulus (kn/mm ) Lacing Angle 80x80x8 Coupon Sample Reference 5L1 5L 10L1 10L Measured Width (mm) Measured Thickness (mm) Cross-sectional Area (mm ) Yield Load (kn) Yield Strength (N/mm ) Maximum Load (kn) Max. Tensile Strength (N/mm ) Elastic Modulus (kn/mm ) ** 18.7 Channels 54x76x8.9 kg/m 50x90x5.5kg/m Coupon Sample Reference 5C 10C Measured Width (mm) Measured Thickness (mm) Cross-sectional Area (mm ) Yield Load (kn) Yield Stress (N/mm ) Maximum Load (kn) Tensile Strength (N/mm ) Elastic Modulus (kn/mm ) Note: ** Low value, this result is ignored Table B. Section Dimensions of Lacing members and Channels D (mm) B (mm) t (mm) T (mm) Area (cm )

37 36 Lacing members Measured dimensions From section table x80x9.66kg/m Channels Measured dimensions for channel at 3.8m strut segment From section table 54x76x8.9kg/m Measured dimensions for channel at 1m strut segment From section table - 50x90x5.5kg/m

38 Appendix C: Design Approaches for Calculating Buckling Resistance of Build-up members 37 C.1 BS5950:Part1:000 Approach The cross-section UB ( i = mm, i = mm, r = 59 mm, r = 75.5mm, x y x y Ach = 49cm ) is used for chords and section L ( i i mm r = r = mm, 5 4 x = y = , x y 4.3 i = mm, i = mm, r = 30.6 mm, r = 15.6mm, u v. h0 = 1000 mm, a = 000 mm, d = 1000 mm ; u v Ad = 1.3cm ) for lacing members; N Ed / N Ed / L d Ad a UB x Ach h 0 y 1000 NEd NEd L=19600mm Fig. C1 Dimension of laced strut 3. Structural steel grade: S355; 4. Calculation of the overall buckling resistance based on BS :000: (1) Check overall buckling resistance of struts about X-X axis a) L E =1.0L λ = L / R = L/ r = / 59 = 76 (1) X E X x p N mm rolled I section buckling curve c c = 196( / ) (, ) () NbRd,, X = = ( kn) = ( ton) = 995( ton) (3) 9.81 b) L E =0.85L λ = L / R = 0.85 L/ r = / 59 = 64 (4) X E X x p N mm rolled I section buckling curve c c = 30( / ) (, ) (5)

39 NbRd,, X = = 11454( kn) = ( ton) = 1168( ton) (6) 9.81 c) L E =0.70L λ = L / R = 0.70 L/ r = / 59 = 53 (7) X E X x p N mm rolled I section buckling curve c c = 60( / ) (, ) (8) 1948 NbRd,, X = = 1948( kn) = ( ton) = 130( ton) (9) 9.81 () Check buckling resistance about Y-Y axis i) Global (check struts member) a) L = 1.0L= 19600mm E h IY = iy + Ach( ) = ( ) = ( mm ) (10) R = I / A = 505( mm) (11) Y Y ch λ = L / R = / 505 = 39 (1) Y E Y p N mm rolled I section buckling curve c c = 95( / ) (, ) (13) Nb, Rd, Y ( global) = = 14691( kn) = 1498( ton) (14) b) L = 0.85L E h IY = iy + Ach( ) = ( ) = ( mm ) (15) R = I / A = 505( mm) (16) Y Y ch λ = L / R = / 505 = 33 (17) Y E Y p N mm rolled I section buckling curve c c = 309( / ) (, ) (18) Nb, Rd, Y ( global) = = 15388( kn) = 1569( ton) (19) c) L = 0.70L E h IY = iy + Ach( ) = ( ) = ( mm ) (0) R = I / A = 505( mm) (1) Y Y ch λ = L / R = / 505 = 7 () Y E Y p N mm rolled I section buckling curve c c = 33( / ) (, ) (3) Nb, Rd, Y ( global) = = 16534( kn) = 1685( ton) (4)

40 39 ii) Local ( L = 000 mm, check single chord ): ch λ = L/ r = 000 / 75.5 = 6.5 (5) ch y p N mm rolled I section buckling curve b c = 340( / ) (, ) (6) NbRdch,, = = 8466( kn) = 863( ton) (7) N N = 169( kn) = 176( ton) (8) Ed b, Rd, ch (3) Check lacing member λ = λ = d / r = 58, λ = d / r = 46, λ = d / r = 91 (9) x y x u u v v p = p = 5 N / mm p N mm rolled angle buckling curve c p = cx, cy, cu, = 86 / cv, 160 N / mm (, ) NbRd,, x( lacing) = NbRd,, y( lacing) = pcx, Ad = 310( kn) = 3( ton) NbRdu,, ( lacing) = pcu, Ad = 35( kn) = 36( ton) NbRdv,, ( lacing) = pcv, Ad = 197( kn) = 0( ton) (30) (31) N V N /.5%.5% 1131( ) Ed brdv,, ( lacing) Ed = = ton (3)

41 40 C. BS EN :005 Approach 5. Calculation of the overall buckling resistance based on BS EN :005: (1) Check overall buckling resistance of struts about X-X axis a) L = 1.0L= 19600mm E The plastic resistance of the cross-section to compression: Npl, Rk = Afy = = 17679( kn) (33) The Euler buckling load: N 5 9 π EI π = = = 1805( kn) (34) cr, X LE The relative slenderness: λ = N / N = (35) X pl, Rk cr, X The slenderness reduction factor: X ϕx = 0.5[1 + α( λ X 0.) + λ ] = ( buckling curve c) (36) 1 χ X = = ϕ + ϕ λx X X The overall buckling resistance of the struts about X-X axis: 9653 NbRd,, X = χ XNplRd, = 9653( kn) = ( ton) = 984( ton) (38) 9.81 (37) b) L = 0.85L E N 5 9 π EI π = = = 4986( kn) cr, X LE ( ) (39) λ = N / N = (40) X pl, Rk cr, X X ϕx = 0.5[1 + α( λ X 0.) + λ ] = ( buckling curve c) (41) 1 χ X = = ϕ + ϕ λx X X NbRd,, X = χ XNplRd, = 1150( kn) = 1147( ton) (43) (4) c) L = 0.70L E N 5 9 π EI π = = = 36841( kn) cr, X LE ( ) (44) λ = N / N = (45) X pl, Rk cr, X X ϕx = 0.5[1 + α( λx 0.) + λ ] = ( buckling curve c) (46)

42 41 1 χ X = = 0.79 ϕ + ϕ λx X X NbRd,, X = χ XNplRd, = 1891( kn) = 1314( ton) (48) (47) () Check buckling resistance about Y-Y axis i) Global a) L = 1.0L= 19600mm E The Euler buckling load: I = 0.5hA = = ( mm) (49) eff ch (Note: EC3 uses above conservative formula; more accurate formula should be I = 0.5hA + i = = ( mm) ) eff 0 ch y N π EI π = = = 67170( kn) (50) 5 10 eff cr, Y LE The relative slenderness: λ = N / N = (51) Y pl, Rk cr, Y The slenderness reduction factor: Y ϕy = 0.5[1 + α( λy 0.) + λ ] = ( buckling curve c) (5) 1 χy = = ϕ + ϕ λy Y Y The overall buckling resistance of the struts: Nb, Rd, Y ( global) = χy Npl, Rd = 14774kN = 1506( ton) (54) (53) b) L = 0.85L E N π EI π = = = 9969( kn) 5 10 eff cr, Y LE ( ) (55) λ = N / N = (56) Y pl, Rk cr, Y Y ϕy = 0.5[1 + α( λy 0.) + λ ] = ( buckling curve c) (57) 1 χy = = ϕ + ϕ λy Y Y Nb, Rd, Y ( global) = χy Npl, Rd = 1553kN = 158( ton) (59) (58) c) L = 0.70L E

43 4 N π EI π = = = 13708( kn) 5 10 eff cr, Y LE ( ) (60) λ = N / N = (61) Y pl, Rk cr, Y Y ϕy = 0.5[1 + α( λy 0.) + λ ] = ( buckling curve c) (6) 1 χy = = ϕ + ϕ λy Y Y Nb, Rd, Y ( global) = χy Npl, Rd = 164kN = 1656( ton) (64) (63) ii) Local ( L = 000 mm, check single chord ): ch The plastic resistance of the cross-section to compression: Npl, Rk, ch = Ach fy = = N = kN (65) The Euler buckling load: N 5 8 π EI π = = = 73371( kn) (66) cr, ch Lch 000 The relative slenderness: N pl, Rk, ch λ ch = = (67) N cr, ch The slenderness reduction factor: ch ϕ = 0.5[1 + α( λch 0.) + λ ] = ( buckling curve b) (68) ch 1 χch = = ϕ + ϕ λch ch ch The local buckling resistance of single chord: NbRdch,, = χ NplRdch,, = 8367kN = 853( ton) (70) The shear stiffness of the lacings: S v 5 nead ah = = = 18646( kn) (71) 3 3 d (1000 ) The design value of the maximum moment in the middle of the struts considering second order effects (M I Ed=0): M Ed I NEde0 + MEd = NEd N 1 N S cr, Y Ed v The design force at the mid-height of single chord should fulfill N M h A N = + N = ton (73) Ed Ed 0 ch ch, Ed b, Rd, ch 853( ) Ieff Then the design axial force N Ed should fulfill (69) (7)

44 43 N 15035( kn) = 1533( ton) (74) Ed (3) Check lacing member Similarly, the buckling resistance of lacing member is calculated: NbRd, ( x x/ y y) = 36( kn) = 33( ton) NbRd, ( u u) = 364( kn) = 37( ton) NbRd, ( v v) = 1( kn) = ( ton) The design force N Ed should fulfill (75) VEd Nb, Rd ( v v) / = 300( kn) I MEd π NEde0 + M Ed VEd = π = L L NEd NEd 1 Ncr, Y Sv N 400( kn) = 467( ton) (77) Ed (76)

45 44 Table C1: Summary on the buckling resistance of laced strut system Failure mode A f y L I Max. Load N Ed (ton) (mm ) (MPa) (mm) (mm 4 ) EC3 BS 5950 Laced X-X Global S E /1147/1314 * 995 / 1168 / 130* Struts Y-Y Global S E /158 / / 1569 / 1685 I-beam y-y Local 4900 S E ** 176 x-x/y-y 130 S E+05 Lacing u-u 130 S E ** 1131 v-v 130 S E+05 Notes: * Different values under different effective lengths L E =1.0L, 0.85L or 0.70L; ** Value under effective length L E =1.0L. If the built-up member is bent about Y-Y plane, the relationship between the shear force V Ed and the design compression force N Ed to the built-up member is shown in following Fig. C by assuming is the initial bow imperfection e 0 as L/ e0=l/ VEd/NEd N Ed (kn) Fig. C The relationship between shear force V Ed and compression force N Ed Design chart for laced struts showing the relationship between the design capacity and the effective length is shown in Fig. C3. If BS5950:Part1:000 approach is adopted the design capacity is limited by the buckling capacity of the lacing member assuming a maximum shear force of.5% axial force. The capacity of the lacing member will govern the design when the strut length id less than 17 m. However, such limitation does not exist if BSEN1993:EC3:005 approach is adopted. This is explained in the following sections.

46 Strut resistance (10 3 kn) EN 1993 BS 5950 Shear resistance control line Effective length, L ex (m) Fig. C3 Design chart for laced struts using UB Grade S355 section

47 46 C.3 EC 3 approach to evaluate the shear force of the axially loaded laced struts An axially loaded column, pinned at the two ends, is showed in Fig C4. Fig. C4 The shear force of axially loaded column Assuming the deflection curve is π y = vsin x l (1) Then the moment is π M = Ny = Nvsin x () l The shear force is dm Nπ v π x V = = cos (3) dx l l Therefore, the maximum shear force at the two ends is Nπ v Vmax = (4) l The maximum moment is (at middle height) M max Thus, Equation (4) can be expressed as = Nv (5) M max Vmax = π (6) l Equation (6) is the formula adopted in BSEN1993:EC3:005 The maximum shear force in the laced strut is depending on the lateral deflection, the applied axial force (i.e, M max ) and the strut length.

48 47 C.4 Example The cross-section UB ( i = mm, i = mm, r = 59 mm, r = 75.5mm, x y x y Ach = 49cm ) is used for chords and section L ( i i mm r = r = mm, 5 4 x = y = , x y 4.3 i = mm, i = mm, r = 30.6 mm, r = 15.6mm, u v h0 = 1000 mm, a = 000 mm, d = 1000 mm ; u v Ad = 1.3cm ) for lacing members; N Ed / N Ed / L d Ad a UB x Ach h 0 y 1000 NEd NEd L=19600mm Fig. C5 Dimension Structural steel grade: S355; Calculation of lacing force according to EC3 (Y-Y axis): The maximum shear force in struts is V M Ed Ed = π (1) LE where L E is the effective length and M Ed is maximum moment in the middle of the struts considering second order effects M Ed N = N 1 N Ed Ed cr, Y e0 N S where S v is the shear stiffness of the lacings Ed v ()

49 48 S v nea ah = = d (1000 ) 5 d = 18646( kn) = 18618( tons) (3) and N cr,y is the effective critical force about Y-Y axis of the struts π EIeff Ncr, Y = = 67170( kn) = 6847( tons) ( L 1.00 ) E = L (4) L E Therefore, the maximum axial force in one lacing member is N lacing = VEd = π N L N E 1 N Ed Ed cr, Y e0 N S Ed v (5) Taking e 0 =L E /500 according to EC3, it obtains N lacing π NEd = 1000 N 1 Ed N N S cr, Y Ed v (6) It can be observed that the maximum axial force in the lacing member is depending on the applied axial load, N Ed, the shear stiffness, S v, and the elastic critical load of the laced strut bending about the y-y direction, N cr,y.