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 2006 (This Report Contains 48 Pages)
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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
Figure 12 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
2mm). 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).
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7.4 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 18.
7.5 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:Part1:2000 of 2.5% 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 8.
7.6 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 16-18. All the
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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:Part1:2000 Clause 4.7.8 [Ref. 1] and in Eurocode 3-1-1:2005 Clause 6.4 for built up
compression member [Ref. 2]. 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), (2) 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:Part1:2000 and EC3 (2005) are given in Appendix C.
The maximum load predicted by EC3-1-1:2005 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:Part1:2000 is 995 tons, 1168 tons and 1320 tons assuming
effective length of 1.0L, 0.85L and 0.7L, respectively. If a shear force of 2.5% 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:Part1:2000 approach is conservative as compared to the
actual failure load of the strutting system.
Clause 4.7.8 (i) of BS5950 Part 1:2000 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 2.5% 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, 2.5% of this load would indicate 18
tons of shear force. However, the measured maximum axial forces in the channel and angle section
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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 3-1-1:2005, 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 3-1-1:2005
based on second-order analysis of built-up compression. The predicted test result is compared with
those obtained from tests in Figure 19. EC3:2005 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.
(2) 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:Part1:2000 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
2.0. The load capacity predicted by the codes is found to be on the conservative side.
2) 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
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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 2.5% of axial force assumed in BS5950:Part1:2000 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 (2000), Structural use of steelwork in building, Part1: Code of practice for
design – rolled and welded sections, British Standards Institute.
2. Eurocode 3 Part 1-1 (2005), Design of steel structures: Part 1-1 General rules and rules for
building, British Standards Institute.
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Table 1: Loading intervals and observations
Total Applied Loads (ton) Observations 100 200 300 400 500 Maximum loading for preloading cycles 600 700 Design load of struts (750ton), no major deformation
noted 800 900
1000 1050 1100 Visible deflection at mid-span 1150 1200 1250 Adjustment of hydraulic actuators, unitisation of actuator
stroke 1300 observed reduction in strains and displacements 1350 1400 1438 Sudden buckling and collapse of strut, end of test
Table 2: 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
2 mm 9 mm 21 mm
Bottom flange deflection
4 mm 8 mm 16 mm
Average deflection
3 mm 8.5 mm 19 mm
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Table 3: Locations, alignment and direction of sensors Measuring Element Location Position Direction Ref in
Fig. Ref in
Data fileAxial 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 D2 1 Rear Waler Right of strut Centre line of load pts Outwards, +ve=Comp strut D3 2 // Left of strut Centre line of load pts Outwards, +ve=Comp strut D4 3
Vertical Strut Right Joint nearer Front
Top channel of side strut Downward, +ve=Defl down D5 4
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
Rear Top channel of side strut Downward, +ve=Defl down D9 8
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 D12 11 Strut Left Mid-span Top Flange, 100mm ext Towards right, +ve=Sway right D13 12 // // Bottom Flange, 100mm ext Towards right, +ve=Sway right D14 13
Strain Strut Right Mid-span Top Flange, top surface Middle of right outstand S1-1 14 // // Top Flange, top surface Middle of left outstand S1-2 15 // // Bottom Flange, bottom surface Middle of right outstand S2-1 16 // // Bottom Flange, bottom surface Middle of left outstand S2-2 17 Strut Left Mid-span Top Flange, top surface Middle of right outstand S3-1 18 // // Top Flange, top surface Middle of left outstand S3-2 19 // // Bottom Flange, bottom surface Middle of right outstand S4-1 20 // // Bottom Flange, bottom surface Middle of left outstand S4-2 21
Strain Channel #1 Mid section Flange nearer front Top channel C1-1 22 // // Middle of web // C1-2 23 // // Flange nearer rear // C1-3 24 Channel #2 Mid section Flange nearer front Bottom channel C2-1 25 // // Middle of web // C2-2 26 // // Flange nearer rear // C2-3 27 Channel #3 Mid section Flange nearer front Top channel C3-1 28 // // Middle of web // C3-2 29 // // Flange nearer rear // C3-3 30 Channel #4 Mid section Flange nearer front Bottom channel C4-1 31 // // Middle of web // C4-2 32 // // Flange nearer rear // C4-3 33 Channel #5 Mid section Flange nearer front Top channel C5-1 34 // // Middle of web // C5-2 35 // // Flange nearer rear // C5-3 36 Channel #6 Mid section Flange nearer front Bottom channel C6-1 37 // // Middle of web // C6-2 38 // // Flange nearer rear // C6-3 39
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Table 3 (continue): Locations, alignment and direction of sensors Measuring Element Location Position Direction Ref in
Fig. Ref in
Data fileStrain Lacing #1 Bolted flange 12mm from edge Top lacing L1-1 40
// // 12mm from other flange // L1-2 41 // Vertical flange 12mm from other flange // L1-3 42 // // 12mm from edge // L1-4 43 Lacing #2 Bolted flange 12mm from edge Bottom lacing L2-1 44 // // 12mm from other flange // L2-2 45 // Vertical flange 12mm from other flange // L2-3 46 // // 12mm from edge // L2-4 47 Lacing #3 Bolted flange 12mm from edge Top lacing L3-1 48 // // 12mm from other flange // L3-2 49
Strain Lacing #3 Vertical flange 12mm from other flange Top lacing L3-3 50 // // 12mm from edge // L3-4 51 Lacing #4 Bolted flange 12mm from edge Bottom lacing L4-1 52 // // 12mm from other flange // L4-2 53 // Vertical flange 12mm from other flange // L4-3 54 // // 12mm from edge // L4-4 55 Lacing #5 Bolted flange Middle of flange Top lacing L5-1 56 // Vertical flange // // L5-2 57 Lacing #6 Bolted flange Middle of flange Bottom lacing L6-1 58 // Vertical flange // // L6-2 59 Lacing #7 Bolted flange Middle of flange Top lacing L7-1 60 // Vertical flange // // L7-2 61 Lacing #8 Bolted flange Middle of flange Bottom lacing L8-1 62 // Vertical flange // // L8-2 63 Lacing #9 Bolted flange Middle of flange Top lacing L9-1 64 // Vertical flange // // L9-2 65 Lacing #10 Bolted flange Middle of flange Bottom lacing L10-1 66 // Vertical flange // // L10-2 67 Lacing #11 Bolted flange Middle of flange Top lacing L11-1 68 // Vertical flange // // L11-2 69 Lacing #12 Bolted flange Middle of flange Bottom lacing L12-1 70 // Vertical flange // // L12-2 71 Lacing #13 Bolted flange Middle of flange Top lacing L13-1 72 // Vertical flange // // L13-2 73 Lacing #14 Bolted flange Middle of flange Bottom lacing L14-1 74 // Vertical flange // // L14-2 75 Lacing #15 Bolted flange Middle of flange Top lacing L15-1 76 // Vertical flange // // L15-2 77 Lacing #16 Bolted flange Middle of flange Bottom lacing L16-1 78 // Vertical flange // // L16-2 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
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Table 4: Axial load capacity of laced strut -comparison of predicted results with test result
A fy L I Max. Load (tons) Failure mode (mm2) (MPa) (mm) (mm4) EC3 BS 5950 Test
, , , 12891( ) 1314( )b Rd X X pl RdN N kN tonχ= = = (48)
(2) Check buckling resistance about Y-Y axis
i) Global
a) 1.0 19600EL L mm= =
The Euler buckling load: 2 2 10 400.5 0.5 1000 24900 1.245 10 ( )eff chI h A mm= = × × = × (49)
(Note: EC3 uses above conservative formula; more accurate formula should be 2 2 8 10 400.5 2 0.5 1000 24900 2 1.416 10 1.273 10 ( )eff ch yI h A i mm= + = × × + × × = × )