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Marios Theofanous
Prestressed HSS Trusses30‐06‐2015
HILONG WORKSHOP:High strength steels in long span structures
Birmingham research team:
• Dr Marios Theofanousm.theofanous@bham.ac.uk
• Michaela Gkantou (Research Fellow)m.gkantou@bham.ac.uk
• Professor Lambis Baniotopoulosc.baniotopoulos@bham.ac.uk
HILONG WORKSHOP 30‐06‐2015
PRESENTATION OUTLINE
1. Introduction2. Prestressed steel structures3. Prestressed members
• Experiments• Numerical modelling
4. Prestressed trusses• Experiments• Numerical modelling• Dynamic response
5. Conclusions
HILONG WORKSHOP 30‐06‐2015
1. Introduction
HSS have increased fy compared to ordinary structural steels
Young’s modulus remains the same, hence stiffness rather than strength ismore likely to govern the design of HSS slender structures
Deflections (SLS) and buckling become increasingly important
As part of the HILONG project, means to mitigate excessive deflections havebeen investigated
HILONG WORKSHOP 30‐06‐2015
1. Introduction
Long span applications (stadia, arenas) considered less stringent deflections limits structure deadweight is a considerable
proportion of the design load aesthetics is an important design criterion
Steel cables are introduced into the chords of tubularsteel trusses and post‐tensioned to provide a moreefficient load carrying mechanism
Particularly efficient for light, long span structures
Post‐tensioning used to erect the structure,minimising or even eliminating the need for craneageleading to safe and rapid construction
HILONG WORKSHOP 30‐06‐2015
HSS long span structure: Friends Arena Stadium, Sweden
1. Introduction Research on prestressed arched trusses dates back to '90s, when the University of Sydney
started a research campaign on the response of prestressed arched trusses with slidingjoints that can be erected through the tensioning of a cable on their bottom chord.
The cable tensioning allows the predetermined gaps on the bottom chord to close, thuscreating the upward curvature of the top chord.
The trusses are unloaded during their assembly, whereas after the erection, the tensioningforces of the cable induce compressive forces at the bottom chord, which after theapplication of external vertical loading is subjected to tensile stresses.
The top chord is subjected to negative bending during the uplift of the truss
Technology developed and implemented S‐Squared (project partner)
HILONG WORKSHOP 30‐06‐2015
• Clarke, M.J. and Hancock, G.J. (1991) Finite‐element nonlinear analysis of stressed‐arch frames. Journal of Structural Engineering, 117:2819–2837
• Clarke, M.J. and Hancock, G.J. (1995) Tests and nonlinear analyses of small‐scale stressed‐arch frames. Journal of structuralengineering, 121: 187–200
2. Prestressed Steel Structures
Case studies designed by S‐squaredLong span applications (stadia, arenas) consideredAviationSportIndustrialMilitaryGovernment + CouncilsMiningResidential developmentsHumanitarian
Advantages of S‐squared technologyUse less carbonSave time right through the project life cycleImprove construction safetyIncrease overall strength and structural performanceLengths up to 150 m without intermediate supports
HILONG WORKSHOP 30‐06‐2015
• http://www.s‐squared.com.au/hopxme.asc
2. Prestressed Steel StructuresCase study 1: Ilshin textile factory (industrial building)
Location: Changshu, Jiangsu Province, China Size: 80m clear span x 88m Ilshin Textile Co. Ltd recognised the long term value and flexibility of a completely column free production
space. S‐squared innovative approach to modifying and improving an existing structural system typically usedthroughout China delivered this, while reducing material requirements by around 40%
HILONG WORKSHOP 30‐06‐2015
• http://www.s‐squared.com.au/hopxme.asc
2. Prestressed Steel StructuresCase study 2: Central west livestock exchange
Location: Forbes, NSW Size: 84m clear span x 116m The innovative, post‐tensioned steel roof solution for the CWLE at Forbes, NSW incorporates clear spans of 84
metres over a site of approximately 9,700 m2 with clear heights of 5.5m rising to 14m in the centre of thebuilding.
Such large spans are unprecedented in livestock facilities and agriculture in general, where traditional saw‐toothdesigns with a forest of internal columns are commonly used.
HILONG WORKSHOP 30‐06‐2015
• http://www.s‐squared.com.au/hopxme.asc
2. Prestressed Steel StructuresCase study 3: Sydney olympic stadium reconfiguration
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Location: Homebush, NSW
Size: Twin 114m clear span North and South roofsections, 7000m2 total covered area
The lightweight, post‐tensioned truss and infill roofsystem was assembled outside the stadium seatingbowl and lifted into position with cranes. TheStadium remained fully operational duringconstruction, with no internal propping from thestands or field required, thus preserving all availableseats and ticket revenue throughout the busyfootball season (NRL, AFL and ARU).
The S‐squared solution delivered a high degree ofsustainability with steel weight reductions of 40%,and was subject to rigorous scrutiny frominternational architects and engineers includingPopulous (then HOK Sport), Sinclair Knight Merz(incorporating Modus) and Arup.
• http://www.s‐squared.com.au/hopxme.asc
2. Prestressed Steel StructuresCase study 4 : Australian equine & livestock events centre
HILONG WORKSHOP 30‐06‐2015
• http://www.s‐squared.com.au/hopxme.asc
Location: Tamworth, NSW Size: 22,000m2 S‐Squared were responsible for the design, engineering and construction methodology for the main arena
(60m clear span), the stud selling area (42m span dome structure) and six stable buildings (25m span each).Each main element represented different sets of challenges, which were overcome through innovation and acreative approach to form and function.
Equine and livestock events are worth more than $45 million annually in the Tamworth region. Since openingin late 2008, over 120 separate organisations have shown interest in using the new centre and it remains fullybooked virtually all year round.
2. Prestressed Steel Structures
HILONG WORKSHOP 30‐06‐2015
Arched prestressed trusses in Hyde Park, London
2. Prestressed Steel Structures
P
HILONG WORKSHOP 30‐06‐2015
2. Prestressed Steel Structures
Means to increase stiffness
The geometry of the arch follows the thrust line of an inverted chain
Pre‐stressed cables in the bottom chord:
Controls deflections
Counteracts self‐weight
Enhances the lateral stability of arched trusses in the case of uplift
Results in less material usage and more efficient design
Tensile forces arise in bottom chord members due to gravity and snow loads
Elastic region in tension can be up to doubled since the pre‐compression in the steel tube must first be
overcome
HILONG WORKSHOP 30‐06‐2015
2. Prestressed Steel Structures
Definition of Popt Prestress level
Popt is the optimal prestress level, for which both the tube and the cable yieldsimultaneously under tensile loading and can be calculated by the following formula:
, ∗
where A is the cross‐sectional area, E is the Young’s modulus, fy is the yield stress and thesubscripts t and c stand for tube and cable respectively
HILONG WORKSHOP 30‐06‐2015
L (mm) Cable Loading Prestress level Grouting
S460
2000 1 Tension 0 No2000 1 Tension 30% No2000 1 Tension 65% No2000 ‐ Tension 0 Yes2000 1 Tension 30% Yes2000 1 Tension 65% Yes
S460
1000 1 Compression 30% No1000 1 Compression 65% No1000 ‐ Compression 0 Yes1000 1 Compression 30% Yes1000 1 Compression 65% Yes
S690
2000 1 Tension 0 No2000 1 Tension 25% No2000 1 Tension 55% No2000 ‐ Tension 0 Yes2000 1 Tension 25% Yes2000 1 Tension 55% Yes
S690
1000 1 Compression 25% No1000 1 Compression 55% No1000 ‐ Compression 0 Yes1000 1 Compression 25% Yes1000 1 Compression 55% Yes
3. Prestressed Steel Members ‐ Experiments
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3. Prestressed Steel Members ‐ Experiments
HILONG WORKSHOP 30‐06‐2015
3. Prestressed Steel Members ‐ Experiments
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Addition of collars for the reduction of buckling length during prestress
• Gosaye, J., Gardner, L., Wadee, M.A., et al. (2014) Tensile performance of prestressed steel elements. Engineering Structures, 79: 234–243
Tensile members
3. Prestressed Steel Members ‐ Experiments
HILONG WORKSHOP 30‐06‐2015
Compressive members
3. Prestressed Steel Members ‐ Experiments
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General Comments for tensile members
Adding the cable increases the capacity of the member
Increasing the prestress extends the elastic range leading to stiffer response (bydelaying the yielding of the tube)
Strain hardening of the tube ‐ higher ultimate load
Failure mode: breaking of the individual cable strands followed by tube fracture
Grout delayed fracture of the cable and led to more ductile response as all strandsfractured simultaneously
3. Prestressed Steel Members ‐ Experiments
HILONG WORKSHOP 30‐06‐2015
Rigid Body constraintBC: u1=u2=u3=φ2=φ3=0
Rigid Body constraintBC: u1=u2=φ2=φ3=0
1st buckling mode
Modelling assumptions
Half section & full length modelled
Shell elements for the tube and solid elements for the cable and the collars
Pin‐ended BC modelled
Model included contact
Prestress ‐ introduced as initial stresses in the cable
Geometric imperfections ‐ introduced through 1st buckling mode ‐ only in the tube
Nonlinear static analysis (general static ‐ displacement control) for both compression andtension
3. Prestressed Steel Members – Numerical Modelling
HILONG WORKSHOP 30‐06‐2015
WP4, Sub‐Task 4.2.4: FEA of post‐tensioned HSS trusses –Modelling of post‐tensioned members
0
50
100
150
200
250
300
0 50 100 150 200 250
Load
(kN)
Mid displacement (mm)
Contact Stresses
Compressive post‐tensioned member with Popt prestress
HILONG WORKSHOP 30‐06‐2015
4 trusses (S460) will be tested: SHS 50×50×5 for bottom chord, SHS 70×70×6.3 for top chord and SHS 40×40×2.9 for diagonal members
Truss 1: No cable Truss 2: Cable with nominal pre‐stress level (5 kN) Truss 3: Cable with pre‐stress = ½ Popt Truss 4: Cable with pre‐stress = Popt
4. Prestressed Steel Trusses‐ Experiments
HILONG WORKSHOP 30‐06‐2015
4. Prestressed Steel Trusses‐ Experiments
HILONG WORKSHOP 30‐06‐2015
4. Prestressed Steel Trusses‐ Experiments
HILONG WORKSHOP 30‐06‐2015
Modelling assumptions
Truss elements for the cable, beam elements for the tubular members
Prestress introduced as initial stresses in the cable
Study of both 2D and 3D models
Various loading arrangements considered
Geometric imperfections ‐ introduced through 1st buckling mode –only in the tubeTruss modelling
4. Prestressed Steel Trusses‐ Numerical Modelling
Investigated parameters Prestress level (no prestress, 0.5Popt, Popt)
Steel grade
Truss shape
Cross section of members
Aim: Determine the truss geometry that maximises the benefit of prestress
HILONG WORKSHOP 30‐06‐2015
Prestress level from zero to Popt prestress level
4. Prestressed Steel Trusses‐ Numerical Modelling
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Varying top chord (80x80x8, 70x70x6, 50x50x5) – bottom chord 50x50x5
4. Prestressed Steel Trusses‐ Numerical Modelling
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Combination of shell and beam elements
4. Prestressed Steel Trusses‐ Numerical Modelling
HILONG WORKSHOP 30‐06‐2015
Truss was made to undergo free vibration by applying animpulse at various locations (Locations 1‐11)
The time‐acceleration response of the trusswas recorded using an accelerometer atvarious locations (Locations 1‐11 )The time‐acceleration
response was transformed into frequency‐acceleration profile using the Fast Fourier Transform function
4. Prestressed Steel Trusses‐ Dynamic response
HILONG WORKSHOP 30‐06‐2015
4. Prestressed Steel Trusses‐ Dynamic response
For most structures the introduction of theprestressed cable leads to a negligibleincrease in both mass and stiffness
The dynamic response of the prestressedstructure is not significantly affected
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mode 1 mode 2Frequency (Hz) Frequency (Hz)
no cable 17.008 47.385no prestress 17.543 45.9150.5Popt 17.585 45.910Popt 17.626 45.903
5. Conclusions
Practical issues during prestressing and design
HILONG WORKSHOP 30‐06‐2015
The general practical issues of post‐tensioning concrete are applicable to prestressedsteel structures
The addition of grouting in prestressed members is advisable, as it not only protects thecable from corrosion, but also ‐as shown for tensile members‐ it delays the fracture ofthe cable, resulting in a more ductile response
The addition of collars in the cable is essential at the prestressing stage, as it reducesthe buckling length of the tubular member, which undergoes compression duringprestress
Prestress losses (instantaneous losses: losses due to anchorage set, prestressingmethod, friction and elastic shortening, time‐dependent losses: Relaxation of theprestressing steel causes) should be carefully considered
5. Conclusions
Practical issues during prestressing and design
HILONG WORKSHOP 30‐06‐2015
Based on the dimensions and type of the selected cable, it should beensured that the there is sufficient space for the grouting
Types of prestressing steel
Wires (single unit made of steel)
Strands (a few wires are spun together in a hellical form: 2‐wire, 3‐wire, 7‐wire)
Tendon (a group of strands or wires are wound to form a prestressingtendon)
Cable ( a group of tendons form a prestressing cable)
5. Conclusions
HILONG WORKSHOP 30‐06‐2015
Adding the cable increases the member resistance even when no prestress isapplied
Increasing the prestress extends the elastic range, leading to a stiffer response bydelaying the yielding of the tube
The stiffness is maximized when the cable and the bottom chord yieldsimultaneously (Popt)
The influence of prestressing on the dynamic response of the trusses was shownto be negligible
5. Conclusions
The main benefit of the prestress is that it increases the truss resistance, whilstdecreasing the corresponding displacement
The benefit increases with increasing prestress level (from zero to Popt prestress)
The benefit of the prestress changes with the truss shape and the cross‐sectionalsize of the members
By delaying the onset of buckling of the top chord the benefit of the prestressincreases
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Thank you for your attention!
HILONG WORKSHOP 30‐06‐2015
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