HILONG WORKSHOP: strength steels in long span structuresnews-sci.com/wp-content/uploads/2015/07/13-hillong-marios... · High strength steels in long span structures. ... Steel cables

Marios Theofanous

Prestressed HSS Trusses30‐06‐2015

HILONG WORKSHOP:High strength steels in long span structures

Birmingham research team:

• Dr Marios [email protected]

• Michaela Gkantou (Research Fellow)[email protected]

• Professor Lambis [email protected]

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


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


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


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)


• 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


• 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%



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.



2. Prestressed Steel StructuresCase study 3: Sydney olympic stadium reconfiguration


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.


2. Prestressed Steel StructuresCase study 4 : Australian equine & livestock events centre



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.



Arched prestressed trusses in Hyde Park, London


P



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



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


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






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



Compressive members



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



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


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


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






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


Prestress level from zero to Popt prestress level



Varying top chord (80x80x8, 70x70x6, 50x50x5) – bottom chord 50x50x5



Combination of shell and beam elements



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


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


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


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


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


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


Thank you for your attention!


Documents

HILONG WORKSHOP: strength steels in long span structuresnews-sci.com/wp-content/uploads/2015/07/13-hillong-marios... · High strength steels in long span structures. ... Steel cables