Modelling, testing, and construction of the firstDuctal® canopy in the world

S.M. Adeeb, B.W.J. Scholefield, T.G. Brown, N.G. Shrive, J. Kroman, V.H. Perry,and G. Tadros

Abstract: Two series of nine architectural shell structures — constructed of a 20 mm thick layer of a new ultra-highperformance cement (UPHC) composite with organic fibres (Ductal®) — were proposed to form the roofs of a new sta-tion for the light rail transit system in Calgary. Each section of the roofs consists of three panels each 6 m × 4.5 mconnected to provide an 18 m continuous system. The City of Calgary required testing of a full-scale prototype panelfor both static and dynamic responses. Finite element analysis (FEA) was used to determine the boundary conditions tobe imposed on one edge of the prototype panel during the tests to simulate the presence of the other two in the actualstructure. The FEA was also used to determine the critical (independent) snow and wind loading cases. The analysesand the testing procedures for the full-scale instrumented panel are described. The panel withstood both full factoredloads without damage. The methods for manufacturing and assembling the canopy and roofs are also described.

Key words: UHPC, shell structure, finite element analysis, static test, dynamic test.

Résumé : Deux séries de neuf structures architecturales en coque, construites d’une mince couche de 20 mm d’un nou-veau composite de ciment à ultra-haute performance et de fibres organiques (appelé Ductal®), ont été proposées pourformer les toits d’une nouvelle gare de train léger sur rail à Calgary. Chaque section des toits était composée de troispanneaux mesurant chacun 6 m sur 4,5 m reliés entre eux pour former un système continu de 18 m. La Ville de Cal-gary a demandé un essai d’un prototype à pleine échelle pour déterminer les réponses statique et dynamique. L’analysepar éléments finis a été utilisée pout déterminer les conditions limites imposées à un rebord du panneau prototype du-rant les essais afin de simuler la présence des deux autres panneaux de la structure réelle. L’analyse par éléments finisa également été utilisée pour déterminer les cas de charges critiques (indépendantes) de la neige et du vent. Les procé-dures d’analyse et d’essai pour le panneau instrumenté à pleine échelle sont décrites. Le panneau a supporté les deuxcharges pleinement pondérées sans subir aucun dommage. Les méthodes de fabrication et d’assemblage de la marquiseet des toits sont également décrites.

Mots clés : BUHP, structure en coque, analyse par éléments finis, essai statique, essai dynamique.

[Traduit par la Rédaction] Adeeb et al. 1165

Introduction

The Shawnessy light rail transit station in Calgary hasbeen constructed with new concrete technology — a thinshell of an ultra-high performance concrete (UHPC) calledDuctal® that was not reinforced with standard steel bars.Both the function of the station and nature of the site dic-tated a long, linear form running along a north–south axis.

Site constraints, above and below grade, restricted the widthof the platform to an even greater degree, minimizing theoverall footprint. A walk-on station was therefore conceivedwith a side-loading platform, staggered to improve passen-ger safety across the tracks (Fig. 1). At-grade access was tobe provided from an adjacent park’n ride facility and busterminal. Given the constraints, the architects desired to cre-ate a structure of light appearance using canopies to provideboth shelter and natural light to the space below. Light wasto be reflected off the smooth underside of the shell throughlouvered glazing to help engender a light and open appear-ance. The canopies would each be supported by a single,slender column that would also serve as a conduit for com-munication, security, and electrical cabling. To realise theirconcepts, the architects chose a canopy shape consisting of apart cylinder rising out at an angle to a slightly curved shellbase (Fig. 2).

The execution of the conceptual design in the conven-tional materials for such a type of structure would have ex-ceeded the allocated budget. To allow the City of Calgary tostay with the architect’s concept without exceeding the bud-get, it was proposed that the structure be constructed from anew ultra-high performance concrete composite, which is re-

Can. J. Civ. Eng. 32: 1152–1165 (2005) doi: 10.1139/L05-065 © 2005 NRC Canada

1152

Received 8 September 2004. Revision accepted 13 June 2005.Published on the NRC Research Press Web site athttp://cjce.nrc.ca on 30 November 2005.

S.M. Adeeb. Transcanada Pipelines Ltd.B.W.J. Scholefield, T.G. Brown, and N.G. Shrive.1

Department of Civil Engineering, University of Calgary, 2500University Dr. N.W., Calgary, Alberta, Canada.J. Kroman. City of Calgary, Alberta, Canada.V.H. Perry. Lafarge North American Inc., Calgary, Alberta,Canada.G. Tadros. Speco Ltd., Calgary, Alberta, Canada.

Written discussion of this article is welcomed and will bereceived by the Editor until 30 April 2006.

1Corresponding author (e-mail: shrive@ucalgary.ca).

inforced (in this case) with poly-vinyl alcohol fibres. Thecompressive strength and modulus of rupture of this materialare typically in excess of 150 and 25 MPa, respectively. Forthe 67 batches of material used in these canopies, the com-pressive strength was 152.3±6.3MPa (mean ± standard devi-ation) and the modulus of rupture was 17.9±3.4MPa.

The intent of the design therefore was to construct thepartial-dome like canopy with a 20 mm thick shell of theUHPC without conventional steel reinforcement. Stresses in-duced by the loading were to remain below the flexural ten-sile strength of the concrete material used. The canopieswould thus be the first shell structures constructed of thismaterial. If such a design could be successfully achieved,there would be a significant reduction in materials used, at afraction of the depth and weight previously possible withconcrete technology. As well, the repetitive nature of theform permitted re-use of formwork in the casting of theshells and columns. The other materials selected for use inthe station were also highly durable, including stainlesssteel, galvanized steel, and glass. While having greater em-

bodied energy than other materials such as wood, the stationis nonetheless expected to require less maintenance over itsservice life than if other materials had been selected.

Roof sections consist of three canopies, each supported bya single column, which are tied together to form a continu-ous structure. Each canopy is 4.5 m wide by 6 m long so thecomplete roof section is 4.5 m wide by 18 m long. Theslanted cylindrical portion of the canopy is connected to theflatter portion of each canopy by a reinforced rib. Each sin-gle canopy unit was designed so that it would consist of twohalves that would be bolted together on site with grout alsoinjected into the connections (Fig. 3). The open end of thecylindrical shell is tied by a steel-reinforced tie beam. Eachcanopy is supported by three struts that are, in turn, sup-ported by a single column. Structural design of the canopyand supports was performed by Montreal-based Strudes Inc.through analysis with the finite element (FE) method.

During the design process, the sharp corner between thecylindrical shell and the flatter portion was revealed to be apotential stress raiser, if the thickness of the shell remained

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Fig. 1. Schematic and axonometric views of the site, showing the offset of the two platforms and the long narrow footprint, courtesyof Enzo Vicenzino of the CPV Group.

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Fig. 2. (a) Structural drawings and (b) schematic of a single unit. (All dimensions are in millimeters.)

at 20 mm throughout. Thickening the connection, whilesimultaneously removing the sharp corner reduced thestresses. A steel reinforcing bar was added in this thick-ened zone to resist the tension that develops circumfer-entially around the base of the cylindrical section underwind loading. The lateral thrust from the arching action ofthe cylindrical shell under snow load and the outwardsthrust from wind pressure are resisted by the flatter sectionthat transfers the loads sideways to create tension in the tiebeam at the back of the canopy and the beam at the front ofthe canopy.

The City of Calgary required the testing of a full-size pro-totype canopy to the level of factored dead and design loadsfor both snow load and uplift from wind, as well as deter-mining the response to dynamic loading. The model devel-oped by Strudes Inc. was used here with the softwareSAP2000 (Computers and Structures 2000) to determine theboundary conditions to be applied in the physical tests, thecritical load cases, and the important locations to instrumentthe canopy. The methods used to make the determinationsand the experiments are described. The results of the experi-ments indicated that the canopy could withstand the criticalwind and snow loading without cracking.

Dynamic tests were performed to determine the modesand frequencies of natural vibration of the structure.Knowledge of the natural frequencies of the structurewould help to predict the effects of potential excitationsources on the behaviour of the canopy. Of particular inter-est was potential excitation caused by vibrations from thetrains passing through or stopping at the station. Possibleexcitations include buffeting from wind generated by pass-ing trains with frequencies up to about 0.75 Hz and vibra-tions transmitted through the ground with potentiallyhigher frequencies. These latter would be transitory excita-tions. The primary mode of vibration was determined to bea front-to-back sway at 2 Hz experimentally and 2.8 Hzfrom the finite element model. Given these frequencies, itwas thought that vibration would not be a problem for thestructure.

After approval from the City for construction, twenty-fourcanopies were cast and erected on the platforms. The cast-ing, assembly, and erection procedures are described.

Finite element analysis

ModelShell elements were used to model the thin shell, while

vertical shell elements were used to model the supportingbeams on the sides and in the middle of the cylindrical shellwhere the two halves of a single unit are joined. The stiff ribaround the cylindrical portion of the canopy was modelledby thicker shell elements to simulate the greater stiffness ofthis part. Solid elements were used in two locations, the firstbeing the supporting beam at the front of the canopy and thesecond being where the tie beam connects to the canopy.Beam elements were used for the tie beam, the three sup-porting struts, and the column (Fig. 4). Stiff beam elementswere used to connect the two halves of a single canopy unitat its middle to simulate the effect of the bolts and the groutused to connect the halves together. Stiff beam elementswere also used to join the three canopy units into a singlestructural element.

Boundary conditions for the single canopy prototypeThe nine live-load cases considered were obtained from

the National Building Code of Canada 1995 (NBCC 1995)and are shown in Fig. 5 (load case 1 is dead load alone). Asingle panel was to be tested and it was thought that an endpanel (of the three) would see more onerous stress resultantsthan a centre panel (supported on both sides). Thus, it wasnecessary to determine the boundary conditions (other thanthe fixation of the column to the ground) that would be re-quired for the physical test on an end panel to simulate theeffect of the two companion panels.

The model of three joined canopies was analyzed for allload cases with the boundary condition of total fixation ofthe columns to the ground. The displacements and rotationsat the connecting nodes between the panels were obtained.These deformations were then applied to a model of a singlepanel (with the column fixed at its base) as boundary condi-tions, and the reaction loads were determined where the can-opies were previously connected. The apparent stiffness foreach degree of freedom was obtained by dividing the reac-tion forces and moments by the imposed displacements androtations. The apparent stiffnesses thus obtained for the in-

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Fig. 3. Assembly of canopy in the laboratory, showing the front-to-back centre beam from each half of the canopy being bolted to-gether.

terface between an end and centre panel indicated that thecomplete structure (three panels) restrains the end panelfrom movement in the x-direction (side-to-side) but not ver-tically (z-direction), nor from front to back (y-direction)(Figs. 2 and 4). Rotations about the vertical and front-to-back axes are also restrained, but not front-to-back swayabout the side-to-side (x-) axis. In practise, continuous re-straint of x-direction motion along the edge of the panelwould also induce restraint of rotations about the y- and z-axes because of the physical size of the edge plane.

The appropriateness of the restraints was confirmed bycomparing the stresses in a single panel subjected to re-straint of x-translation and then x-translation combined withconstraint of y-rotation, at all nodes on the connectingboundary, with the stresses obtained in the same panel whenit was part of the complete (three panel) model. The maxi-mum (tensile) and minimum (maximum compressive)stresses at the top and bottom of the shell were compared foreach load case. The number of nodes where the magnitudeof the stress in each restrained case was different from thecomplete model by more than 1 MPa was summed. Thevalue of 1 MPa was chosen as it is about 5% of the expected

modulus of rupture of the material (25 MPa) and thus laywithin the expected accuracy of the FE model. For the firstconstraint (x-translation only), 0.12% of all nodes in all loadcases had stresses more than 1 MPa different to the completemodel, whereas for the second constraint (x-translation andy-rotation), the value was 0.04%. The restriction of x-trans-lation along the edge face was therefore deemed satisfactoryto simulate the effect of the adjoining panels. The results ofthis analysis dictated the choice of a steel frame as a lateralsupport, as described later and shown in Fig. 6. The steelframe was designed to restrict the canopy from moving inthe x-direction. With the actual vertical dimensions of thisrestraint at the point where it meets the edge of the canopy(Fig. 6), the second constraint (rotation about the y-axis) wasalso achieved. The length of the steel frame also constrainedz-axis rotation.

Critical test loadsThere were two objectives in analyzing the load cases.

The first was to find which case (of 2, 3, or 4) was criticalfor snow load and which (of 5–10) was critical for wind (up-lift) load. While snow loading is gravitational over the upper

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Fig. 4. (a) Structural elements in one panel of the model, (b) front elevation, and (c) side view.

surface of the canopy, wind loading is pressure normal tothe undersurface of the shell. The loadings are thus not theinverse of each other; the wind load has a horizontal compo-nent, as it is normal to the cylindrical portion of the canopy.

The second objective was to find a factored value of thecritical load case equivalent to the factored dead and liveloads, so that the actual loading to be applied experimentallywas an appropriate multiple of the live load. Thus, the sec-ond objective was to determine the value α, such that

[1] αL D L D= + −1.25 1.5

where L is the live load and D is the dead load. The deadload was subtracted from the factored load because it al-ready existed in the form of the panel itself in the experi-mental work.

The critical load case was deemed to be the one thatcaused the largest number of high stresses in the model. Forthe snow loads, each load case was simulated with the FEmodel and the principal stresses at the top and bottom of theshell, due to dead and live loads, were obtained. Nodes withstresses less than 1 MPa were excluded from further consid-eration. For snow loading, for each node on a shell element,the maximum value of factored stress, the load case, and thefactor α were identified. With snow loading, the dead loadalways increased the peak stress induced by live load, sincethe two loads act in the same direction. However, for windloading, the dead load can either increase or decrease thepeak live load stress. Thus, where the dead load increasedthe peak stress induced by the uplift loading, eq. [1] wasused. Where the dead load counteracted the effect of the liveload and reduced the stress induced by the live load, 0.85dwas used in eq. [1] rather than 1.25d.

The critical load cases were identified, and the loadingfactor α for both the snow and wind loads were obtainedfrom this analysis. For snow loading, 65% of nodes havingmaximum principal stresses higher than 1 MPa were due tosnow loading case 2. The average factor α obtained for case2 was 1.75. Wind load case 6 induced high stresses at 34%

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k k

k

k

k k k

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-k

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k

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k

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k k

k

k

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Fig. 5. The nine live load cases considered for modelling drawn on a series of plan views of a single unit of the canopy, where positivesnow loads (s) are vertical downward pressures and negative wind loads (w) are uplift pressures normal to the surface of the canopy.

Fig. 6. Lateral stiffener.

of the nodes but case 9 had high stresses induced at 50% oflocations. The average factor α obtained for case 9 was 1.7.A variation of case 9 was therefore used as critical (Fig. 7),as only three different pressures could be created with theavailable equipment. The two pressure areas on the right, 1.2and 1.1 kPa, were combined into a single area and the aver-age pressure of 1.15 kPa was applied.

The stresses induced by the critical load cases were nextexamined to determine where the actual canopy should beinstrumented during testing. The results (Figs. 8 and 9)showed that the rib connecting the arched part of the shell tothe flatter section needed to be instrumented on both upperand lower surfaces.

Physical testing of prototype

Static test setupThe tests were performed in the structures high bay of the

Calgary Centre for Innovative Technology, University ofCalgary. The column to support the canopy was the first toarrive to the laboratory and was bolted to a 2 m cube of con-crete to ensure its stability (Fig. 10). The canopy was deliv-ered to the laboratory in two halves and assembled on theload floor with the aid of two steel frames in the same wayas it would be assembled at the production plant (Fig. 3).Strain gauges were placed on the canopy after assembly butbefore hoisting into place above the column and supports.The gauges were either 90° rosettes, where the principalstresses from the FE analyses varied with the loading cases,or uniaxial gauges, where the direction of possible criticalstress was more consistent.

The FE results (Figs. 8 and 9) indicated that the locationslikely to have high strains were those on both the dome sideand the flat side of the stiffening rib running along the domeand (or) slab connection, on the top of the canopy. On theunderside of the dome, gauges were only placed on thedome side of the beam, as the analyses indicated that tensilestresses that might result in cracking were unlikely to de-velop on the underside of the slab part of the shell — undereither snow or uplift loading. Gauges were also placed nextto the centre front-to-back beam (bolted together during as-sembly (Fig. 11)) and beside a bolt hole connecting the HSSsection for supporting the louvres to the dome around theunderside of the dome (Fig. 11b). Such bolt holes would bestress raisers for the tensile stresses induced in the domefrom the wind loading. The accuracy of the gauges and thedata acquisition system was ±10µε.

Once gauged, the canopy was swung over the struts, low-ered into place, and the connections welded (Fig. 12). Thecanopy is held to the single front strut by four lengths ofrebar embedded in that strut, which pass through matchingholes in the canopy. The holes are subsequently grouted andthe rebar welded to a plate on the upper surface of the can-opy. The rear struts are welded to plates on the canopy.

Boundary conditions for actual testA special steel frame was constructed to provide the x-

translation restraint, consisting of a steel plate supported byfour braces bolted to the load floor (Fig. 6). To permit verti-cal and front-to-back motion, oversize holes were cut in theplate where the bolts would normally connect one panel to

the next. Sheets of Teflon® and grease were placed betweenthe plate and the edge of the panel and between the bolt“washer” and the plate (Fig. 13). The bolt (and thus thepanel) was free to move within the oversize hole. With thenut hand tight on the bolt, the panel was restrained frommoving laterally (i.e., normal to the steel plate, the x-direc-tion). Rotations about the vertical and front-to-back axes arealso constrained, but small rotations about the transverseaxis can occur so long as the movement of the bolts can beaccommodated in the oversize holes. The grease and (or)Teflon® combinations provide very low friction and thusmovements parallel to the steel plate can occur freely (verti-cal and front-to-back translations, rotation about the trans-verse axis).

Loading sequenceThe sequence of loading chosen was aimed at being able

to fulfill the acceptance criteria should cracking occur. Aslinearity was required to unfactored design load for both up-lift and snow, the following sequence was applied:(1) critical case (snow) to unfactored design load (linearity

expected)(2) the less critical wind case (uplift) to unfactored design

load (no cracking expected), and then factored designload (cracking permitted)

(3) snow to factored design load (cracking permitted)With this sequence, the linear response to both loading

cases would be obtained, even though cracking might subse-quently occur.

Flexible rubber mats were used to simulate snow loading.Each mat applied a gravitational distributed load of 0.18 kPaover its area. Thus, multiple layers were required to simulatethe overall required load (Fig. 14). Uplift was applied withair bags, which were placed between the underside of theshell and a reaction frame (Fig. 14). Bags were connected inseries where the same pressure was required. Three differentsets of bags were used to create the different pressure areasfor the critical uplift case. Pressure was supplied to eacharea through pressure control valves. Pressures were raisedstepwise and in proportion to the unfactored design load andthen up to the factored load.

Static test resultsWhen the supporting wood was removed from the panel,

such that it was supported solely by the struts and column ofthe space structure, there was a small forward motion com-

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Two areasthat wereaveraged andtreated as asingle area

k

k

k k

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Fig. 7. Plan view of a single canopy unit showing the applieduplift load.

bined with a small rotation about the transverse axis. Thebolts that connect the canopy to the designed lateral steelframe at the rear rose in their oversize holes and those at thefront descended, with all bolts moving forward. Strains dueto assembly were recorded and were minimal; in the order of20µε or less.

For the first test, five layers of mats were placed withreadings taken from the gauges after placement of eachlayer. The final distributed load applied was therefore0.9 kPa, just under the unfactored design load of 1.0 kPa.For the second test (uplift), pressures were raised stepwiseand in proportion to the unfactored design load, and then upto 1.70 times that load. The final static test was the applica-tion of snow loading to 1.75 times the design pressure of

1.0 kPa. Mats were applied in layers again with readingstaken at 3, 5, 6, 7, 8, 9, and 10 layers. The final total loadwas thus 1.8 kPa, slightly higher than the calculated factoredload.

Strains measured during loading were linear to the load(allowing for noise), with most being of inconsequentialmagnitude. The linearity demonstrated a lack of cracking inthe structure due to loading. The distributions of strain gen-erally confirmed the patterns expected from the FE model.The strains from uplift loading were not the mirror of thoseof snow, again as expected. The strains predicted from theFE model were small and were in the same range as thestrains recorded by the gauges mounted on the canopy. Forexample, during experimental snow loading, the maximum

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Fig. 8. Stresses due to the critical snow load on the canopy: (a) on the upper surface, (b) on the underside.

Fig. 9. Stresses due to the critical wind load on the canopy: (a) on the upper surface and (b) on the underside.

strain increase was about 70µε, recorded on the upper sur-face of the flat part near the strut connected to the canopy.The maximum strains at that location predicted by the FE

model were in the range of 40µε. The experimental strain ishigher because the experimental loads were factored loads(1.8 times the design load). As the values of the experimen-

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Fig. 10. (a) Concrete block to stabilize the canopy and (b) column being bolted to the concrete block.

Fig. 11. (a) Strain gauges on the upper surface and underside of the canopy (the arched HSS section to support the louvres can beseen on the underside of the dome in the left figure and (b) numbering of strain gauges drawn on plan views of the canopy.

tal recorded strains and those from the FE model were bothsmall but comparable, there is some assurance that the FEmodel was providing a correct sense of structural behaviour.

The gauge that produced the most interesting strain wasthe one on the underside of the front strut just above thejunction of the two arms of this strut (Fig. 15). This gaugeshowed rapidly increasing (tensile) strain with snow loadingand a total change of some 220µε over the full snow load of1.8 kPa vertical distributed load. The tensile strain developedbecause of the rigid fixation of the strut to the canopy. In

side elevation, the canopy appears to form a portal framewith the struts, with the front strut at a sharp angle. Never-theless, the transfer of moment from the beam of the frame(the canopy) to the side legs induces tensile stresses on theoutside of the frame legs — the underside of the front strut.The rear struts do not demonstrate the effect to the same ex-tent, as there are two of them. This structural behaviour wasconfirmed by an individual walking forwards and backwardsalong the centre line of the canopy while the strain was con-tinuously monitored. The largest increase was when the indi-

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Fig. 12. Fixing the canopy to the supports; (a) rear struts and (b) front strut.

Fig. 13. Greased steel plate with enlarged holes, Teflon®, and steel washer.

vidual was at the middle of the canopy, and the least effectoccurred when the individual was at the front or rear edge.In the latter situation, no moment is induced in the portal.

Dynamic tests

The frequencies of natural vibration were determined fromboth a dynamic analysis of the FE model and dynamic testson the prototype panel. This test was conducted before apply-ing the static loads to the structure, as it was not known a pri-ori what damage the static loads might cause. Had there beendamage to the structure during the static load testing, the dy-namic test results would have been affected by the change inthe stiffness of the structure. For the dynamic test, accelerom-eters were placed on the canopy and on the rear of the columnat mid-height. Excitation was provided by pulling with a ropein the front-to-back direction, hitting the structure with a mal-let at different locations, and jumping on the structure. Oscil-lations were recorded for all excitations.

The first mode of free vibration obtained from experi-ments was front to back sway from the base of the column(Fig. 16). The column and the front beam had zero phasedifference, while the rear was out of phase by π radians(180°). This corresponds to the results obtained from the FEmodel. The canopy is essentially a mass on the end of the

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Fig. 14. (a) Snow load and (b) wind load.

Fig. 15. Front support before the canopy was put in place. Ar-row points to the location of tensile strains.

Fig. 16. The shape of the first mode of vibration obtained fromfinite element analysis.

Fig. 17. The front strut was modelled by the combination ofbeam elements shown.

column. Because this mode develops from the base of thecolumn, the canopy is rotating about the longitudinal x-axiswhile moving backward and forward. This primary modeoccurs at 2.0 Hz experimentally but at 2.8 Hz in the FEmodel.

The difference between the model and experimental re-sults indicates that the model is stiffer than the actual struc-

ture. The frequencies obtained from the model decreased invalue when the main column was simulated as a series of ta-pered rather than prismatic members. The frequencies de-creased further when the stiffeners at the joints in the strutswere altered (Fig. 17). The natural frequency thus decreasedfrom 2.81 to 2.62 Hz with these modifications. Modelling ofthe actual stiffness of the column, the struts, and their con-

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Fig. 18. (a) Half canopy in steel form and (b) complete mould in one position during curing.

Fig. 19. (a) Columns in place on platform and (b) a canopy being swung onto the temporary scaffolding.

Fig. 20. Canopies set on temporary scaffolding and struts attached.

nections is therefore important to the vibration response. It isparticularly important for the first mode of vibration, wherethe canopy is the mass on the end of the cantilever of thecolumn and struts, because the stiffness of the cantilever hasa primary influence on the frequency. In general, the modeshapes obtained from the model resembled the mode shapesobtained from the accelerometers in the physical test. An ex-act comparison, however, was not possible because of thelimited number of accelerometers and their orientation; eachaccelerometer could only measure the acceleration in one di-rection, therefore, the complete motion of the canopy wasnot captured. The higher modes often involved various con-figurations of the flatter part of the canopy “flapping”.

Manufacture and installation of the actualstructure

The precast canopy components were individually castand consist of half-shells, columns, tie beams, struts, andtroughs. The columns and half shells were injection cast inclosed steel forms (Fig. 18); troughs were cast through dis-placement moulding, while struts and tie beams were pro-duced using conventional gravity two-stage castings. For thehalf-shells, a series of pilot tests was performed to determinethe best inlet geometry, pressure, and flow rate. To avoidshrinkage cracks, the moulds were rotated at specified timesduring initial curing and the half shells demoulded 12 h aftercasting.

With the experience from the laboratory, the right- andleft- half shells, along with the tie beams, were preassem-bled in the plant and then transported to the site. At the site,the columns were installed first and temporary scaffoldingwas assembled (Fig. 19). The canopies were placed onto thetemporary scaffolding, and the struts were attached to theshells and the columns with welded connections (Fig. 20).The completed structure is shown in Fig. 21.

Discussion

The FE models were used to aid in both the design of thecanopy and formulating the test parameters. In the tests, ten-sile strains were measured on the underside of the front sup-porting strut under snow loading. This was not expected.Further tests revealed that this tensile strain was largestwhen point loads were applied in the middle of the span be-tween supports and least when point loads were applied overthe supports — even the front strut. This behaviour indicatestransfer of moment from the canopy into the struts, suggest-ing the canopy and struts act as a portal frame in a front-to-back vertical plane. Tensile strains were not predicted by themodel on the underside of the front strut. The sensitivity ofthe model was therefore assessed in terms of the way thestruts and their fixities to the canopy were modeled.

The actual front support consists of two struts that meetbelow the connection to the canopy and was modelled by acombination of beam elements. However, if the configura-tion of the model is changed in this area, distinctly differentbending moments can be induced at the upper end of thestrut. For example, results showed that there is a substantialchange in the bending moment in the upper part of the strut

through simply removing one of the small beam elements(Fig. 22).

The obvious sensitivity of the model to the way the strutconnection is modelled may be in part why the frequency ofthe first mode of vibration was 2.8 Hz, as opposed to the2.0 Hz in the actual structure. The second mode of vibrationfrom the model involves rotation about the front-to-back (y-)axis. This mode would not be observed experimentally be-cause of the lateral restraint on one side only and is unlikelyto occur in practice because of the restraining effect of theneighbouring panel. The higher modes in the model appearto involve forward and (or) backward sway with differenttypes of flapping of the flatter slab sections of the canopy.This was confirmed experimentally when analyzing the datafrom the dynamic testing. The amplitudes of the highermodes, however, are unlikely to be large, but the forwardand (or) backward sway of the primary mode may well beexcited by gusting wind or vibrations from train movement.

The FE models were extremely useful in indicating wherestress concentrations might arise in the canopy and how thecanopy should be designed to minimize these. The modelsalso provided insight for the testing loads and boundary con-ditions. The tensile stress induced in the front support wasexplicable with the help of the models and showed that anarea of the model that was not of prime interest in terms ofcanopy design could, in fact, create significantly different re-sults for that part of the overall structure. The tensile stressesinduced by the factored design load were about a quarter ofthe flexural tensile strength of the material. Finite elementmodelling of the shell structure was therefore highly benefi-cial from both the design and testing perspectives. The ac-tual canopy withstood maximum snow and uplift loadswithout damage. The FE model was also helpful in explain-ing the tensile stresses induced in the front strut during thephysical test.

This project was the first of its type in the world for theuse of this UHPC for thin, architectural curved canopies.While this solution demonstrates many of the benefits of thenew material technology, it is apparent that the true benefitshave yet to be fully recognized. Optimized profiles could be

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1164 Can. J. Civ. Eng. Vol. 32, 2005

Fig. 21. Completed installation.

developed for this material, so much progress and manymore applications based on this first one are anticipated.

Acknowledgements

The authors gratefully acknowledge the help of the Trans-portation Planning Office of the City of Calgary, Terry Chowof Lafarge North America, and the technical staff of the De-partment of Civil Engineering, University of Calgary fortheir help in executing this project. Strudes Inc. supplied

their FE model of the canopy — many thanks. The authorswould also like to thank Enzo Vicenzino of the CPV GroupArchitects and Engineers Ltd. for supplying the site plan andthe canopy drawings.

References

NBCC. 1995. National Building Code of Canada 1995. NationalResearch Council of Canada, Ottawa, Ont. pp. 144–146.

Computers and Structures. 2000. SAP2000 [computer program].Computers and Structures, Inc., Berkeley, Cal.

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Adeeb et al. 1165

Fig. 22. Large changes in bending moment (newton-metres) at the front support for small changes in the model, demonstrating thesensitivity with respect to how the support is modelled.