A parallam and stainless steel compositionEpp, Gerald A.1, Woudstra, Brian M.2

INTRODUCTION

The recent Pacific Canada expansion at the Vancouver Aquarium involved enclosure of a central courtyard, which serves asa feature welcoming space for the entire facility. In keeping with a West Coast theme, the architect chose exposed heavytimber (Parallam and turned Douglas Fir logs), to form the surroundings for the magnificent new Pacific Canada pool centeredwithin the space. The desire was to create an exposed wood structure which was more refined than normal heavy timber, andwould also endure a salt-water marine environment. As a result, polished stainless steel was introduced as part of the structure,and much attention was paid to refining the structural connections.

To facilitate quality control, a fast schedule, an extremely tight site (already busy with demolition and renovation work), andan operating facility, StructureCraft Builders prefabricated all the trusses, beams, purlins, and even bow-truss windowmullions, and all their fittings, in the shop for speedy erection by their crews on site. This paper will describe the creation ofthis unique timber structure, from design conception to the fine-tuned installation of the window mullions.

DESCRIPTION OF STRUCTURE

The new roof structure is supported on three sides by the walls of existing buildings, and on the fourth (west) side by newbeams and log columns which frame a long glass facade overlooking the feature killer whale pool. The structure clear spansapproximately 20m x 25m over the newly created Pacific Canada Pool and exhibit space, and consists of exposed timberdecking on parallam heavy timber used as purlins, beams, truss members and as beam trusses in combination with stainlesssteel. (See Fig. 1 for plan layout)

The architect chose to place a large vaulted pyramidal structure directly over the new pool. This pyramid is truncated witha skylight on top, and is approximately 13m x 17m in plan at the base and 2.5m high. Its three sides form a 3-dimensional“folded plate” truss which works with three supporting king post beam-trusses to support itself and surrounding roof areas. Each planar truss in the pyramid is up to 4m in width (measured in its plane) and consists of chords and webs made of doubleparallam timbers, with connectors hidden between. The edges of these trusses join together to form the hips of the pyramidand carry large compression forces which are resolved in three thrust blocks at the corners of the base.

The pyramid truss with its three boundary beam-trusses, is supported at one of its corners by the log columns, and at the othertwo by heavier beam-trusses, which transfer the loads to the three existing walls. Vertical beam reactions from the flat roofsurrounding the pyramid must be carried into the plane of the pyramid truss, resolving horizontal components through specialconnections into the roof diaphragm.

The five beam-trusses consist of double parallam beams trussed at a single point in the span with a large stainless steel “yoke”,connected to stainless steel rods as tension members. (see Fig. 3) The rods are anchored at the beam ends to large stainlesssteel pins which bear against plates timber-riveted to the beams. The two larger beam trusses are heavily stressed, and requiredpre-stressing of the rods to change the stress distribution in the beam, and allow higher capacity. Timber rivet connections,mostly hidden, played a prominent role in both the beam-trusses and the pyramid truss.

While the pyramid and supporting beam-trusses are located somewhat asymmetrically in plan, they support a very regular gridof flush, concealed-hanger mounted parallam beams and purlins in the flat roof area, which extend to create a large overhangover the west facade. At the point where little backspan exists on the cantilever beams, round timber struts prop the beams 1 Gerald A. Epp, President, StructureCraft Builders Inc. #201-1672 West 1st Avenue, Vancouver, B.C. V6J 1G12 Brian M. Woudstra, Project Manager, StructureCraft Builders Inc. #201-1672 West 1st Avenue, Vancouver, B.C. V6J 1G1

against the log columns. The flat roof is covered with 38mm tongue and groove timber decking, and the pyramid with 64mmdecking. The entire roof is sheathed to resist diaphragm forces with 13mm plywood.

The west glass facade is up to 4m high, and consists of double butt-glazed units with silicone joints. To provide lateralstability to the glazing, the architect and structural engineer devised a vertical bow-shaped mullion design composed of astainless steel “Tee”, to which the vertical glazing joints were siliconed, and a custom made, hand-rail-shaped vertical parallampiece which curves as a bow into the room, but joins the stainless Tee at top and bottom. The intent was to create a structuralelement with a furniture quality.

DETAILED DESIGN AND SHOP DRAWINGS

Following award of contract, StructureCraft Builders Inc., the fabricator-erector of the roof and mullion structure, provideda complete detailed design of all the connections, and assisted the architect with refinement of some of the exposed details.

The unusual three-dimensional geometry, and the variety of components and materials, coupled with the need to conform tothe existing building layout, created numerous challenges. One interesting problem was the development of connectors forthe pyramid bases which would transfer large forces and simplify fabrication, shipping, and erection.

StructureCraft supplied shop drawings for all the timber and steel components to ensure precise fitting of the numerousinterdependent parts. Shop drawings totaled over 130, many of which were full-size sheets. This shop drawing processreinforced the importance of accuracy and coordination in the production of prefabricated components.

SHOP FABRICATION

All of the timber components were pre-fabricated in StructureCraft’s shop, which was equipped with two cranes. The craneswere extremely useful in handling the large beams and trusses (up to 3400 kg) required for this project. Machined stainlesssteel, fabricated stainless steel, and hot-dip galvanized mild steel were produced in trusted local shops, and delivered to theStructureCraft shop. As many components as were practical were preassembled in the shop prior to shipping. Great care wastaken to ensure accuracy, so that the “kit of parts” would fit together well on site.

Flat roof beams and purlinsThe flat roof was sloped up to 1m, which had to be accounted for in the geometry both of beam and purlin end cuts, and theconnectors. Purlins were 89 x 241 parallam and supporting beams 133 x 457. Proprietary hidden beam hangers had to bespecially ordered with modifications to suit the various conditions. These hangers were inserted into mortised slots in the beamand purlin ends, which were cut using a special chain mortising tool. Lengths of members were calculated to provide aconsistent 10mm reveal at the flush connections, to allow for some erection tolerance, and enhance the appearance of theconnections.

Composite parallam/stainless steel beam trussesAs discussed, there are two types of beam-trusses. The three which form the base of the pyramid truss are made of pairs of133 x 457 parallam beams of varying length, and double 32mm dia. stainless steel rods fitting with stainless steel clevises ontoa double “yoke” (see Fig. 5) at the centre, and threaded through 102 mm dia. stainless steel pins, which pierce through thedouble beams at the ends. The force from the pin is transferred to the wood through bearing on the 6mm hot-dip galvanizedtimber rivet plates, which are riveted to the inside (concealed) face of each beam with 89mm timber rivets. Stainless steelretainer rings hold the pin in place.

The two beam-trusses supporting the corners of the pyramid, and indeed, much of the roof, were more heavily stressed, andconsisted of double 178 x 457 parallam beams and triple 45 mm dia. rods. The triple yokes are located off the centre of thebeam, below the point load from the pyramid corner. On these trusses, heavy forces required larger end pins, and double10mm timber rivet plates on both beams. There are 216 - 89mm timber rivets installed in each of the plates of these trusses. In keeping with specifications, the rectangular cross-section rivets were installed with their longitudinal axis aligned with thegrain of the wood, and they were placed starting with the outside ring of holes, moving progressively inward. This methodprevents any tendency to split the wood, and provides confinement.

The beam-trusses were assembled in the shop in the upright position, with the beams supported by blocks at their ends. The

rods were reverse threaded to allow for tightening, which was accomplished using milled flat spots near the ends. They werethreaded into the pins and custom clevises using molybdlenum grease, to prevent freeze-up. Pre-cambering of the beamsupwards approximately 40mm was achieved by lifting on the centre of the 16m span with the crane while the rods were beingtightened. The result was a tightly sprung truss without sagging rods, ready for installation.

All of the stainless steel components were custom-designed. Since there are not an abundance of stainless steel shops equippedfor or accustomed to fabricating large structural pieces with an architectural finish, selecting the right shop took some time. The pins, clevises, and associated hardware are of type S.S. 303 stainless machined on a CNC automatic machine from theoriginal AutoCad shop drawing files. The triple yokes are made of 25mm thick S.S. 316 stainless plate. The smooth profileof the yoke legs was cut using the water-jet method, which gives the cleanest cut for such thick material. In order to savematerial, each triangular leg was formed out of three pieces, fused together in low stress locations with full strength fusionwelds. The welds became imperceptible after polishing, which was a costly and involved process.

Pyramid trussesThe three planar, trapezoidal trusses which form the sides of the structural pyramid consist of chords along the boundaries withinterconnected webs. These members are double 68 x 241 parallam (separated by a gap) with timber rivet plates fastened tothe inside face of both pieces at the end connections. The galvanized plates were routered flush into the parallam, and thenumber of 38mm rivets vary with the size of the connection force. The plates were fabricated with bolt holes in their centres,to align with holes in the parallam, so that when the trusses were assembled in the shop, the accurately calculated angles ofthe 10mm thick connector “fan” plates would define the geometry of all the members connected to each node (see Fig. 4). Thus, single bolts in double shear, steel to steel, could be used for factored connection forces as large as 220kN.

The shop drawings were very helpful in allowing the carpenters in the shop to, with confidence, bevel-cut the members so thatthey would mate with a constant reveal between them. Cuts as shallow as 15 degrees were made, and the fit was very close. In order to prevent damage to sharp ends of such members, the cut was designed to allow for a “blunting” cut, leaving athickness of about 4mm at the tip. It was very rewarding to see the large trusses come together so accurately.

The long (up to 18m) and tall (up to 4m), but thin profile of the individual trusses created handling problems, even with thecranes, in the shop. The out-of-plane flexibility allowed by the single bolt joints were also to prove a later problem duringerection on site. Each truss was shop fitted with connectors along the hips and bases, in keeping with the sequence of erection,which would allow its placement with minimal difficulty on site.

It was perceived early in the design process that the geometry associated with the completely unsymmetrical shape of thispyramid could create very complex timber connections, and it was decided to resolve the complexity in the steel connectorsrather than the wood. Thus, geometrically complex steel thrust blocks were designed to receive the end bearing compressionforce of the hip-members, and to transfer the forces at the three corners of the pyramid base into tension in the beam-trussesbelow it. These thrust blocks were each fabricated in two pieces so as to fit onto the individual truss ends in the shop. Thethree beam-trusses had extensions of the glulam rivet plates which were fitted with bolt studs to connect to the thrust blocks. Extra glulam rivets were required to take eccentric joint forces.

Log columns and strutsThe three turned Douglas Fir log columns are 457mm dia. and support large 273 x 711 deep parallam beams spanning up to12m over the window head. All bearing surfaces of beam and column were prepared to receive the shallow beam slopes. Knife plates and countersunk bolts were used to connect bases and tops of the columns. The four 150mm dia. turned strutssupporting the cantilevered roof beams are connected to the columns and beams with custom polished stainless steel brackets. The struts are tapered at the ends to meet the connections.

Window mullionsMuch thought was given to the final design of the seventeen bow-shaped vertical window mullions, which stand up to 4m tall. Proportions and shaping were critical to the appearance of these highly visible pieces. Further, every one is a different length,due to the slopes in the glazing head beam. Each mullion consists of a curved parallam member, approximately 89 x 48mm(mushroom shaped in cross-section), with a straight stainless steel Tee, connected as shown in Fig. 2.

Parallam stiffness dictated the use of 13mm thick strips in the gluing jigs for the 10m. radius curve, which was kept constantfor the various billet lengths. Special shaper bits were made to create the desired shape, which was selected after looking with

the architect at a number of alternatives. Following shaping, the curved parallam billets were filled and sanded prior tostaining. They were later shipped to the site, to be fit to length in accordance with field measured conditions of both head andsill.

The stainless steel tee whose stem fits in the silicone joint between the 1400 wide double-glazed panels, is connected to theparallam billets at top and bottom with a profiled fillet piece, whose shape was adapted in three types to the various lengthmullions, because end geometry was surprisingly different. Three rods intermittently connect the two components betweentheir ends. Fabrication was done with stitch TIG welds for appearance, and to minimize distortion with plates thin as 5mm.

SHIPPING AND ERECTION

Shipping of the wide loads was done at night, to meet City of Vancouver requirements. There had been some concern thataccess down the rear Vancouver Aquarium loading ramp could be a problem, and it was only due to the skill of the driver thatthe trailers were placed within reach of the crane.

A 60 ton mobile crane was used for erection. It was placed strategically for erection in a location which balanced a numberof constraints. Reach and load limitations required a location which initially interfered with a portion of the roof, and wasat the same time the heart of the Aquarium’s customer activity. Work began early every morning, but overlapped with openhours. Safety was paramount, and pedestrian traffic was carefully controlled. Erection sequence was as follows:

• Ledgers with field-measured pre-drilled holes were mounted on anchor-bolts in existing concrete block walls on threesides. Precision was required, because some of the ledgers are exposed to view, and yet difficult to achieve because ofsite conditions.

• The two heavy beam-trusses BT4 and BT5 were placed on brackets in the three walls. BT5 was the truss which governedthe crane size, because of its weight and the reach required (3400kg at 20m. reach.)

• The pair of log columns below the corner of the pyramid were place, as were the large adjacent window head beams.

• The remaining three beam-trusses were placed on top of BT4 and BT5 and on the beam over the pair of log columns. These trusses played a crucial role in creating a stable base for the pyramid erection.

• Beams and purlins for the flat roof were erected, beginning at the south side. Due care was given to special-purpose andskewed connectors at the interface with the pyramid. Slowly the shape of the roof began to emerge. A portion of themembers near the north west corner of the roof had to be temporarily left out because of crane interference.

• Before stability of the pyramid could be achieved, all three pyramid trusses had to be in place and fastened together. Thus, the first two were temporarily shored, in accurate locations, before the third was erected. As the grueling processof erection of these planar trusses began, it became immediately apparent that the lifting of the trusses in a near horizontalposition was going to create problems because of out-of-plane distortions. The final positions of the two largest trussesare only 40 degrees off horizontal. Careful choosing of pick locations and use of a “whipline” were necessary to combatthe problem.

• With the pyramid complete, the crane could back out to complete the balance of the flat roof. Timber decking andplywood were then installed, completing the roof structure.

• The window mullions were installed, with due regard to the need for complete plumbness to receive the glazing, mademore difficult because the fastening of the parallam bow tended to affect the plumbing of the Tee.

CONCLUSION

This unique exposed timber structure required a level of refinement not normally seen in North America. The design dependedheavily on pre-planning, production of accurate shop drawings, precision shop craftsmanship, and innovative site installationprocedures. Yet it also demonstrates that a whole new range of heavy timber design possibilities exist within our markets.

Installing timber rivets in the shop. Pre-assembled beam trusses ready to leave the shop.Note pyramid trusses in background.

Curved Parallam mullion billet. Lowering final pyramid truss into place.Note temporary bracing shores.

Exterior view. Interior photo.Note window mullions in the background.

Note pyramid truss framing. Clevis-Yoke connection.

Close up of the window mullions.

Photographs courtesy of Roger Brooks Photography and Martin Tessler Photography