bend the steel › Using a 3 point smooth bending machine › Using a brake press › Heat applied bending facet the building to give the appearance of curves while using straight members cut curved forms out of plate material
Member type Orientation of member Length of member Shipping considerations Sourced out work Accuracy of curve Multiple curves
Different shapes are more or less “easy” to bend Tendency for buckling on tighter curves Thin steel likely to buckle Heavier steel harder to bend
Dies have to be tooled for each shape and changed between jobs. Smaller runshave higher costs. Bender must have the dies in order to do the job.
There is always a straight piece at the end as a result of the bending process thatis cut off and recycled. So the length of “raw” member must be longer than thebent one.
The straight steel that is shipped INTO the bending facility may NOT fit on the sametruck for transport back to the fabricator. Detailing must account for this andinclude splices if required.
These curved tubes for the Canadian Museum for Human Rights were to the limitfor the bending facility. These will be AESS4 quality in the finished project as the“Cloud Rails” will be used to support the specialty glazing system.
The curved stairs on the Art Gallery of Ontario were fabricated using round HSS. Therewere difficulties in ascertaining approval of the splices as unavoidable deformationshappen when bending tubes, so guarantees on the welds were difficult.
When splicing tubular steel so that the joins are not evident, it is typical to use aninset sleeve to form the backstop for the weld. Given the angular splice anddeformation of the material, this proved to add a challenge to the splice.
Although workmanship was not a large issue in making the splices, the contractorcould not use plates or bulkier methods as the cladding for the stair was to be verytight to the steel.
The Abilities Centre in Whitby, Ontario, uses curved steel to create the top andbottom chords of these large, long span trusses over the rink area of the sportsfacility.
The connection between the curved “vertical” truss and the long horizontalmembers have been done as to be invisible. Although similar methods are used asin the AGO stair, the AESS4 level of this project requires impeccable workmanship.
It is not always necessary to hide connections. Here plates are used between thejoining elements of the truss to accentuate the detail. This is easier to accomplishthan a fully blended weld, and truly adds elegance to the detail.
Frank Gehry’s Experience Music Project in Seattle, Washington used W sections tocreate the complex curves for the project.
Although the steel exposed, it is quite high up and the aesthetic might only call foran AESS2. Here more evident splices were permitted as they added to the ruggedaesthetic of the interior.
In brake forming, sheet steel is carefully marked with “lines” and then the brakepress puts pressure on the lines to create creases. Much skill is required by theoperator to determine the correct pressure and line placement.
These oversized cylinders have been created using brake forming. The lines areevident on the interior but not the exterior. Brake forming is used for oversizedmembers as well as plate that has complex or non uniform curves.
Here on the AGO, brake forming was used to form the large plate sections into thecomplex curves required for the stair. Weld seams can be seen to join the wedgesof the large flat portion that will provide support for the steps.
The grand stair at the Boston Society of Architects was created using brakeforming. The stair (tread portion to handrail) was installed/shipped in one piece.The side piece/hanger is separate. Bolted joint visible at left.
The treads were fabricated separately – brake forming used for the curves. Thetreads were then welded to the side panels and the welds completely concealed.This is AESS4 level workmanship.
The stadium designed by Peter Eisenman may appear to be curved, but is actuallycreated from all flat sections. Faceting is used in the design to give the illusion of asmooth curve. This saves greatly on fabrication and erection costs.
It can be seen that the exposed members are quite straight, the curved geometrybeing resolved towards the exterior cladding by a sequential decrease in the spandistance.
The Chinese National Theatre in Beijing is a large “egg” shape. The exteriorcladding is faceted. The interior structure uses cut sections of plate to create truecurves.
Each of the trusses that forms the amazing structure for the theatre is created fromplate sections that have been cut to true curves and welded to create largerentities.
The welds within the trusses have been cleanly done, carefully filled and sanded toAESS4 levels in order to provide a clean, seamless appearance. They are in closerange to sight and touch.
The round bracing members that connect the plate trusses are attached via a“ball” joint as it most readily adapts to the continually changing geometry of thestructure.
Here you can see the “secret” behind this joint! Here a fully welded “appearance”was desired, but a bolted connection has been used – subsequently covered withfiller, sanded and then painted.
Cast iron or steel used to be synonymous with the requirement for high levels ofornamentation, made cost effective through the use of articulate forms. This is nolonger the case. Castings are now used to SIMPLIFY connections!
Used for special connections Can be one-of large pieces formed with expendable molds (i.e. Structural) Can be smaller die cast pieces made in great quantity (i.e. glazing attachments) Can be solid or hollow depending on size and purpose
It is said that when the fabrication costs for a connection become 4 times as expensive as the materials used to create the connection, then castings begin to make economic sense.
Some of the first large structural castings to be seen in Canada were used in theconstruction of the atrium for the Caisse de Depot et Placements in Montreal.
The castings at the CDP were connected to round HSS that comprised thebalance of the vertical trusses using variations of non hidden welded connectionsor evident welded connections. There was no attempt to hide the connections.
This steel “tree” at the University of Guelph was created using hollow castings tojoin the branches. The branches were made from mechanical pipe rather thanHSS, for both structural and textural reasons.
The main casting node was attached at the shop as it is easier to have access forpreheating. The connections are welded. Contemporary castings are quiteweldable.
The casting is hollow in the centre but the wall thicknesses are not uniform. Theyvary in order to locate the steel where the load transfer stresses require it.
The expendable molds are often made from sand in resin, based on a form in thiscase driven by a CAD/CAM device. The casting technique leaves the surface ofthe casting with an orange peel like texture.
This close view of the casting reveals the very different surface. In the case of theGuelph Tree, it was important that the connection between the casting and thebranch was seamless, so this required extra care in fabrication and erection.
For these unique connections it is important that adequate testing be carried out.The steel must go through very controlled cooling in order to prevent the build upof problematic stresses in the material.
The cast connection used at Hauptbahnhof Station in Berlin does not try to hidethe connection between the casting and the connecting HSS members. Thisreduces the cost and difficulty of site fabrication to an extent.
These tree like supports were repeated throughout the terminal. There is greatereconomy when producing repetitive elements as there are savings in fabricationas well as testing.
The renovation to King’s Cross Station in London uses tree forms as columns.Castings are used to attach the branches to the trunk. A reveal is used as theconnection detail.
The cast node in this instance has an unusual geometry and is not as smooth oruniform in its geometry. The level of quality in the project will depend on the qualityof the translation of the geometry of the design smoothly into the casting.
At Shanghai International Airport, Terminal 2, castings are combined with curvedsteel and tension trusses to create a vibrant structural system.
The attachment of the diagonal to the roof member is achieved through acasting. This created a much smoother and better looking transition. The pinconnection also improved constructability.
Tension members are used to hang the spiral ramp system for the Reichstag fromthe ribbed steel dome.
Smaller die-cast connectors are used at the Reichstag in Berlin to attach thetension support system.
Three different cast connectors are used in the system. The one at the left thataccepts two incoming members is quite innovative and the detail allows for a verytidy connection. This project also uses a significant quantity of bent plate steel.
Started as diagonal reinforcement to a portal frame typology Evolved to exclude the braced frame Elimination of vertical columns Replacement by diagonal members In theory material savings in the range of 20% over traditional braced frame structures
Similar to a space frame, the diagrid is formed by connecting straight members to nodes Nodes can be fixed (moment resisting) or hinge-type (non moment resisting) Fixed most often used for fabrication and erection control
The Royal Ontario Museum, by Studio Libeskind, used fixed nodes to join the straightsegments and required very little in the way of temporary shoring during the constructionof the project. Intermediate members were sized for eccentric loading during erection.
Most current diagrid buildings use fixed or prefabricated nodes Preferred as it makes erection simpler Reduces need for temporary bracing Allows for repetitive shop fabrication Minimizes on site welding of the node itself Introduces some stiffness into the overall structure during erection
In a “simple” diagrid structure, the diagonals take the place of columns Diagonals form a tube that braces the exterior Beams at the edge of the floor connect to the diagrid and serve as braces The floor plate is clear of columns, the floor framing system spanning from the core to the exterior diagrid Hearst Building | Foster+Partners w/ ARUP 2006
The Hearst Tower uses a diagrid frame whose diagonals span 4 floors of thebuilding. The exterior beams for the floors serve to brace the diagonals. The nodesare shop fabricated for maximum accuracy.
http://www.e-architect.co.uk/new_york/hearst_tower_new_york.htmThe prefabricated nodes of the Hearst Tower were attached to the straight Wshaped segments of the diagrid using bolted connections. This made for very fasterection with little “extra” access for fixing the connections.
The diagrid framing for the Hearst Tower was not intended to be exposed, so thechoice of member shape and the bolted connections of the straight segments tothe nodes, had no aesthetic considerations.
Foster+Partners’ Swiss Re Tower in London was constructed shortly after the GLAand made use of a true diagrid. Here the outside diagrid formed the primarystructure to support both floor plates and facade.
http://www.architectureweek.com/The plan of Swiss Re shows the open floor plan between the exterior diagrid andthe central core.
The South Face of the Bow Encana Tower DOES expose its diagrid, so where thesteel is exposed to view, it needed to be of AESS4 quality.
The straight segments have been welded to the fixed nodes to meet AESS4expectations. Only the two sides of the triangular members that are exposed to theinside of the atrium have been fabricated to AESS4. The hidden rear face has not.
Here you can see the weather protection provided at the nodes to allow the highquality welding to be accomplished. This obviously adds cost to the project overbolted connections. But the aesthetic requirements overrule in this case.
The diagrid on the north side of the Bow Encana Tower is concealed by finishes, so an entirelydifferent set of criteria has been applied to member selection, erection and fabrication. Weldedconnections are still used, but the straight members are simpler W sections.
The fixed nodes of the Bow Encana were fabricated in Hamilton and trucked toCalgary. Bracing between the stubs ensured accuracy through transportation anderection.
The straight segments, although extending over 6 floors in height betweennodes, needed to be cleanly spliced at the mid point due to shipping limitations.Not all diagrid straight sections span over the same number of floors. This varies byjob.
The Aldar Headquarters in the UAE, designed by MZ Architects and ARUP, is the firstcircular diagrid building.
https://picasaweb.google.com/lh/photo/EUz-8DR3HES38c4l6LbqVgThe straight sections of the diagrid frame extend only over 4 floors in this case. Theyare attached by fixed nodes. Placing the nodes closer together allows for theincorporation of the curves in the design.
Aldar Headquarters’ diagrid was designed to be concealed, so member choice was based onstructural considerations. You can see how the exterior floor beams are being used to brace thelength of the angular diagrid. There is a clear (albeit short) floor span from the concrete core tothe edge.
http://innovativebuildings.net/2010/06/20/capital-gate-abu-dhabi/It is the strength of diagrid framing that allows Capital Gate Tower in the UAE , byRMJM Architects, to lean backwards 18 degrees.
http://www.zawya.com/story.cfm/sidZAWYA20090114101526/Here, due to the unusual eccentricity of the structure, we see that the fixed nodes happen atevery floor level and that the straight members are much larger than we saw on Aldar. Theconcrete core runs vertically up the building and must offset the floor loading.
http://www.panoramio.com/photo/46687499The eccentric geometry of the building has forced a very tight floor plate, with aninefficient ratio of core to usable area. For this signature building, this was obviouslynot a concern!
The white lines indicate the location of the diagrid. The glazing has beensubdivided into triangles, some of which you see are open permitting naturalventilation.
A sun shading screen is a major feature of the south facade. Round HSS is beingused to attach the facade back to the diagrid structure at the nodes.
The Orbit Tower for the 2012 Olympics in London uses a diagrid type structuremade up completely from prefabricated nodes. These are joined to each otherwithout any straight interconnecting members.
http://www.flickr.com/photos/arcelormittalorbit/However complex the sculpture might appear, it is reduced to the replication of this element.Designed by artist Anish Kapoor and structural engineer, Cecil Balmond, the 115m highArcelorMittal Orbit will be the tallest sculpture in the UK and will offer unparalleled views of theOlympic Park and London’s skyline.
http://www.flickr.com/photos/arcelormittalorbit/Although the nodes were prefabricated with joints between each node, largeraggregated elements were prefabricated prior to lifting to make the constructionmore efficient.
http://www.flickr.com/photos/arcelormittalorbit/As with all projects it is important that the designers keep in mind the issues of scale– size of elements and types of connections – to be sure that the design of theconnections is appropriate to the AESS level of the project.
The new Peace Bridge in Calgary, designed by Santiago Calatrava, uses a diagridtype of “Chinese Finger Tube”, in a fully welded scenario, to create a very slenderspan across the Bow River. Span 130 m.
The end of the tube has been substantially thickened to provide support at theend. The bridge is created from sections of bent plate with internal reinforcement.
The floor has been built into the tube (constructed first). Plate steel ribs have beenwelded to a plate steel floor to provide reinforcement. The bottom of the bridgeshows the small HSS stubs that have been welded into the diagrid halves.
The smooth finish of this AESS4 structure, in combination with the large open panelsand translucent glazed panels, give a light and airy feeling to the bridge interior.The interior was to have been painted white.
Very small traces of show through weld are visible, indicating where the internalreinforcements are positioned. The rounded edges of the final product would havemade the proposed white painted interior rather impossible to achieve.
Night lighting can be strategic, but it will have impact on the requirement of theforming and finishing of high level AESS work.
A note of caution when choosing both a forming method and a finish. Nightlighting can highlight imperfections in the steel that are not evident with regularday lighting. Here the lines of the brake forming translate through the finish.
Many unusual geometries are possible through tension support systems Tension vs. compression can allow designers to differentiate member size and type Systems can include: › Rods › Cables › Smaller sections Standard or high strength structural steel Stainless steel
Tension systems can be used for far more than the support of fabric structures asseen in the tent canopy at the Grand Arch at La Defense in Paris, France.
What is interesting in the use of tension systems at La Defense is the ability of thetrusses to both push and pull the fabric into shape. Clear differentiation of tensileversus compression members is evident.
The Olympic Stadium in London is using differentiated member sizes and types to createan energetic exposed steel structure to support the roof. Within the truss, larger HSS isused for compression members. Actual tension cables are used as well.
The practice of using cables and masts to support “cantilever” situations iswidespread.
The large glazed canopy at the Aria Hotel in Las Vegas, by Cesar Pelli, uses acomplex tension system to “assist” the cantilevered glass.
For the most part the main tension members that assist are hidden from view. Theceramic frit on the glass also makes it difficult to see through and limits theunderstanding of the support system.
Such detailing requires the use of very fine fittings, both to allow for on site erection,potential replacement of broken panes, and tension adjustment of the entiresystem during erection.
The relatively lightweight pedestrian bridge platform is achieved by the use oftension cables to support and thereby break up the effective span.
The railing design on the “support” side of the bridge is slightly more opaque and designed tohide some of these details. Tension cables in exposed conditions must be protected fromweathering. This is often done by overwrapping both cable and connection.
Rather than using a more typical die cast anchor, this bridge creates theconnection point through welded plate “boxes” that hide the more standardbolted connection inside (also weather protected).
This fully glazed room achieves a clear, column free span through the use of atension assisted roof. The tensile members criss-cross the room and are use to pushup through the centre point, allowing the compression members to be quite light.
The custom fabrication for the central support system is quite elegant. If you lookclosely you can see the weld lines on the roof beams as they were attached to the“star”. London, England. No snow loading!
Much of the detailing and expression that we see in tension systems came about inthe architecture of the High Tech Movement. We need to learn from the mistakesmade here to show that steel is indeed a durable material.
The design and detailing on Waterloo Station, designed by Nicholas Grimshaw, areinnovative and exemplary – but this fussy, exposed, exterior structure wasimpossible to maintain, and the train shed is set to be demolished.
The train shed at Hauptbahnhof Station in Berlin uses an exterior tensile structure, but thedetailing and materiality are far simpler than for Waterloo Station. The choice of colour here alsoimpacts cleaning. The glass on the shed is able to be cleaned using fairly standard methods.
The trusses continue on the interior to allow for the clear spanning curved beams tobe lighter through this “assistance”. The square steel framing that supports the glassroof is also reinforced with X type tension reinforcing.
Here you can see the many different levels of structure in the station. The largertension system is comprised of cables with cast clevis attachment systems. Thestation is recent so it will be interesting to see how easy it is to maintain.
The large glazed facade at Poly Plaza in Beijing, designed by SOM, is supported bya cable net system. This eliminates mullions. The “box” at the left is supported by alarge cable system, to effectively hang the structure.
Looking through the glass the ghost of the hanging system can be seen. The majorlines on the facade glass are the connection points of the facade to thesuspension system.
Looking up the atrium the cable net system for the glazed facade is visible and thesuspension system for the “box” barely visible. It is up 15 floors.
Zooming in, you can see the large cast clevis like element that is being used tohang the floors.
The cables that are used to hang the floor system are bundled. The cable netsystem for the facade is attached to the larger cable to provide wind bracing.
The scale and high level of finish of the clevis can be seen in this close up image.
Castings combined with custom fabricated elements are used in this instance toprovide a very clean and controlled appearance, function and assembly for thestructure.
http://www.som.com/content.cfm/the_new_beijing_poly_plazaThis detail of the clevis attachment for Poly Plaza illustrates the type of tighttolerances and complexity when creating such an innovative project.
Steel design today is far different than it was even 10 years ago Architects present fabricators and engineers with complex problems With a good understanding of the goals of the project, and the tools and standards available, high quality work is quite achievable! A high level of COMMUNICATION between the Architect, Engineer and Fabricator is essential!
Terri Meyer Boaketboake@uwaterloo.ca“Understanding Steel Design:An Architectural Design Manual”is available from Amazon.com