Exeter University Forum: No room for error in a challenging multi-material project

The whole structure of the Exeter University Forum roof, including timber, was modelled by SH Structures in Tekla, and all details and general arrangement drawings were produced directly from the model.

This challenging multi-material project had no room for error, and the issues facing the project team meeting the fabrication tolerances and erection methods must have been immense, said the jury of the Tekla UK BIM Awards about the winning building information modelling work. Geometry information shared with the glazing contractors from the model must have been invaluable was another comment by the judges.

We used both Tekla and AutoCAD in the project, and there was full collaboration between the design team and fabricators using building information models that were exchanged between the 3D systems, says Ian Mitchell, Drawing Office Manager at SH Structures, who was in charge of the connection design, modelling, detailing and fabrication for the composite timber and steel roof to the Forum.

Tekla is the only modelling and draughting package used within our offices for the production of general arrangement (GA’s) and detail drawings, says Dave Poole, Project Manager at SH Structures. Other CAD packages are used in order to prepare models for importing, or for further developing models issued to us from clients or main contractors, but due to the complexity and lack of repetition, Tekla was the only real option for this project.

Glulam timber members with steel nodes

The various buildings are linked by a stunning new grid shell roof, which is formed from a series of triangular bays consisting of glulam timber members with steel nodes. The complex structure has no standard parts with all the bays being unique. It covers around 2,000 m2 and has been built on a large load-bearing ‘birdcage’ scaffold that required rigorous dimensional control to ensure that the various components fit together on site. The roof sun screen is supported by a 193.7 mm diameter tube with steel tee sections welded to it. The ends of the primary flitch plates were site-welded to this perimeter tube, which, together with a series of internal columns, creates a dramatic central feature of the new development.

The steelwork and timber modelling commenced in June 2010, and completion on site took place in October 2011. SH Structures Ltd. provided the service for structural steelwork frames, column and nodes and prefabricated timber beams, and Constructional Timber supplied over 2000 pieces of individual glulam elements. Most of the members were twin flitched members that were dowelled together and secured with pairing screws to enhance the load capacity of the joints. Due to climatic conditions, 400 mm long stainless steel screws were also used. The main elements were GL32C grade European Whitewood with the secondary members being GL24H grade.

Full collaboration between the design team and fabricators

SH Structures used the Tekla Webviewer application to develop details between contractors and the design team.

Our Tekla model was seen as the primary reference model, from which all other geometry would be derived.

Dave Poole, Project Manager at SH Structures

Exporting 3D DWG models was useful in helping other trades develop their details, such as cladding, glazing, and ETFE cushions, as well as defining the final geometry to the point where our Tekla model was seen as the primary reference model, from which all other geometry would be derived Dave Poole says.

The basic primary node positions were developed using coordinate information supplied by the structural engineers Poole explains.

We copied a reference target out from model zero using the X, Y and Z vectors supplied. This formed the basic geometry for the structure. Once the primary geometry was established, the architect’s faces model was imported as a reference model to determine secondary member positions as well as to provide a visual check for the coordinate-derived geometry. After all the primary and secondary members were modelled, we used various user joints to form the node tees, timber connections, flitch plates, dowels, and reinforcing screws on the different types of member, and manually finished off the attachments to the nodes and secondary brackets interactively.

There were certain elements relating to the supply, particularly of the vertical screen printed glazing, which could not have been determined from any source other than our model, so in that respect the model was vital to the integration of the supply chain. Accurate steel and timber procurement was also critical – especially when materials, both steel and timber, were coming from outside the UK, Poole concludes.

Sub-assemblies used to good effect

As a newer feature of Tekla Structures software, subassemblies were used, and the multi-user function was consistently used across the contract and regarded invaluable.

We used the sub-assembly feature to great effect when creating the Tees fitted to the ends of the paired timber primaries and node cans, thus allowing these to be fabricated and galvanised in advance of the main assembly drawings being completed, says Poole.

It was also beneficial in generating all the plates and Tees that went into the jigs, from which all the 162 different nodes, each carrying 6 primary timber connections, were produced. There was a total of 14,724 drawings produced, which contained the details of 37,794 assemblies and subassemblies, including the jig plates, which equated to 174 tonnes of steel, and over 180 cubic meters of timber.

The only way is modelling

There was in excess of 7600 man hours expended in the modelling and detailing of this bespoke and complex structure, which could not realistically be done in any other way than using a solid model, says Dave Poole.

The modelling and detailing of this bespoke and complex structure, could not realistically be done in any other way than using a solid model.

Dave Poole, Project Manager at SH Structures

There was no repetition, no finite geometric pattern, as the whole roof structure was free form, and the fact that all the nodes and primary members were tipped over to suit the dihedral angle between adjacent triangular facets meant that modelling was the only possible way forward.

In future projects, there would be very little difference in our approach to a project of this nature from a modelling and draughting point of view, as all our projects are geometrically challenging. We have honed our modelling practices to suit intricate and unusual circumstances. The main learning point from our perspective was the sub-assembly feature, the numbering, setting the hierarchy, and getting the drawing sheets and call off in the parts lists to work correctly, Poole concludes.