IStructE Model Competition

The Competition

The student model building competition run by the Institute of Structural Engineers is an annual event hosted by one of the ten chartered Universities in Scotland on rotation. This year the competition was held at Dundee University and attended by teams of three students in a structures related course from the majority of the Scottish universities.

The idea behind the competition is to engage structural engineering students in a fun and informative experience of an extremely fast paced design and build project. The projects are designed to be a condensed version of a real construction process and as such replicates to some extent the stress and strain (structure related puns intended) within the teams that those working in the real world encounter when faced with fixed deadlines and non-negotiable constraints.

The teams of three all produced structures fulfilling an identical brief, the structures are then tested appropriately up to failure, and a formula applied which represents the clients priorities in the project designs.

Prizes are then awarded to the teams for first, second, third and' most deserving' based on their score coming from the criterion formula.

Myself and two other third year Civil Engineering students from Strathclyde decided to make the trip up to Dundee and show them how it's done!

The Brief

Each year the teams are given a different brief which their structures must fulfil. These include aims for the structure, constraints on aspects such as cost/weight/dimensions/foundation locations/etc. and a criterion formula used to determine the winning structure.

This year the brief was to design a support structure for the chandelier hanging in the Odesa opera house. The chandelier is suspended from above a false ceiling with no extra load bearing capacity. The false ceiling had a 500mm square plan area with a 150mm structural zone around its perimeter on which our support structure could rest. The supports for the structure had to be designed such that a 50mm vertical clearance around the edge was achieved at the edge of the false ceiling. The chandelier had to be suspended from a point directly over the centre of the false ceiling with a minimum clearance of at least 100mm vertically from the false ceiling to the support. The maximum height of the support structure was 150mm at all points within the 800mm plan area 'roof space'.

Failure would be classified as having been reached when the vertical deflection of the supporting structure had reached within 50mm of the false ceiling.

We were provided with materials for construction including: 2 thick lengths of Balsa wood, 10 slender lengths of Balsa wood, string, masking tape, a glue gun (with glue) and scalpels. We were also provided with a pre-weighed square board representing the area of the roof in which we could build, with the false ceiling represented by hatched markings designating the 'no-build' area.

The formula from which the scores determining the winning structure would be calculated was also displayed.

The Design and Build

My group began the design process by looking at several concepts which could conceivably carry the a load over the centre of a void. These ranged from structures employing supports on four sides of the void, to single span bridges which would would support a concentrated load to the midspan points.

Inspection of the winning criterion formula indicated that the score would be higher with increased capacity, but more influentially by decreasing the weight of the structure. This led us towards a single span structure rather than a four-way support, as instantly we would half the amount of material required.

Having settled onto a single span solution we continued to approach the structure as a bridge, something we had some experience of designing in the past. Assessing the design constraints we quickly realised that it would be the light structures in the competition that scored the highest due to the squared value in the criterion formula and so attempted to design a light-weight bridge.

From experience we knew that balsa wood was a strong material in both tension and compression, however the lengths of compression elements in any structure would have to be carefully braced to avoid Euler buckling or kept as short as possible. However balsa wood was the only material with any compression strength available to us and so had to be used, despite it being the heaviest of our available resources. With this in mind we looked at utilising the minimum vertical clearance at the edge of the false ceiling. The supports we designed became A-frame trestles through the desire to split the load down as many paths as possible so as to reduce thee buckling load in each individual member. As can be seen to the right we settled on string as the perfect material too provide tension resistance between the trestle legs as it's tensile strength is extremely high. further movement of the support on the construction area was removed by adding stops at the base, attached to the test-board and a diagonal bracing strut to avoid horizontal displacement at the top of the trestle.

In order to bridge the gap between our two lovely new support trestles we initially looked at creating a single span triangular cross section truss. This would be supported at the ends on its point, so as the two upper long elements would both be in compression, rather than a single long element as would have happened with the point of the triangle on top. This design evolved into a peaked triangular truss, where the truss rose to a point at the span midpoint at the maximum height restriction which would have provided greater strength as long as the trestles were sufficiently restrained from lateral movement. Furthermore with the addition in elevation at the centre, extra tension members could be added bellow the truss at the level of the minimum height restriction.

After construction began we realised quickly that the full triangular truss with dimensions limited by the spacial restrictions of the brief would still use a significant amount of balsa wood, and so would weigh more than a more slender structure. We decided to move in the direction of this slender support structure appreciating that a greater propensity for buckling out-of-plane would limit our maximum capacity. To combat this we added long slender lateral restraints to the design whose sole purpose was to provide a small degree of lateral restraint close to the midpoint.

We decided to continue with our triplicate long element design, with a flat support end cross section and a 20mm tall triangle meeting a mirrored copy of the same structure at the midpoint. This joint was strengthened with cross joint joists attached to the sides of the top-most beams. These would resist deformation about the joint in several directions and would be more effective due to the higher contact area afforded them by being attached to the side of the beam rather than directly between them. The folded paper tension ties were attached similarly to the outside of the lower beams. Paper was chosen for it's high tensile strength and folded to double it. It was also extremely light and did not stretch in the same was as string would have in the same position. As can be seen to the left, the long torsion resisting lateral restraints were attached through the structure to sit securely against the joists on the far side of the beams from their supports. Finally a bracket was constructed at the point of the joint to allow housing of the cable from which the testing weights would be connected.

In the end what we ended up with was something very close to a commonly used structural timber A-frame as seen in roof and ceiling structures all over the world and we had a lot of fun putting it together!

Testing

Before testing all off the structures and their test boards were weighed and the pre-measured weight of the boards removed so as to give the weight of any added structure by the teams. This was recorded on an excel spreadsheet with the winning criterion formula pre programmed. The board was then placed between two tables, a cord placed over the midspan point, a hanger attached to the two ends of this cord and then weights added until the supporting structure failed. The recorded maximum load capacity was that of the last total weight the structure could hold prior to failure.

Sadly my camera work was not as good as our bridge building and I managed to miss the point of failure. However the final measured weight of our structure was 41g, the total load 12lbs, and an arbitrary criterion score of 3.5 (ish) placing us in second place overall!

Design Review

Having time to review any structure design is an important educational tool, especially if it fails! Looking back at our design, it obviously had some weak points which could have been rectified with the gift of foresight.

First however, looking at the mode of failure. The picture above shows a large break in the principle beam spanning across the gap at approximately two thirds of the distance from the support to midspan. It can be seen that this is roughly half way between the closest lateral restraint, in the form of the long slender lateral supports, and the point at which the separate beams converge into the same horizontal plane. Although not captured on video, it was obvious from watching the failure live that the structure suffered from a tortional buckling failure at this point of weakness. This was due to poor judgement on our part, as we made the decision to place the single central beam on the top of the A-frame truss rather than flip it 180 degrees about its lengthwise axis giving two upper members and one lower. This would have better spread the compression loads through the truss and potentially have allowed us to increase capacity.

From inspection it also became obvious that we had provided more than enough lateral support close to the centre span, indicating that most of the balsa wood used was unnecessary added weight. This gave us an insight into the true lesson of the evening.

Given the brief, there were numerable options with regard to design options, many of which would successfully carry a load much greater than what ours ultimately did, as was the case with several designs utilising stayed pre-tensioned string to span the gap, which in the end failed due to vertical deflection rather than failure. However the winning structure was not selected by maximum load. The winning criterion clearly specified that weight would be the influential factor in the success of any design, and crucially, that there was no MINIMUM load which the structure was required to manage. Therefore even the lightest and most unstable cantilever structure could (and did) win even though it could only support around 2/3 of our second place (heavier) structure.

All in all I am glad I took part and definitely took away a valuable lesson in not only adhering to constraints, but also to review very closely what the ultimate measure of success of a design is and to aim for this in at least equal measure to client wishes and personal preferences.