Friday, 23 November 2007
AKT has run an initial FEM analysis on the roof structure, and well...we've got a bit of work to do.
Though significantly lighter than conventional concrete, the Fibre-C is still concrete, and the self-load of the material is a bit more than our geoemtry can handle at the moment. Basically, with the tilt/lean of the roof structure, the self-load of the structural material, and the non-continouous nature of the structure....our notch joints are taking a hell of a load. The moment forces that these joints are taking is very high. We are also experiencing high stresses and deformation in the profiles due to bending in their weak axis. AKT has told us we need to reduce the stresses by a factor of about 6 (no kidding...my stress factor has got to be +100 these days).
We've got a few options for reducing these stresses, but it looks like it's going to be a combination of them all:
1. Decrease the tilt angle of the roof structure. By reducing the angle by 15 degrees we gain a factor of 2. We also lose some of the dynamic formal movement and drama of the structure.
2. Introduce bracing, especially at the sides of the pavilion. This could be done by infilling flat sheets of fibre-c stiffeners between profiles at various locations. This could also detract from the visual and structural purity of the pavilion.
3. Add more primary ribs at the side faces. This might alter the visual effect of the moire pattern, but could also be interesting if done as a gradient of spacing. I also find it interesting to architecturally express where the structure is performing the most.
4. Increase thickness of material at primary ribs to 13mm, and decrease thickness of material at secondary profiles to 10mm (reducing the self-load).
As I said, we've got some work to do. All of these decisions need to be looked at before we can make a call on them. We need to maintain our design concept, while finding a structural solution that suits. But in particular...we need to so so ASAP!
Here's a look at the revised wire model with spliced profiles and intersection nodes we sent over to AKT today for a quick analysis. In this version, we have reduced the tilt angle by 10 degrees on the front face, and 5 degress on the back.
On another note, once again we have also revised the bench geometry again. Don't worry, it won't be the last time. As you can see below, we are using the 3.6m x 1.2m standard sheet size of Fibre-C as the module for sizing our profiles.
In light of this, we have now lowered the height of the seatback/counter, so that it drops within the 1.2m width of the sheet material. this allows us to cut the entire bench profile in the primary direction, in one continous piece. this is a very minor change, but one which saves us huge amounts of hassle and detailing.