Spider Web concept University of Oxford 11 Jan 2011 SLHC meeting in Edinburgh

Centre ring Toggle bar – in released position Toggle bar (4-off per ring) – in locked position Kevlar fibre strand (8 off per ring) for pre-tensioning 2 nd ring 3 rd ring 4 th ring Outer ring

Toggle bar in release position Toggle bar in locked position Toggle bar pivot Pre-tension force from Kevlar keeps toggle bar in locked position

The 2 vertical toggle bars support the full gravity load on rings 1-4. The total load from the first 4 cylinder is approx. 400 Kg (4KN). Assuming that the middle ring takes 1/5 of the total gravity load, this makes each of the two vertical toggle bars taking on 1/10 of the total gravity load, or 0.4KN. Kevlar strand made up of 10 fibre, each 100μm will stretch by 2% (10mm for a fibre length of 500mm) when toggle bar is in locked position. The Kevlar strand will thus provide a pre-tension of 9.5KN. The pre-tension load is then sustained by the 4 toggle bar, each taking nearly 20KN compressive load The gravity load on the toggle bar is small compared to the pre-tension load, and can be ignored.

Pcrit = 1.0 * π 2 * E * I x-x / L 2 We need to establish the minimum X-section moment of area for the toggle bar to avoid buckling For L = 0.6m ( longest toggle bar length when in locked position) ; E = 70 GPa (aluminium, say); P crit = 20,000N  I x-x = P crit * L 2 / ( 1.0 * π 2 * E )  = 1.04 x10 -8 m 4  This minimum value also applies to I y-y Obviously the toggle bar needs restraining in the y direction (along the cylinder axis). This can be achieved with some minor modification to the toggle bar cross- section or other A-frame arrangements. 30mm Ix-x = 1.48 x10 -8 m 4 Iy-y = 4.5 x m 4 The toggle bar needs to be rigid enough to avoid buckling from the pre- tension load. The pre-tension load is approx. 20KN on each of the toggle bar. The critical buckling load P crit for a bar pinned at both end with a sectional moment of area of I x- x and length L is:- Min. X-section for the toggle bar:

Remarks on this concept The toggle bar design provides an active toggling arrangement and as such the pre-tension from the Kevlar keeps it in locked position unless an external applies to de-toggle it; Nevertheless, a safety catch will be needed to prevent it from an accidental de-toggling; All 5 rings could be pre-mounted. No tool is needed to pre-torque the rings ….. The toggling is done ring by ring and makes the system self- sustaining; No inner most cylinder needed for mechanical reason. It could also be a ring. Ring X-section needs to be sufficient to sustain the pre-tension loads from te Kevlar wire; Toggle bar X-section needs to be carefully profiled to ensure it can be fitted in the free space between the staves; It requires longitudinal restraining (along the cylinder axis) to prevent it from “flipping” as any highly tensioned structure would, unless the pre- tensioning arrangement is itself provides a self-centering stability. This requires further thoughts