huangyhg

how much tension is in a sail roof?
hello,
a building i am working on has a small sail awning. does anyone know roughly how much tension these carry? i have seen a few bust loose.
thanks,
tom
you should be able to do some simple calulations to approximate the tensile force by idealising the sail as a perfectly flexible cable with its ends pinned to rigid supports
roark's formulas for stress & strain gives the following equation:
p = (w*l^2) / (8*y)
w: unit load along element (in your case consisting of self-weight and some allowance for wind pressure)
l: unstretched length
y: maximum displacement
y = l*[(3*w*l)/(64*e*a)]^(1/3)
surely wind and water are going to be your biggest enemies?
how much tension it will have to take will be determined by local weather conditions.
limit states are probably:
water + self weight
wind + self weight (if it's angled so the downwards force of the wind will be worse)
wind - self weight (if it's angled so the upwards force of the wind will be worse)
the wind and water values will depend on the area and location of the canopy.
usually these things don't tear in the middle they break free at the supports so that should probably be your area of focus.
you'll also need the pretension in the cables, not insignificant in my experience.
cheers
greg locock
i designed foundations for a tensile fabric canopy for a hospital in the midwest. non-uniform, free form geometry. the loads were large at the tie down locations. the main support was an angled hollow column which had significant loads for overturning.
small fabric membrane structures typically carry about 5 pli of pretension in the fabric. mst structural fabrics have an allowable capacity of 15 - 40 pli, or even larger. (check with supplier or manufacturer of the material for a tangent moe and tensile strength; use a factor of safety of eight on the latter.) dependng on the structural arrangement these may be in addition to forces induced by applied loads, or may be relieved by the other loads as they are applied, until the prestress is equaled, then proceed to go up.
complex structures can be highly non-linear in behaviour. the stress changes as the fabric stretches under load. in a simple-span structure, this is conservative; you calculate your tension by w-l>2/8*y as shown above, then calculate the sretch in the fabric, then you have a new "y" based on the fabric stretch, then repeat until the change is satisfactorally small. if the structure has two or more panels arranged in such a way that the two panels of the membrane can affect each other, this stretch can end up making the "simple" analysis unconservative.

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