from Roark 7th Ed, table 11.4.17, pg519
for an equilateral triangle, simply supported sides, constant thickness, with a uniform pressure load
maximum stress = 0.207q*((L/t)^2)
where q = applied pressure
and L = side length
(the equation is in terms of a, the perpendicular height, = L*sqrt(3)/2)
and max displacement = -0.019q(1-nu^2)L/E*((L/t)^3)
nu = 0.3 ... (1-nu^2) = 0.91