Gordon's formula is the best in a general way for calculating the area of struts or pillars in iron or wood: - f - intensity of pressure to crush short column of the material in tons per square inch; a - constant deduced from experiments on actual breaking weight of long columns; h - least transverse dimension in inches: I = length of pillar or strut in inches; P = total pressure on pillar in tons; p = pressure per unit of sectional area tons per square inch; S = total sectional area in square inches; p=f/(1+a(l/h)2) or P=fS/(1+a(l/h)2). Factor of safety, say, from 6 for short pillars to 10 for long pillars; f= 36 for cast-iron, 16 wrought-iron, 26 mild steel, 2.5 fir timber, 3 oak timber; a - for timber 1/250, for square or rectangular sections, 1/187 for circular "sections merely flattened at ends take 3/4 a in the formula, when rounded one end and fixed the other take 9/16 a, and rounded both ends 1/4 a; a = for wrought-iron or mild steel, 1/3000 for solid rectangle, 1/2250 for solid cylinder, 1/1500 for thin round tube or pipe, 1/1500 for angle with equal sides, and 300 for rolled joist section where S = sum of flange areas and b = area of web; when rounded or jointed at ends take 1/4a; a=for cast-iron, 1/40 for round hollow pillars ends flat and fixed, 3/800 for H-seetion, 3/400 for cross (+) section.

It will be found of service to get an approximate section first, and then to calculate by the formula to ascertain if it is strong enough. For this purpose a fir post may be considered capable of sustaining safely 4 cwt. per square inch, or failing with 2 tons per square inch, and an oak post 6 cwt. and 3 tons respectively. A round cast-iron hollow column with a thickness of 1/12, diameter may be safely loade 1 to 5 tons per square inch up to 10 diameters long, 1 tons from 10 to 15,3 tons from 15 to 20, 2 tons from 20 to 25, 1 1/2 tons from 25 to 30, and £ ton from 30 to 35.