The tee-steel, or tee-iron, ranks perhaps next to the angle in general utility. Its general form is shown in Fig. 11.

The proportions of top table to stem or web are very variable, and the error of misdescription of the dimensions is one very frequently found on drawings, rectified, it may be, by dimensioning the members, but the young draughtsman will do well to remember that a 6" X 3" tee is by no means the same thing as a 3" X 6" tee. He will probably ascertain this to his cost if he specifies the one in mistake for the other, in the absence of a figured section. The width of top table is the dimension first quoted. The top table and stem are usually both slightly tapered, and connected by roundings of small radius, the corners of the extremities of the limbs being sometimes rounded and sometimes square. A variation sometimes found is when the top table is of uniform thickness and the stem tapered, or vice versa.

The British standard section of tee has the top table and the web tapered, the edges of the top table being rounded off beneath, while the edge of the web is square.

Tees are commonly used as beams, as in the case of purlins, secondary bearers in fire-proof floors, and the like. As struts they are a favourite section for the compression members of roof trusses of moderate span, lattice girders, etc., also as stiffeners to the webs of plate girders. As ties their use is more limited, there being some difficulty in making such an end connection as will effectively bring into play the whole cross-section of the metal. Tee struts will be further referred to in the chapter on columns.

The proportions of tees to be adopted in any particular detail will, apart from the value of their moments of inertia when used as beams, or of their least radius of gyration when used as struts, be frequently ruled by the dimensions and spacing of their riveted or bolted connections. Thus, to take a familiar example, the tee stiffener to the web of a plate girder will require a width of top table or flange sufficient to take the rivets required in a joint of the web plating, which again will be ruled by the shearing stresses in the web, and the number and diameter of the rivets required. Or supposing, in the case, let us say, of a footbridge with timber floor secured to tee bearers by bolts or coach-screws, the top table of the tee must be of width enough to receive screws or bolts of the diameter required, with (a) a sufficient amount of metal outside the hole, and (b) sufficient space between the stem and hole for the nut of the bolt or head of the coach-screw. Such elementary considerations may bear the aspect of truisms, but careful attention to points of detail such as these will always be found to characterize sound ironwork design.

Tees 12

Fig. h.