The main chord (11) is made up of two members i.e. the left-hand member (17) and the right-hand member (18), each being made of an angle iron, in which apertures are spaced for receiving locking pins. The two members (17, 18) are also joined by the fastening devices where they are overlapped. The fastening devices are individually made up of a wedge member and a locking pin. The wedge member is made of a plate having its central portion defining a sloping surface in which there is a hole and a length-wise slot which allows the rims of the slot to fit into pin grooves when the wedge member is driven by a hammer. Thus the locking pin is securely wedged in the lengthwise slot of the wedge member.
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