The following will be valid for cables or rods in structural applications, which I take it is where your tirants are. Note however that a tirant can be something else in a different context, without which I cannot guarantee that the following is correct.

A chape in this context is a two-pronged steel attachment, a clevis/fork etc. as diversely detailed below. The two prongs each have a hole drilled through them. The prongs fit over a "lug", also with a hole drilled through it. The lug will be welded or otherwise attached to some structural member. By fitting the fork/clevis/yoke etc. over the "lug" and putting a bolt through the holes, you can fix two structural elements together;

The yoke/clevis etc. can be attached (swaged) to a cable or can be threaded onto a rod, which presumably is the case here. A locknut is threaded onto the rod first, then when the clevis is in the position required, the locknut is tightened against it, thus preventing it moving.

Kiwipedia has:
Chape ... knuckle joint, clevis, yoke [Chambers, McGHill], fork and eye, pin joint, 'split' hinge [MitchellS&F2,p.326], yoke [NCEI,4.03,p.14]
Chape/oreille Clevis/lug, fork and eye (voir Chape)


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Note added at 49 mins (2011-06-24 19:21:59 GMT)
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There's a nice picture of these things here, showing the clevis type fitting threaded onto the end of a bar, with the locking nut tightened against it, and also of the individual components.
http://www.flying-wires.ch/flying-wires/img/wires_1.jpg

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Note added at 51 mins (2011-06-24 19:24:36 GMT)
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Flying wires complete with clevises, clevis pins, cotter pins and lock nut
http://www.flying-wires.ch/flying-wires/pricelist_en.html