Graphical Construction Glossary >> Roofs and roofing. >> Roof Trusses >> Pratt truss

Pratt truss

A roof truss with vertical web members to take tension forces and with angled braces to take compression

French truss

A roof truss based on two Pratt trusses and a bottom chord. A way of making a larger truss out of pre fabricated elements of smaller trusses,

These roof trusses are simple in design but very effective and cheap to build.

The ones in the photo above are in a heritage building of around the 1950's, (Fannie Bay Gaol, Darwin NT). This type of truss was made in their thousands in a kit form. They consist of easily transportable sections of steel angles and flats that are bolted together on site.

Note in the photo above the braces between the bottom chord of the truss and the columns, known as knee braces.

It is just possible to make out that the central vertical member is a 60x5 MS flat. Members in tension are quite often flats or even bolts.

In the same family is the French truss, which is in effect two Pratt trusses and an extra bottom chord section. Another way of using prefabrication components to create different spans.

P- girder trusses apart from having a long history in bridge building are seen in residential construction. A good carpenter or steel wall frame builder might brace the framing over wide windows and doors in this fashion.

There are many hybrid timber and pressed metal beams in roofs and floors that use this pattern.

The purlins bolted directly to the top chord of the truss.

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Adam Smith 1723-1790

"When we build, let us think that we build for ever."John Ruskin 1819-1900