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Queen Post Trusses In N.American Framing (Timberbee)

From the Quicksilver Metaweb.

The Queen Post Truss consists of two upright; Queen Posts. These Queen Posts sit upon a Continuous Tie. Generally they are either connected to the Tie with a mortise and tennon joint, Steel Straps, or Both. A Strong joint, able to withstand fairly great loads, is required, as the joint is placed in tension as soon as the Truss is loaded.

The Queen Posts are separated by a Straining Beam. Which sits, anywhere from 8 – 14” from the top of the post.

Running from either Post, at, or below, the level of the Straining Beam, to the Tie, in the direction of the outer wall, is a the Brace, often as large, in cross section, as the Post, and/or the Straining Beam.

The Last element of the Truss is the Purlin Plate.

The Actual Geometry of the Truss is dependent upon a number of factors;

  • First is the Pitch of the roof.

  • Second is the Height of the Knee wall

  • Queen Post Trusses are generally used in buildings which have a Drop-Tie, they are not used in buildings whose Ties and Plates are level with one another – such as structures which employ “English” Tying Joints. They are also not used in Aisled Buildings, where the Purlin Plates are supported by continuous Posts, which stretch from Sill (Long or Cross), to Purlin Plate. One example of this type of aisled Building, is the typical American Dutch Barn.

An example of a Traditional Timber Framed Structure which Does use a Queen Post Truss is the “Gambrel” Barn, another is the typical New England, Greek Revival Church.

The manner in which the Truss functions is that when it is loaded, i.e.; * The Truss bents are assembled * Dropped into place, pegged, and/or strapped to the Tie * The Purlin Plates are lowered, and pegged, onto the queen Posts * The Common rafters are added * and, finally, the roof is sheathed and shingled.

At this point the roof can be considered to be “Loaded”, though all the slack may not yet have been taken out of the Truss until it receives either wind, or snow load.

When the Truss is loaded, what happens is that force is transmitted down through the Posts, from the Purlins, which share with the plate in bearing the dead, and live, loads of the roof. The Purlins take the bulk of the load.

The Purlins attempt to depress the posts, putting them in compression, as the roof seeks to fall inwards, and outwards.

The Posts, bearing nearly the full weight of the roof, put the Tie in Bending. This puts the upper surface of the tie in compression, the lower section in tension, and the central section remains neutral – By the Way, this is why steel I Beams have their familiar shape.

Meanwhile, the Tie deflects, until the Braces are engaged. These Braces work in compression, as do all the braces in a typical Timber Frame.

Once the braces are engaged, and are placed in compression, they attempt to push, both against the Tie, and the Post. The resulting thrust against the post is the reason that the joint is not placed directly below the Purlin Plate itself, as there needs to be a certain amount of Relish between the top of the Brace mortise and the top of the Queen Post, in order to resist the thrust of the Brace.

This places the ends of the Ties in Compression – which is why this joint is also set back, inside the building, providing relish here as well.

The Section of the Tie (The Bulk of the Ties’ overall length) between the two Braces, undergoes ‘’’Some’’’ tension.

The Braces, unable to fail at the Tie/Brace connection, transfer their force upwards, to the Posts, lifting them, and forcing them inwards.

This then places the Straining Beam into compression. The Post is not allowed to fall to the center of the building, and it’s upward movement is resisted by the weight of the roof, which keeps the Truss under “Load”

All of the Post below the Brace Joint is now in Tension, while all of the Post, 8-14”, or so, ‘’’Above’’’ the Brace joint is now in compression.

The Post is now exerting Lift on the Tie. This is why so much strength is required in the Post/Tie connection, the Post is attempting to lift the Tie, and all of the floor.

The Tie is mainly neutral now, with the very ends taking the greatest load, as Almost ‘’’All’’ of the load exerted by the roof, through the Purlin Plates, and down the Queen Posts, is bearing on the Braces, which, in turn, transfer that load to where they meet the Ties.

The Ties are, generally, seated in Wall Posts. Generally but not always, as Stone and Brick Buildings may use Queen Post Truss roof Systems, as well as Timbered Structures.

Further, Braces, whether 45 Degree Braces (Braces which mainly resist Racking, 60/30 (Which act to transfer some of the vertical load into the wall, the steeper the brace, the more efficient the transfer), or some other combination, are often employed to “Brace” the building against racking forces, as well as to transfer some of the load from the Tie to the Wall.

That, in a nutshell, is how Queen Post Trusses work.

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