What is a Tension Tie?
Asked by Anonymous on September 02, 2013
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What is a tension tie?
Answered by Doug Friant
Good question! All trusses have two simple forces that need to be resolved – compression and tension. The compression is developed when a roof truss is loaded, for instance when it snows and puts a gravity load on the roof. That gravity load tries to make the roof flatten and push the walls out. Of course the walls can’t push out, they have to be held in by tension, the other force in a truss. A tension tie does that work. All trusses loaded by gravity have tension in the bottom chord. Sometimes the tension tie is a solid wood bottom chord, other times it is a steel rod, like in the image shown below.
2 comments
just wondering if a tension chord is always necessary in a hammer beam when buttresses are not present. any ways to avoid having to install one?
Hi Roland,
A tension tie can be avoided depending on the span of the truss, the size of the columns and top chords, the height of the lower brace, and the location of the job. It all comes down to loading. A hammer beam works by either buttressing, or by moving the internal tension down to floor level. The path from the floor to the roof starts with resolving the “kick” at the bottom of the columns by using a steel base boot, resisting the pressure on the column from the brace by beefing up the column, making sure the hammer tie is big enough so it won’t fail in bending from the roof load, and resolving the tension in the upper braces with steel rods. The top chords also have to be beefed up to resist bending from the upper tie.
It becomes harder to eliminate the tie in areas with a lot of snow or wind. Sometimes the budget is limiting too, so a tension tie is an inexpensive solution. In reality, each hammer truss is unique so we analyze them individually.
Doug