Saw two variants of the W-shaped pieces in a truss roof. In one, boards are nailed ON the truss arms. In the other variant, studs are BETWEEN the arms. Both variants ultimately create the W-shape. But what could the difference be between them? Easier, faster, and cheaper with the board variant? Stronger with spacers?
There is no other difference between them than that the W-truss with 'intermediate' studs in the Vs is intended for factory manufacturing with pressed nail plates over the nodes. The W-truss with two outer boards from the ridge to the bottom chord is intended to be built on site. There, nail plates over the nodes are not recommended.
Thank you very much! I was thinking about one more thing: Do the bottom chord and top chords in a truss need to have the same dimensions? Can I use 45x190 for the bottom chord and 50x150 for the top chords?
Thank you so much for the answers!
I would probably have reversed it because the rafters are exposed to bending moments due to self-weight, snow, and wind loads, while the lower frame (not arm, that is between the elbow and the wrist) is exposed mainly to tensile forces (dimensioned for self-weight + the weight of an underroof and insulation, which do not create significant bending moments and are also supported by the nodes p.b.s where the V's meet, resulting in a small span).sommarbygge said:
You can view a W-truss statically in the following way:
The rafters (those on which the roof covering lies) are subjected to the weight of the roof covering, snow, etc. They are supported at the eaves and the ridge. At the ridge, they meet and exert opposite compressive forces on each other. This causes the supports at the eaves to want to push outward, similar to a house of cards.
The bottom chord pulls the eaves together.
The short strut that goes up and meets the rafter is subjected to compressive force because it 'pushes up' the rafter. The same compressive force is present at the bottom where it meets the bottom chord.
To prevent this compressive force from burdening the bottom chord, a diagonal brace has been drawn from the meeting point with the bottom chord up to the ridge. This brace is subjected to tensile force which in turn burdens the ridge, making the rafters want to bow outwards even more, which the bottom chord also resists.
A W-truss is thus a perfect example of a structure that, so to speak, lifts itself by its bootstraps.
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