How do struts affect king post trusses?

Emerger · October 11, 2021, 06:51:04 AM

I've been playing with some of the calculators on the site and I've hit a snag. How do struts affect a truss and what calculations should be used to account for them?

Don P · October 11, 2021, 08:24:33 PM

Hmm, this could be a deep question or an easy one. It depends on your understanding of trusses.

The short and easy answer is the web members halve the total top chord span length. The 2 angled web members are in compression, they are posts that typically support the top chord members at midspan.

In a "true" idealized truss every panel point is an intersection of all members, no member passes unbroken through a truss node, each and every one is a pinned free to rotate connection.

The simplest truss is a pair of opposing rafters with a continuous ceiling joist nailed to each rafter foot. Although there is only a nail at every joint, they are free to rotate, a triangle is rigid. It cannot change shape. Do the same pin connection with 4 or more sides and you have just built a machine, it can and will change shape under load.

Lets say the size increases and the ceiling joist gets long enough and thin enough that it begins to have a sagging problem in the middle. The top of the truss at the peak is a rigid point, drop a chain down, hook it to the center of the ceiling joist and snug it up. There's the kingpost, it isn't a post at all, it is a tension member, a rope or chain or a stick of wood that is hanging from the peak.

Here we go, this was in my gallery. Red is compression, blue is tension. Members in blue could be a cable.

The feet of the web members are hooked to kingpost, or kingrod which is a more correct term. Those stiffening posts run up to the underside of the top chords and cut their span in half, allowing them to be lighter or span further.

This is the deflected shape (exaggerated for view) superimposed on the unloaded truss. It doesn't hurt to start with the king pulling a slight upward crown in the bottom chord so the in service deflection is straight.

Notice everything is triangularized, locked.

Chapter 2

The assumption in a perfect theoretical truss is that loads are only applied at the panel points in the drawing above, at the intersecting tips of the triangles. Then, the members, the straight lines (your wood), experience only compression or tension pushing or pulling from the ends. There is no bending load applied to the members.

Back in the real world it isn't that simple, the wind or snow obviously hits the whole rafter and bends it... AND the same rafter is experiencing a compression load. The engineering term is combined bending and axial loading, or the old term was beam-column. A supporting post that's also being pushed on from the side.

You cannot use all of the bending strength of the member nor can you use all of the column capacity. As the bending load increases the ability to carry load along the lengthwise axis of the beam decreases, and vice versa. Anyway, you work both equations simultaneously and the combined stresses must not exceeed what the beam can handle in compression or tension and bending. A beam interaction equation. That's your top and bottom chords, there is stuff nailed to them and carrying load.

Back to where we started, the internal web members. Nothing pushing on them from the sides, there is no roof load or ceiling load trying to bend them. The webs are in pure compression, a column. The kingpost in pure tension. Easy peasy. Often in a lightweight 2x truss the web members are a #2 or even #3. If you are full size and crawling through the trusses while setting them don't put much faith in the webs at midspan, stand on the metal plates when possible, the panel points.

I attempted to make a kingpost truss calc, never really finished it. The main thing I learned is there is a lot going on. Help yourself and feel free to redirect or whatever. Its hard online to know if you are yawning or yearning.
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