An Overview of Truss Designs

Introduction

I want to close the recent examples of trusses by providing a sampler of truss designs. This is far from encyclopedic. In fact, this post is limited to planar trusses.

First, however, let me give you a link to an online calculator. I checked it out on the Howe truss with a snow load.

As you can see, I scaled the loads by a factor of 10. I had to, for the program – not a big deal.

There are plenty of websites with information. You can search for yourself… you could start with the usual wiki article – the merit of which is that it has a lot of external links.

Personally, I have kept a link to bridge trusses in western PA and a link to roof trusses by an Australian contractor.

Okay, what do we have?

Oh, I relied on J.L. Meriam, “Statics”, John Wiley & Sons, 1966 – the book from which I took Examples 3 (Howe) and 4 (Pratt). I constructed the last five drawings in this post (“sign and cantilever trusses”) from his examples 107, 108, 110, 112, and 114.

In addition, he had a convenient one-page display (p. 128) of commonly used trusses in another book, “Engineering Mechanics: Statics and Dynamics”, Wiley, 1978, ISBN 0 471 01979 8. I made my own drawings based on his.

Finally, there is a very recent book, “Engineering Mechanics: Statics, Student Value Edition by J. L. Meriam and L. G. Kraige”, ISBN-13: 978-0470499771 (paperback). I haven’t held a copy in my hands, but based on two of the three reviews, this latest edition is like the earlier ones: it has tons of examples.

My examples were roof trusses… there are, as I’m sure you know, bridge trusses… there are also cantilever trusses… and there are trusses for signs… and trusses need not span two points at the same elevation….

Roof trusses

We have seen two forms of a Howe truss:

We have seen a simple Pratt truss, and there is a more complicated one:

I’m sure that both of these designs can be extended to more beams in the obvious way.

Notice the distinction between the Howe and the Pratt: whether the diagonals which touch the center do so at the top or at the bottom.

We have seen a simple Fink truss… and it, too, has a generalization:

The joints at L and N… and the horizontal beams there… this is not a straight-forward generalization.

Then there is a Warren truss – which I want to compare to a Howe:

Bridge Trusses

There are Howe, Pratt, and Warren bridge trusses. Let me compare them to the corresponding roof trusses. Here are the Howe roof and bridge trusses:

Isn’t that odd? The diagonals go the other way.

Here are the Pratt roof and bridge trusses:

Again, the diagonals go the other way.

Here are the Warren roof and bridge trusses… of course, the diagonals go the other way.

I would, naively, have expected the pair

to have the same name, but they don’t: that’s a Howe roof and a Pratt bridge.

What’s going on?

The roof trusses are designed for continuous loads on the top chord… the bridge trusses are designed for continuous loads on the bottom chord. Imagine that we take a Howe roof truss and turn it upside down to start on the Howe bridge truss. Turning it upside down flips the diagonals – and then the diagonals have the same relationship to the expected load.

At least, that’s how I figure it. (No pun intended, I swear.)

Then there are some bridge trusses for which I have not seen roof trusses.

Here’s a K truss.

Here’s a Baltimore:

Here is a Wichert truss. It is apparently intended as a statically determinate alternative to two trusses:

Maybe I’ll work that out someday, the Wichert versus a pair of trusses.

Sign and Cantilever Trusses

Here is a truss for a sign…

It expects a continuous wind load from the right on beams BC and CD. Oh, it’s anchored at A and G.

Here is a truss which I think could be used for a sign:

If so, it expects a continuous wind load on beam FG. Oh, of course it’s anchored at A and B.

Here is a cantilever truss:

It’s anchored at A and J.

Here is an interesting truss: it is fixed at A and E. I imagine it supports a load at F. The key is that the anchor points are at different elevations.

Here is another interesting truss: it is fixed at A and H. Again, the key is that the anchor points are at different elevations – and, in fact, one is on a vertical wall. I expect that this one supports a load at E.

I also suspect that the last two trusses are actually 3D. They seem rather complicated for 2D supports. But I’m just guessing.

And that’s probably it for a while on trusses.

About these ads

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 49 other followers

%d bloggers like this: