Flat timber truss

Hi, I am designing a timber 24' Ridge truss which is to support a floor beam via two steel rods so that the floor beam can clear span a garage area.

See article for example...

http://www.timberframeengineeringcouncil.org/images/pdf/TFG%20Article%20December%202007.pdf

I have a really high tension load at the bottom chord column connection, does anybody have a solution for this truss style.

Thanks.

Ideally the forces are resolved in the bottom chord by landing the toe of the strut well inside the post to develope the required shear planes. Of equal concern is compression load at the top of the queen ties. Also consider braces under the bottom chord. In matters of truss design, get an engineering review.

Roger - thanks for the reply. I am a structural engineer (I work with a building components company) I am reviewing this truss design and I am looking for some field knowledge of this truss as I have never designed it before.

The magnitude of the tensile/compressive loads in the truss arrangment (approx. 25,000 lbs) are proving problematic to resist with dowels in double shear. My span is 24' (28' trib width) with a 44 PSF factored snow load. The literature I referenced mentions spans up to 60', so there must be a strategy for this design.

I was thinking the compressive loads are of less concern becuase I will have wood on wood interaction and can use this to assist with load transfer at the joint - comments?

For the tension joint we are discussig, I have 12x12 white pine members w/ red oak dowels. My preliminary calculations indicate that I will require 25 dowels to resist this tension. I suppose I could use a knee brace to add additional tensile strength at the joint as you suggested.

Any other thoughts. Thanks again Roger.

Can you post up a sketch or two? I'm not sure which image in that PDF you are referencing.

And, in many cases, folks use steel connections for the tension loads. Lots of ways to take the compression loads with wood to wood joinery, sometimes backed up with lags or through bolts to hold things together (adding in a mortise and tenon may actually weaken areas of timber where you just need brute surface area.

Mike, I think it's fig. 4.

Scott, in timber framing design heavy load transfer should occur in the abutting surfaces of the joints not by dowels. Issues exist in the difference in bearing capacity of end grain to side grain such as might happen at the top of the queens where the end grain of the strut and the end grain of the top chord pinch the side grain of the queens in a everlasting grip.

post a picture of the design your considering. I am interested in your design. I always like to see what others are doing.

It is typical in modern timber framing for any significant tension connection (in our work this means anything over 2 kips) to employ steel whether it be side plates (not my preference), knife plates, barrel connectors, or any number of other schemes.

It is possible to design a solution to the problem, but in this case, I would try to design the problem away.

In historic examples, as Roger points out, these types of connections were avoided by configuring the truss or frame to use compression joints only (sometimes easier said than done). In my simplified view, I think in terms of making the truss to be self supporting, where it just sits on top of the supports rather than having it pulling or pushing on the supports. This sometimes requires designing the truss to be a separate, complete component rather than integrated into the supporting structure.

For some historic examples of how the joinery was configured in parallel chord trusses, look at covered bridge truss designs.

I hope some of this helps.

First of all, thanks to everybody for sending me their feedback. I am interested in hearing potential solutions to the joinery issues I have.

I am trying to post a sketch of the design so you can see what it looks like but I can't seem to figure it out. The FAQ says to use the file manager which I can't find anywhere.

Any suggestions so I can post the PDF file?

Thanks.

You will need to post the file on something like DropBox or Google docs. Then share a link here.

[img:center]https://docs.google.com/viewer?a=v&p...M0&hl=en_US[/img]

This drawing is a rough sketch of the proposed frame. The ridge truss is used to support the floor beam via steel rod in a gable style 24'x28' attached garage. From the sounds of it, I will need to use steel to connect these members. Is there a way to conceal the steel connections?

Can you send me the file? That link isn't working - either the image isn't available, or you don't have your sharing setup correctly for others to see it.

Mike

Mike,

Could you post the sketch for everybody to see? I like to see as many opinions on this before I continue my analysis.

Thanks.

Heres a link:
https://picasaweb.google.com/lh/photo/tw9y7PNm607GeEzcQvHJEA?feat=directlink

And the pic:

Scott, I hope I'm not jumping to far with an assumption but previously you said trib of 28 x 24 and building size 28 x 24, so I'm looking for tributary area of 14 x 24, thus bringing down the load by half.

Roger - I think I had meant tributary width of 14', 28' building width. Frame to span 24'. The loads I show on the drawing are factored (Limit States Design) and should reflect 14' trib (approx).

JOINT A - 26,500 lbs T
JOINT B - 30,500 lbs C
JOINT D - 3000 lbs T

I wanted to show these loads to show magnitude. I can post more accurate numbers tomorrow if you want more detail. Mike came up with a few great strategies using steel connections for my high tensile loads.

Scott,

Your design looks very similar to the Queen truss w/steel rods found in many clear span dairy barns here in the midwest. Although there are no Queen posts in these barns just Queen rods.

You might be able to eliminate the Queen posts (that are in tension)in your upper truss design and instead add a shorter rod that is dedicated to the suspension of the truss lower cord, and then have a longer rod that runs parallel to the shorter one to suspend the lower floor system. I would probably hang both rods from the same point or plate/ washer. The truss diagonals would then need to either be notched into the top-cord, or have an extra beam laminated to the underside of the top-cord that the diagonals then compress against.

Good luck,

Rooster

Interesting - thanks Craig.