Truss Calculators

jschuff32 · September 07, 2015, 11:31:45 AM

I'm going to be leading a team to build a church in Mexico next month, and I'm trying to finalize the plans. The main issue I'm running into is the truss design. The structure will be 18'w x 50'l with 10' walls made of 2x4 stick framing. We would like to use a 5/12 scissor truss design to keep the ceiling from feeling too low, but I can't seem to find any calculator that does the calculations. Our initial thought is to use 2"x6" instead of 2x4's for the trusses for increased strength. Being set in Mexico, there is no snow load, but being near the coast there is a strong wind factor.

Is there any help you could offer, or any suggestions on where I might be able to find help? Thanks

Jeremy

Don_P · September 07, 2015, 12:42:19 PM

Hi Jeremy,
Just another thought, if you call one of your local truss plants and explain the situation I'd be willing to bet they would provide an unstamped design.

September 07, 2015, 12:58:58 PM

What are you plans as far as the truss plates? Wood gussets with nails and glue? Nail size and glue type?

jschuff32 · September 09, 2015, 02:07:43 AM

Yes, planning for 3/4" plywood gussets with 8d 2 3/8" nails and construction adhesive.

Medeek · September 09, 2015, 10:01:51 AM

Just out of curiosity I would take your design concept to the truss plant and see if they will quote you on plywood gusseted trusses or provide you any sort of shop drawings or engineering. Let me know what their response is either way.

September 09, 2015, 10:49:35 AM

Thanks, I will contact one of the truss companies locally and report back.

Medeek · September 21, 2015, 09:00:29 AM

Still working on adding the point loads into the truss calculator, but at least I've got the matrix analysis portion working correctly now. If you enter in point loads for the fink truss it will correctly compute the moments in all of the chords. At one point this weekend I was stumped for nearly 36 hours. Turns out I was using the wrong equation to distribute the shear into a fixed end beam. I now have a lot more respect for the programmers of STAAD and RISA. Not entirely sure how they program the software to be completely open ended but I think I've kind of got a glimpse of that while programming this weekend.

The tough part was making it flexible enough to be useful (ie. point loads can be in any position along the top chord of truss). I'm assuming that most solar panel applications would have the loads on the top chord so that is where I have applied them. Interestingly it doesn't take much weight to cause an upsize in grade or size to the top chord of your typical fink truss.

Medeek · September 26, 2015, 06:54:35 AM

Under "Advanced Options" you can now specify the lumber species, grade and size for the top chord, bottom chord and webs, this should allow one to check any fink truss "as built". I still have not configured a method to specify the plate sizes at each joint but that should not actually be that difficult. For now the plate are auto sized until the JSI requirement is met.

Medeek · October 02, 2015, 03:25:51 PM

Seems like there is always more to do. Updated the deflection section to reflect the new provisions of the ANSI TPI1-2014 with regards to the creep factor. Deflections are now accurately taking into account point loads. Also the repetitive stress increase can be manually switched or set to auto.

October 03, 2015, 02:21:12 AM

Added SketchUp 3D (.rb) file output for the truss geometry. This file, when copied into the SketchUp plugins folder, will create a menu item within SketchUp allowing for unlimited creation of the given truss geometry within SketchUp. I think this feature will be particularly interesting to those DIYers who wants to draw up their own model and plans using SketchUp.

Medeek · October 03, 2015, 04:03:35 PM

I've updated the code so that the plugin now allows for user input in order to specify number of trusses and spacing of the trusses. For example 4 trusses @ 24" o/c would give you:

The actual truss geometry cannot be altered within SketchUp it is hard coded into the plugin when it is created by the calculator. For different truss sizes and shapes it is simply a matter of creating and storing separate .rb files for them. Dropping these files into the SketchUp folder sets up the menu item. This method seems to be the easiest for interacting with the SketchUp API.

Medeek · October 04, 2015, 02:15:02 PM

After giving it some thought I've realized that from a design standpoint the average SketchUp user probably justs want a plugin that they can drop into SketchUp and configure the truss fully within that program. I will have to give this some more consideration.

After programming the Ruby API for this small plugin I've realized that a lot more can be done here. I'm still trying to determine what is the best method for insertion of the trusses (rather than at the origin), selecting a point with your mouse or keying in a coordinate. Also changing the direction (E-W) vs. (N-S) of the trusses would be another helpful feature. Not that it is too difficult to rotate bodies but then it involves another step.

Don_P · October 04, 2015, 10:53:23 PM

Sorry not to be able to provide feedback, I haven't played with rubies yet so am still reading up. If they can be like a single component that I can then manipulate and place on my layouts in the model, move and copy, etc, it would work best for me... the end trusses are rarely on layout. I'm not sure if rubies work that way

.

I did help a friend design a 30' fink about a week ago using this program

. It will be site built with plywood sideplates, we referenced back to the AWC connections calc... adding to the wish list

Medeek · October 05, 2015, 05:34:26 AM

I was actually messing around with this again instead of working on my real work this morning, way too much fun. Anyhow, you can select all of the trusses and then right click and group them. This is like creating a block in AutoCAD then you just simply manipulate the block as a whole. As a group I could then move, copy and rotate the whole pile of trusses.

I've spent about two weeks this last month working on point loads and I'm still not quite done. Turns out that when you throw point loads on the top chord of the truss especially near the peak all sorts of unexpected load conditions pop up. Bottom line, the truss internal forces go from a nice symmetric situation to one where every panel of the top and bottom chords are loaded differently. What this means is that I have to implement the full analysis of each section of truss, just a bunch more checks, nothing new.

Once I have that out of my hair the next big thing is to include the C&C wind loads in the analysis and any other load cases that are currently missing. At that point the calculator will then be providing a "fully" engineered analysis of the given truss. Right now it is a partial analysis.

After that I would like to include plywood gusset plates as an option to replace metal plates. I've had a number of people ask me about it and I think there is enough interest to make it worth the while. I've spent a good deal of time talking to other engineers and researching this topic so I think I can run with it.

Then finally once all of the above is complete and I finally have a full solution for a common fink truss I will then begin the tedious task of applying the code to all of the other common truss types. At some point I hope to include attic and scissor trusses as well.

October 05, 2015, 08:36:15 PM

Really...

This is how I usually import something. It would help if it just flew in from the suburbs rather than hitting the origin. I'll bring it in and set it in a pile just outside of the building, carpenters

.
I then made it a component, made a copy and moved and did the x times copy of that. If desired I can then make the whole assembly a component. That's just how I do it at my minimal level.

Hmm, is it possible to model a "typical" solar array load in the worst location as a default "solar ready" truss button?

Medeek · October 06, 2015, 12:04:09 AM

I'm not as up to speed on SketchUp as I should be anymore, I've had my head buried in AutoCAD for the last couple of years. I'm not exactly sure what the difference is between a group or component but it looks like once you have the geometry you can manipulate it quite successfully, very nice.

Today I had someone from one of the SketchUp board ask me to implement the output for the king post truss so even though it does not do the engineering for this truss type you can now get both AutoCAD and SketchUp geometry for a King Post Truss:

Medeek · October 07, 2015, 10:58:15 PM

Queen Post truss AutoCAD and SketchUp output added tonight.

Medeek · October 08, 2015, 10:35:48 AM

I'm trying to make this easier to use for the SketchUp user. I've created a separate plugin that is used to generate the truss geometry (no engineering or loads). Please feel free to beta test it here:

http://design.medeek.com/calculator/sketchup/

Drop the .rbs file into your SketchUp plugins folder. Suggestions or bug notifications are welcomed. Currently only the fink truss type is available but I will add the others shortly.

Medeek · October 08, 2015, 09:39:41 PM

What is the most typical raised heel height? Depending on the height the typical treatment is one of three options:

1.) Wedgeblock: Butt cut BC depth and top chord just touches bottom chord.
2.) Slider: Butt cut is larger than BC depth (3-1/2") and heel height is less than 12". Bottom chord and top chord do not touch.
3.) Vertical with Strut: heel height is 12" or greater.

I'm thinking about adding energy trusses into the Truss Plugin. However, I don't have a lot of resources on this subject. Anyone with shop drawings of trusses with energy heels who would be willing to share would be greatly appreciated.

Don_P · October 09, 2015, 06:59:11 AM

For me typically #2 is what I've seen, I'll look through old records and see if I kept any shop drawings, if others have any please step in.

October 10, 2015, 01:02:15 PM

I'm currently working on an algorithm for determining which option to show for a raised heel (wedge, slider or vert. web w/ strut). Sliders and wedges typically seem to be either 2x4 or 2x6 members. The length of the wedge or slider is still somewhat of a question. Most sliders seem to average between 3'-5' in length. Wedges extend about 6 inches.

The question of length of these two elements is important because it will determine the scarf of the heel and this determines the panel lengths and hence the web placement.

An interesting feature of the additional strut for higher raised heels is whether they are beveled to their centerline or is this step is omitted. When the angle between the strut and the chord becomes to shallow the bevel appears to be omitted, at least from a number of truss drawings I've reviewed.

Medeek · October 12, 2015, 02:58:48 AM

I have the raised/energy heel working now for a fink truss where a vertical member and strut is required (heel height greater than 12" approx.). Still working on the wedge and slider cases, they are actually easier to calculate and program, but I figured I would tackle the difficult one first.

When the angle between the strut and top chord exceeds 10 degrees I then apply a scarf cut to the strut at its centerline (try a raised heel height that exceeds 24" and you will notice the difference).

Here is an example of a fink truss with a 18" raised heel. Notice there is no scarf cut at the top of the strut where it meets the top chord.

Medeek · October 12, 2015, 10:07:45 PM

For the fink truss all raised heel types are now active:

The algorithm is now smart enough to determine when to use a wedge, slider or vertical member with strut. Depending on the heel height, and the pitch a wedge is either a 3.5" or 5.5" deep. Likewise the slider is also auto selected to be either a 3.5" or 5.5" member.

Don_P · October 13, 2015, 07:25:23 AM

Wow!, You've been busy Thanks

I haven't caught up yet but this looks awesome.

If I'm understanding correctly, on the strutted raised heel at the connection between the web member, top chord and raised heel strut... everyone is in compression? The reason for not scarfing below 10 degrees is the length of the joint requiring plate?

Medeek · October 13, 2015, 09:06:17 AM

I haven't looked computed the forces yet with the addition of the strut and also the moments at the heel of a raised heel with a vert. member. This will significantly change the analysis as compared to a non-energy truss. This sketchup plugin is only dealing with the geometry.

The reason not to scarf probably has to do with practicalities of trying to make a cut that is almost a rip cut down the centerline of the member. At some point it just does't make sense.