Development and Updates for the Medeek Truss Plugin

[Fred H]

Hopefully a few of you will take me up on this offer since I am really curious to see how the plugin is actually being used by real architects and designers.   

I'm not an architect or designer (in recent decades I've done mostly software), but I have a purpose and your plugin has served me well. We are determining whether we can afford to build a second house on some land we own in Vermont. To do this we need a design and it has to serve two purposes: Cost and aesthetics; your plugin has served the second for us. Here's what we did:

We found a design on the Internet that we liked. One of its failings was too low a roof pitch for our taste (we live in a cape). Another was its ceiling height; since the house has to be small, we felt that 9' ceilings would help it feel more spacious. So I learned enough Sketchup to model the house. We tried different wall heights and roof pitches to be sure that we will like the way it looks. As the overall size and inside layout changed, we repeated the tests. That's now past and we're refining the inside design.

So far, we've used it for a preliminary quote of building and erecting the shell with SIP walls with trusses including sheathing. This appears to be the lowest cost method of getting the SIP benefits, although we won't go this route if the builder resists. But it was a good place to start; we now know that we're in the ballpark.

The quote was done with two alternatives: (1) 9' walls with 5" energy heel trusses; and (2) 9.5' walls with 5" drop-heel (or, we've also seen, dropped chord) trusses. The latter, for it's higher wall costs, was touted as providing easier and more complete air sealing. We don't know yet which alternative our (unknown) builder will prefer, if either.

I mentioned the drop-heel trusses for completeness. It's apparent that you (like me) are driven by this, so it's yet another potential item to add to your lists.

Thanks for all your efforts,
Fred

[Medeek]

@ Fred H

Thank-you for your posting on your recent house project.  People actually using the plugin for "real" work is what makes me think all the hours I have dumped into this aren't totally in vain.

I am interested in these drop-heel trusses you have described.  Please feel free to email me some pictures or drawings, I am not familiar with this configuration but if it something that I'm missing from the plugin I would definitely be interested in adding it in.

Hopefully the plugin made it quicker and easier to generate the roof geometry, that is the intent at least.  I really need to get to work on the wall plugin to round out my plugin family.

[Medeek]

I think a lot more people fab their own trusses than we realize.  I don't disapprove but I do think that quality control is imperative.  One bad joint in a truss can cause the entire truss to fail which in turn may cause a catastrophic failure of the roof system.

Some jurisdiction will require an engineer's stamp on any site built trusses, as they should, but there are many situations (ie. agri structures, small buildings) where there is no oversite by a building official or engineer.  For those situations it would be useful to have an easy to use truss calculator that even the lay person could utilize and understand. 

From my side there is some risk and liability associated with this, especially since I have no way to confirm the workmanship of the construction.  If someone were to utilize my calculator and design a truss with plywood gussets, and assuming that they built it per the calculations and input the correct snow loads etc... but the construction was shoddy and it failed would I still be liable?  Probably so.  That is why I have hesitated on releasing the wood gusset portion of my Truss Designer (web based).

[Medeek]

I've seen a lot of great site built trusses.

I actually think a well constructed plywood gusset plated truss with glue and nails will out perform a metal plated truss. It's not the craftsmen that worry me its the DIYer who has limited experience building things and does not take the time and effort to do a quality job.

Trusses are unique structural elements that require special attention to the connection between members (joints).

One method to address the lack of quality control is to overbuild the truss slightly. This method is actually used to some degree in the manufacturer of MPC wood trusses.

[Medeek]

Enabled mod warren webs for the double pitched flat truss configuration:



The truss shown above is an asymmetric double pitched top chord bearing flat truss with a mod warren web configuration and cantilever ends.  That is quite a mouthful.

I have not added the top chord bearing option and the mod warren webs to the other flat truss configurations yet (flat, monopitch, pitched), however I will if specifically requested. There a much hotter items on the plate.

[Fred H]

Hopefully the plugin made it quicker and easier to generate the roof geometry, that is the intent at least.  I really need to get to work on the wall plugin to round out my plugin family.

The plugin was crucial for us to choose a roof pitch. It was why I purchased the plugin and I'm very pleased.

The wall plugin (which I haven't looked at yet) would be useful if we don't use SIP walls. Although, this detail will probably be handled by our contractor.

[Fred H]

I am interested in these drop-heel trusses you have described.  Please feel free to email me some pictures or drawings, I am not familiar with this configuration but if it something that I'm missing from the plugin I would definitely be interested in adding it in.

I've lost the references to drop chord trusses. I do a lot of searching on my phone while kind of watching TV. The phone doesn't keep the records for long and I researched this weeks ago. But I remember the description and the points -- although I'm not technically able to evaluate their correctness, so I'd appreciate it if someone else did.

Description: A chord below the top of the wall that the ceiling is attached to. Think of a long squared U (e.g. └───┘). Of course, some spans would require additional vertical members for support.

Benefits claimed:

   1. No bracing or blocking needed (vs. energy heels, particularly more than 5.25") as the wall provides the necessary structure.
   2. Air sealing should be easier as the complexity of top plates, etc. is above the sealing level; just the ceiling and ceiling to wall seals.
   3. Avoids ceiling lift in winter because the structural bottom chord (my terminology) is the same temperature as the top chord. Note that this requires the drop to contain enough of the insulation to expose the structural bottom chord.

My potential application:

   * To be certain of all three benefits I'd need a drop equal to my insulation depth -- 15+" for R50 cellulose -- to expose the entire structural bottom chord to the same temperatures as the top. This adds that same 15+ inches to the top of my SIP walls to leave the bottom chord above the insulation. My estimated cost of doing this is $515/inch, I doubt that it would be worth it.
   * If I use a drop of 15 - 4.625 (minimum 9/12 heel) = 10 then the top of the bottom chord would be close to the surface of the insulation, and might yield benefit 3. But this would cost about $5K, which is probably not worth it to me.
   * If I use a drop the same as the energy 5" heel I get the first two benefits. This might be worth the cost of $1575.

Note that my cost analysis is entirely dependent on my use of SIP walls. Other construction has much lower material costs, which is what effected my costs. This analysis makes me more interested in investigating non-SIP walls. (Particularly after I found InsulationDepot dot com this morning, which sells recycled insulation.

Implementing drop chords in the plugin will require more point identification than a common truss. Obviously, the inner points of the walls will need to be marked -- as these are the ends of the dropped chord. But overhangs are possible too: my design has an overhang above the front door. (This is easy with a Fink truss, as i just extended the roof line.) I don't see any easy way of doing this with a drop chord truss; I hope you do.

[Fred H]

I found the references again. The most complete is from the Alaskan Housing Finance Corporation, https://www.ahfc.us/files/8813/5553/5112/building_manual_ch_07.pdf, page 13. It has advantages and disadvantages, with drawings showing their use. This was the most complete information I found

Foard Panel has some good detail on what they term a "dropped heel truss", which seems identical, http://www.foardpanel.com/wp-content/uploads/2014/11/28.pdf.

This: https://softplan.com/?page_id=1786 shows a dropped chord in a flat truss.

I found a few other references to them, but they are redundant with the above information.

Best,
Fred

[Medeek]

In the global settings I've added a setting to switch between metal and wood connector plates.  Initially I will only enable wood plates for some of the more typical common trusses (ie. fink, howe, queen and king), if this feature proves to be popular I may enable it for more truss types.

[Medeek]

Version 1.9.1 - 06.09.2017
- Added plywood gusset plates for common fink trusses.
- Enabled a "WOOD" plate type option in the global settings.



*Note that the wood gussets are currently only available for the Fink truss, if anyone needs them enabled for a different truss please let me know.

[Medeek]

Added plywood gusset plates for raised heel common fink trusses.



View model here:

https://3dwarehouse.sketchup.com/model/b21431bc-c3b6-4038-a81e-f27fc78f69b1/WGC-Truss

Note that there are currently no calcs associated with these WGC trusses.  If you are going to build your own trusses I would strongly suggest you find a local structural engineer to assist in specifying the plates sizes and fasteners, especially if the truss span is greater than 16 feet and your snow loads are greater than 25 psf in your region. 

When I engineer WGC trusses I will also call out a waterproof glue as well however I don't take the glue's strength into the calculations.  This provides another margin of safety and also the rigidity of a glued and nailed gusset plated truss is superior in my opinion to your typical MPC truss. 

Every member is critical in a truss, however some are more stressed than others.  Your top and bottom chords should never be anything less than a No. 2 DF.  I would suggest No. 1, No. 1 & Btr. or Select Structural.  The webs of a typical truss are usually stressed quite a bit less than the chords and sometimes you can get away with stud grade lumber but I would suggest DF No. 2 on all webs as well.  A strategically placed knot can easily ruin an entire truss.

Just as important as the materials is the level of care taken in cutting the members and assembling the truss.  All joints should fit tightly together so that members in compression can bear directly on their adjacent members.  Also realize that tension members may experience load reversals in high wind or seismic events so even tension members are no exception to the rule.

The correct fasteners and fastener size is critical.  The nails will be in single or double shear and the joint connection is relying entirely on this shear strength of the fasteners to hold together.  Fasteners too close to the edge or ends of a member are not as effective, so fastener placement is also critical.

I am curious how the two truss types would compare in a burn test, which one would stay intact longer.  My suspicion is that the plywood gussets would char and slowly burn however the metal plates would heat up and loose their strength quickly and fail.

I know most people don't do this but I would personally stress test each truss before installing it on the intended structure.  Mostly what you are looking for is a uniform deflection across all the trusses so that they can load balance properly.  If a truss is defective this test should help ferret out this data even though visually the truss may look acceptable.

[Medeek]

Just for the record the algorithm to size and position the mid top chord to web plate (Plate 2) is a real piece of work:

Code Select Expand

gamma2 = 3.14159265358979 * 0.5 - @Phi
length2 = @Pldim2 * 0.5 -1
p3x = @Swx4 + length2 * (cos(@Theta2) / cos(gamma2 - @Theta2))
p3y = @Swy4 - length2 * (sin(@Theta2) / cos(gamma2 - @Theta2))
p2x = p3x - (@Tcd + length2) * cos(gamma2)
p2y = p3y + (@Tcd + length2) * sin(gamma2)
p1x = p2x - @Pldim2 * cos(@Phi)
p1y = p2y - @Pldim2 * sin(@Phi)
length4 = sqrt((@Strx4 - p1x)*(@Strx4 - p1x) + (@Stry4 - p1y)*(@Stry4 - p1y))
zeta2 = atan((p1y - @Stry4)/(@Strx4 - p1x))
length3 = length4 * sin(zeta2 + @Theta3) / sin(3.14159265358979 * 0.5 + @Phi - @Theta3)
p4x = p1x + length3 * cos(gamma2)
p4y = p1y - length3 * sin(gamma2)

if @Theta2 > gamma2
  p2x = p3x - ((@Tcd + length2) / (cos(@Theta2 - gamma2))) * cos(@Theta2)
  p2y = p3y + ((@Tcd + length2) / (cos(@Theta2 - gamma2))) * sin(@Theta2)
end

Actually had to use the Sine Law:  https://en.wikipedia.org/wiki/Law_of_sines

Its funny because my 15 year old daughter just covered trigonometry in her math class and she told me she would never use this stuff so why learn it. 

[Medeek]

I've been thinking about site built trusses some more and trying to apply some rational thought to how best to nail together a WGC truss.  If you consider nails in single shear, 8d nails spaced at 4" between fasteners with a row spacing of 1" and and end spacing of 1.5" you end up with something like this.



Those are 8d common nails so they aren't very big but spaced at 2" between nails in a row I would be afraid of splitting that bottom chord. 

Looking at this I'm fairly convinced that going with longer nails that are in double shear and clinched give more bang for the buck, allow less nails and thereby less chance of splitting the truss members.  Thoughts?

[Medeek]

Nailing from both sides is not reasonable in my opinion, there is no way to line up the nails easily from one side to the other, better to just nail from one side which bring me back to nails in double shear. 

Using a 10d common nail (.148 x 3) and with 1/2" of protruding nail to clinch I get this layout:



with the backside looking something like this:



I probably don't need a 2.25" end distance (15D) on the plate edge toward the inside of the truss but I am using Table C11.1.6.6 from the NDS Commentary (NDS 2012).

[Medeek]

If you compare the capacity of a single shear and double shear connection in most cases you get double the capacity, see results for a 10D nail into DF and Structural 1 Plywood below:



So returning to our previous example we now have 2180.9lbs/176.3lbs = 12.4 => 13 fasteners in double shear, rather than 25 fasteners in single shear, a much more reasonable number of fasteners