## Truss Calculators

Started by Medeek, March 12, 2013, 06:33:08 AM

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#### Don_P

No, don't go back to the drawing board, leave the MathJax app in place. I have an unusually slow connection, most don't and mine will get better one of these days. I appreciate having all the documentation that you've been including so I can see what the calc is doing.

#### Medeek

I had a couple engineering friends of mine request a metric version of the snow load calculator, so for the metrically challenged:

http://design.medeek.com/resources/snow/snow_calculator_si.pl

I have absolutely no feel for how much a kN/m2  is supposed to be, but I guess some people do.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Don_P

My mind still doesn't work in metric although I know once immersed in it that it is an easier system. funny my FIL was a Dutch mechanical engineer. we worked through problems a few times and he would convert to metric in his head, solve, and then convert back for us Americans.

One thing that might be helpful for "what if" modelling is a reset button on the solutions page or a link back to the beginning. I tend to go round and round for a few iterations while seeing what happens as I modify different variables.

#### Medeek

Quote from: Don_P on December 31, 2013, 08:16:55 AM
My mind still doesn't work in metric although I know once immersed in it that it is an easier system. funny my FIL was a Dutch mechanical engineer. we worked through problems a few times and he would convert to metric in his head, solve, and then convert back for us Americans.

One thing that might be helpful for "what if" modelling is a reset button on the solutions page or a link back to the beginning. I tend to go round and round for a few iterations while seeing what happens as I modify different variables.

I was raised in British Columbia so my first system of units was metric.  After 20+ years living in the States though I'm much more familiar with the US customary system.  Also my engineering education was mostly with the imperial system (BYU).  The problem with the metric system is that they don't have a comfortable unit of measure like the foot.  The metre is too large, the centimetre is too small and who ever uses decimetres?

I have an idea for the reset/edit button on the results page, but it will require a bit of coding to make it work.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### MountainDon

Wind loading on a structure would make a great topic unto itself. Many DIY'ers give wind no thought at all, IMO.
Just because something has been done and has not failed, doesn't mean it is good design.

#### rick91351

What is the standard?  Here it is 90 mph in the mountains.
Proverbs 24:3-5 Through wisdom is an house builded; an by understanding it is established.  4 And by knowledge shall the chambers be filled with all precious and pleasant riches.  5 A wise man is strong; yea, a man of knowledge increaseth strength.

#### ben2go

Quote from: MountainDon on January 03, 2014, 03:09:11 PM
Wind loading on a structure would make a great topic unto itself. Many DIY'ers give wind no thought at all, IMO.

I was dealing with some heavy wind loading last night.50mph sustained gusting to 65mph.

#### Don_P

We had the same last night.
Most of the country has a 90mph design windspeed, the west coast off the coastline has the lowest at 85mph. As you approach the coasts it can go up to 150mph and there are several SWR's, Special Wind Regions, that are on a case study basis. I live in one, above 4,000' elevation here, in gorges and on ridges it usually has a 100 mph design windspeed. Imagine driving your house down the road at 90 mph, that is the design load. We've been close to that here I suspect, the 70mph guage blew apart in a regular nor'easter. To help visualize this maybe hold a clipboard out the window at 90 to get a feel for it, maybe not a clipboard you really want  . Now imagine that scaled up to trying to hold a single sheet of plywood out there. Then think about how many sheets are on each side of the house.... then think about those few nails trying to keep the axles under that house flying down the road at 90 mph. I felt pretty good after one of our houses got hit by a hurricane at above design windspeed and survived with only a few shingles gone, I also wonder how much fatigue damage it sustained... could it do it again? Harder to say once you've gone into your safeties.

#### MountainDon

Like Don_P said.... couldn't waste my keystrokes...

Quote from: rick91351 on January 03, 2014, 03:28:58 PM
What is the standard?  Here it is 90 mph in the mountains.

90 mph covers most of the country. The west has many special wind zones; read higher usually. Gulf and east coasts are higher too.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_3_sec001.htm?bu2=undefined
...scroll down 40% of the page to FIGURE R301.2(4)A BASIC WIND SPEEDS
Just because something has been done and has not failed, doesn't mean it is good design.

#### rick91351

It does surprise me that they only are requiring 90 up here because in the winter time minus this year they can get some violent winds.....
Proverbs 24:3-5 Through wisdom is an house builded; an by understanding it is established.  4 And by knowledge shall the chambers be filled with all precious and pleasant riches.  5 A wise man is strong; yea, a man of knowledge increaseth strength.

#### Medeek

Wind load calculator is in the works but before I get too muddled in that I'm going to try to complete the interactive ground snow loads map:

The only problem I see with this is some states have snow load equations that override the ASCE values and they don't seem to coincide at all.  For example the State of Utah uses this document to establish ground snow loads:

So I'm not really sure what the value is in such a map published and maintained by the ASCE when it seems that you need to consult with the local building dept. to get the "actual" ground snow loads for your location/elevation.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

#61
For Utah the ASCE map is more or less useless, so I created an interactive Ground Snow Loads map for the State of Utah:

As time allow I will also try to determine the snow load requirements for each state and list them here as well:

You will note that Montana has their own online calculator which allows a user to input the lat. and long. to arrive at a ground snow load, however their is a governing 30 psf min. roof snow load regardless of the ground snow load at that location.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

#62
Added a front end to the Montana Snow Load Finder so that you don't need to first determine your latitude and longitude, simply click on the map:

Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

I've done some work analyzing and programming a truss calculator for MPC wood trusses however I've put that project on hold for now and I'm looking at trusses made with plywood gussets. I'm not finding a significant amount of information online regarding these types of trusses. Does anyone have any links or even books they could recommend?

I would like to create a truss calculator for this type of truss that is similar to the previous calculator I started working on, however this time I will incorporate the wind and snow load cases as well as fully engineer the gusset plates (possibly ring shank nails and glue) and their connectors.

I think there may be a few people who would find this sort of application useful.  It would be good to provide a fully engineered solution for those wanting to fabricate their own trusses without having to simply wing it, which I myself have been guilty of when I built a small shed a few years back.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Don_P

A quick look at what I have doesn't give much in the way of specifics. The AITC manual and I believe online tech notes do have do's and don'ts regarding eccentricity at connections. There is a midwest plan services book that I never purchased on design and construction of trusses similar to the ag plans online, might be worth getting. The help desk at APA, introducing yourself as an engineer and asking for resource material would probably be worthwhile, they have been helpful. The awc.org connections calc does list allowable design values for plywood/wood connections, for small simple trusses that would do it. As spans and loads increase and the plate sizes get large the danger of cross grain problems as the connections try to rotate is probably worth thinking about.

#### Medeek

This is kind of what I have in mind however I have not yet determined the proper engineering/research to size the plywood gusset plates, number of nails and correct spacing and edge distances as well as orientation of the grain of the plywood with respect to the joint.   I will be using the ANSI TPI 1-2007 and matrix analysis for a template as far as sizing the members, similar to what I've already done with the current truss calculator.   Where I am kind of in the dark is with the gusset engineering, however working with plywood gussets is not too dissimilar from using metal gusset plates in that the same checks (Laterial Resistance, Tension, Compression, Net Section etc...) probably still apply.  This is where this project gets interesting...

Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

I can't believe it has almost been a week and I'm still not onto the new truss  designer yet.  I figured I would finish up the current truss calculator and add in the plate engineering that I had wrapped up back in October.  I almost have all of the calcs in for the plates.  Note that only the fink truss currently has any engineering associated with it.  Still trying to wrap up the current calculator so that it is mostly complete.

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

I also added in the design option so that one can specify the load duration factor.

After reviewing the NDS 2005 Sample Calculations I found some pertinent examples that will guide me in sizing the plywood gusset plates and the fasteners.  I am planning on using 10d nails so that they are in double shear and clinched thereby eliminating the concern with withdrawal and maximizing the usage of each nail.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

I should have done this a year ago but better late than never:

http://design.medeek.com/calculator/changelog.html

The addition of the CSI and JSI limits makes it possible to further customized the engineering of the truss if a more conservative approach is required.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

Now that I think I have the fastener portion figured out I'm now looking at the failure modes of the plywood gusset plates themselves in tension and in shear.  ANSI TPI 1-2007 describes the critical dimension as the gross width of the plate measured parallel to the joint line (section 8.4.3 and 8.5.3).  What I've drawn below are these gross widths for tension and shear for each joint.

I am trying to consider all failure modes.  In particular Joint (3) has possibly 3 failure modes but tensile case 2 and tensile case 3 may be redundant.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

Version 1.0.5 - 07.26.2014

- Tension perpendicular to grain check added for joint (6) and (7).
- Updated AutoCAD drawing generator so that it now draws the metal connector plates at the correct sizes.
- Limited heel joint plate selection algorithm so that the heel plate is single, symetric and does not exceed bottom chord depth in height.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

#70
I'm finishing up the plating algorithm of the truss designer and was a little stumped with the bottom chord web joint (see diagrams below).

The second image is for the same joint but with a 12/12 top chord pitch.  At this pitch the angles the two webs make with the horizontal is approximately 70 degrees.  All of the fink trusses that I have seen or can find photos of online show this connector plate with the slots running parallel to the horizontal as shown in the top diagram.  My question is does anyone have photos or truss output from a truss manufacturer showing the plate(s) in the orientation shown in the bottom diagram for a high pitch fink (W) truss.

It would seem to me that for high pitch fink trusses this would be a better way to plate the joint but I'm trying to stick with what is common practice.

Any actual photos of such a configuration would be immensely appreciated.

Another point I forgot to add is that with configuration (2) the requirements of TPI 1-2007 Section 7.5.3.3 are more easily met especially for heavily loaded trusses.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

If the forces on the truss are large and you are wanting to plate the truss with the slots parallel to the horizontal and the plate manufacturer only supplies plates where "h" is never greater than "l", then you end up with a geometry problem.  In addition you have large unsupported areas of the plate which are prone to shear buckling.

Perhaps the diagram below clarifies what I'm getting at.  Is Option C a possibility from a manufacturing standpoint and plate availability?

Option B and C also give larger contact areas onto the webs and hence better lateral resistance and net section (h').
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Don_P

Interesting, I don't recall seeing them any way other than "A". What is the difference in design values when the orientation changes?

I have had trusses with pretty large unsupported areas. In the field they are a real snagging hazard, I've hammered more than a few over to close the gap between the plates... probably not the brightest idea. I've wished they would fill those areas with deadwood. If the connection forces are getting high would it be better to step the web members up a size, not for member size requirements but for connection?

I can't imagine that they can't make a plate in either orientation. Maybe an email to MiTek or Alpine?

#### Medeek

A few minor updates to the truss designer:

Version 1.0.7 - 05.12.2015
Design values for select Eagle Plates and Simpson Strong-Tie Plates added from ESR-1082 and ESR-2762.
Added logo and link to evaluation service report for each plate manufacturer.
Client Information and Medeek Toolbox options added to further customize output.
Usage limit enforced so that regular users are encouraged to purchase a Medeek Toolbox Key.

Version 1.0.6 - 05.05.2015
Minor formatting issues resolved in (.dwg) output.

The last few months have been very chaotic with a new business so my time to devote to this project has been limited.  However, there has been quite a bit of interest and emails from the engineering community.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

#### Medeek

Today I added in the MWFRS wind load cases into the truss designer.  These load cases are used to determine the max. horizontal reactions at the truss bearing points as well as the max. uplift, useful for sizing the hurricane ties between the trusses and the double top plate of the wall.  Typically I don't see many cases that need anything in excess of your standard Simpson H1 but there have been cases with larger trusses where a H10 or H8 was required.

The MWFRS wind loads are calculated using the Envelope Procedure (ASCE 7-10, Chapter 28) Part 1.  I could have used the more involved directional procedure (Chapter 27) but I noticed that most Mitek shop drawings show the Envelope Procedure so I followed suit.  Most of the code for this addition to the truss calculator was borrowed from my existing wind load calculator so the programming was actually minimal and only required some minor tweaks to incorporate it into the main program.

A couple days previous I also added an algorithm for calculating the self weight of the truss based on the NDS values and the volume of steel from the plates.  Interestingly the steel plates actually do account for about 5-6 lbs on your typical fink truss, more than I would have thought.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer