Fixities In MiTek® 20/20


We are often asked what the Fixities switches do in the MiTek 20/20 engineering software and why a person should pick one or the other.  The purpose of this article is to help you understand what the options are and what the affects are of these options are on designs.


This option is found on the Design Info screen:
These are the Fixities options that you will find:
The first thing that you will need to understand is what it means for pitch breaks to be Fixed, Pinned or Semi-rigid. 
Let’s begin with a Pinned joint which can be described by looking at a heel joint on a truss behaving like a door hinge.  If you had an open face jack truss with a pinned heel joint, the top chord would just fold down onto the bottom chord because the heel would not have any resistance to rotation or moment.  Now let’s look at a Pinned joint in a common truss where you know the top chord is in compression and the bottom chord is in tension.  In this case, the plate at the heel joint resists these “axial” forces of compression and tension but does not resist any moment from the loads in the middle of the panels.  Therefore the moments would all be carried by the lumber and the plates would only be resisting the axial forces.  This will, in theory, cause higher lumber CSIs and lower plate CSIs.  If we have a Rigid joint at the heel of that same common truss, we allow no rotation to occur between the top and bottom chord so some of the panel load in the top chord is be transferred to the bottom chord resulting in lower lumber CSIs and higher plate CSIs.  In a Semi-rigid joint, you guessed it; we are somewhere in-between where the plate at the heel would only resist a portion of the rotation or moment. 


So why would you use one of these over the other and what does MiTek recommend? 

MiTek recommends that you go with the most economical, realistic and still safe option.  We think the most realistic option is Semi-rigid joints which is probably the most economical choice for most truss designs without compromising safety.  This option gives a nice balance between keeping chord lumber grades low without a significant increase in plate sizes so that the overall truss is competitively designed. Some people feel that heel plates are fairly large compared to other pitch break plates and therefore heel joints should be designed as fully rigid with the rest of the pitch breaks Semi-rigid.  This option is available as “Semi-rigid pitch breaks with fixed heels”.  This feature exists on the Manufacturer, Job and Truss level and should be set on the Manufacturer level.


The next choice to be made is the span to be entered under “Min. span for selected model”. 
What this option does is pin all of the pitch breaks for trusses shorter than the span entered here.  This is helpful because short span trusses will typically not require higher grade chord lumber even if the pitch breaks are all pinned and therefore, you can pin these joints to get the smallest possible plates. The best length to be entered here is really dependent on the minimum chord grade lumber used for your designs, the loading and the panel lengths; however a good guess is between 15-25 feet. 


The “Automatically fix smaller truss instability” should always be checked as it will make a small un-triangulated truss work without compromising safety or the competitiveness of the design.


 This page last modified on 5/28/2009