User Specified Ridgeline Location

This new feature will allow a user to specify the location of the ridgeline for the truss being designed. There are cases where a truss profile (as built) does not represent the roof profile This is an issue when designing trusses that have piggyback trusses that sit on top of them. Currently the program loads the truss based on the truss profile, which can lead to conservative loading in the unbalanced load cases for the base truss of a piggyback system. First we will look at the unbalanced loads on a common truss, and then look at the same roof profile using a base truss (hip truss) and piggyback truss.

This truss represents the roof profile as a single truss:

   

The next two trusses are used to create the same roof profile. The base truss, which has a hip profile, and the piggyback truss that will sit on top of the base truss.

 

Let’s look at the individual truss profiles and compare the unbalanced left snow load cases for the common and hip trusses. There are multiple left snow load cases for ASCE 7-05 because of the truss profile:

 
Let’s also look at the individual truss profiles and compare the unbalanced left load cases for the common and hip trusses.
 

We added a new tab to Loading called Geometry:

The new tab will allow you to input additional geometric information about the roof plane so that the loading on that truss can be modified to match the roof, rather than truss, profile.

To activate the feature you will need to select the “Use the following Geometry information for creating unbalanced load cases” check box:

Here is an overview of the fields inside the Geometry tab.

Truss Profile: The program can generate information based on the profile of the truss and then populate the tab fields based on that information. There are currently three choices: Not Set, Common and Hip. Additional profiles will be added in later versions. If the profile is set to Not Set, the program will not modify the unbalanced loading. The user will need to select the profile. In most cases the Common shape can be used to get the desired loading changes.

Information Verified/ Verify Information button: There are two states for this button: inactive with the wording “Information Verified” or active with the wording “Verify Information”. This button is to let the user know that something has been modified on the truss that could affect the loading (for example, the shape of the truss was changed). If a user attempts to analyze a truss that needs the Geometric information verified they will see the following warning and will need to go to the Geometry tab to verify the information and hit the Verify information button before they can analyze the truss.

Truss Profile Information: This section contains information about the truss profile.

·        RL:  This is the distance from the left most point of the top chord to the ridgeline

·        RR:  This is the distance from the right most point of the top chord to the ridgeline

·        SL:  This is the slope on the left side of the ridgeline

·        SR:  This is the slope on the right side of the ridgeline

Roof Plane Information: This section is used to provide information about the roof plane that the truss profile is part of.

·        W: This is the width of the roof parallel to the truss; it is measured from eave to eave

·        ERL: This is the distance from the left eave to the ridgeline

·        ERR: This is the distance from the right eave to the ridgeline

On the right side of the tab is a graphical representation of the truss profile and roof planes. These are not to scale and are only to give a general idea of how the truss profile is part of the roof plane. For example, the hip truss is not shown running from eave to eave but in most cases this will be the case.

To start using this new feature you will first need to select the “Use the following Geometry information for creating unbalanced load cases” check box. The program will then populate the fields in the Geometry tab with information based on the truss profile. The user then needs to verify this information.  In most cases the program does not have information about the Roof plane, so the user will need to modify the ERL and ERR. For a typical hip profile this information will be correct.

Let’s get back to the original example where we have a common truss that matched the roof profile and a combination of a hip and piggyback truss that also match the roof profile.
First we will look at the loading for the common truss; this is the loading we want to see on the hip and piggyback trusses.
 

We will begin with the hip profile. When we go to the Geometry Tab and check the “Use the following Geometry information for creating unbalanced load cases” check box we get the following information in the tab:

This information is correct so we can select OK and proceed with analyzing the truss. If we go into Special Loads we will see only one unbalance snow left load case and that it matches the one for the common truss. It is unbalanced about the ridgeline.

 
If we also look at the Unbalanced Construction load case we will see that it is also unbalanced about the ridgeline.

Now we will modify the loading for the piggyback truss. When the feature is turned on the program recognizes the profile as a common.

 

The program does not have the needed information to specify the correct ERL and ERR for the roof plane. If these values are not changed the loading will be identical to the loading used when not using this feature. So to correctly load the piggyback truss we will need to specify the ERL and ERR. In this case the piggyback truss sits on the hip truss we looked at earlier and will have the same ERL and ERR values.
 

 
We can now look at the loading on the piggyback truss, starting with the unbalanced snow load.

 

You may have noticed that the unbalanced load on the piggyback is 48.28 plf and it is 48.73 plf on the base truss and original common truss. This is caused by the program starting the load at the center of the level cut on the top chord extension. The program is assuming that this level cut provides bearing as it does. If you would like to see matching loads, then the ERL and ERR should be increased by half the length of the level cut section.

The unbalanced load case for construction loading is not affected. The program already unbalances about the peak of the truss.

The Geometric Tab settings are truss specific and exist only at the truss level, but it is possible to turn the feature ON and OFF using Job Global Changes.
 

If you have multiple trusses in a job that need to have the loading modified by the Geometry tab it is recommended that you select the trusses in the truss list window and right click “Global Changes…” to access the Global Changes window.  You can then switch the feature ON or OFF for those trusses. As noted earlier if the truss has a hip profile and spans from eave to eave all that is needed is to batch those trusses to get the modified loading. If the profile is not a hip profile you will need to go to the Geometry tab for each truss and modify the information to match the roof plane.

 

In future releases additional profiles will be added to the feature to simplify the loading procedure. At this time if the program is not able to recognize the profile of the truss you are working with and the roof plane has a common shape. Then the truss profile can be manually set to Common with the Truss Profile and Roof Plane information modified as needed.

This feature will be available in version 7.1.3 (scheduled to be released in April). If you are interested in evaluating this feature in version 7.1.2 (scheduled to be released in February) please send an email to wfigi@mii.com for further information.