Ø  Adjustment of Dead Load for Slope

Compliance with TPI design criteria requires, When dead loads are applied on a projected horizontal basis, the effect of the pitch shall be taken into account.  Because gravity always acts straight down, Dead Loads (DL) must be applied vertically.  Therefore, when there is a high slope, more material is present for a given horizontal projection (plan view).  For instance, the DL for a 12/12 slope is greater than for a lower pitch because there is more material applied to the steeper pitch. When the TPI 1-2002 design criteria option is selected (Figure-V.01), VIEW automatically increases uniform DL according to the slope when necessary.

Dead Load Determination:

Has your historic standard DL been based on all of the materials applied to the trusses at the job site, including the wood in the truss?  (Wood Shakes, ½” Plywood Decking, Felt, 2x4/2x4 trusses @ 24”o.c., for example).  This is an important question to consider, since VIEW adjusts the dead loads automatically when using TPI 1-2002 design criteria.  This means the loads you used in the past could now be adjusted beyond what is actually necessary.  Weights of Material charts generally indicate loads based on a horizontal (i.e. Flat) projected area.  Determining and applying the actual dead load is not only be more accurate, it may also provide you with a competitive edge!

Consider the following example:

Using the material indicated above (Wood Shakes=3.0 psf, ½” Plywood Decking=1.5 psf, 3 Ply Felt=1.5 psf, and 2x4/2x4 trusses @ 24”o.c.=2.6 psf), the total roof DL equals 8.6 psf.  Many times for this roofing condition, 10 psf top chord DL is used no matter what the pitch.

In Figure-V.02, assume you used 10 psf in the past for the example materials.  Although 8.6 psf is all that is required for our example material, the chart indicates 10 psf is adequate up to a 7/12 pitch.  Beyond 7/12 would require the DL to increase. 

But when TPI 1-2002 design criteria is selected, and 10 psf is input as the DL, it is adjusted for the slope starting at a 2/12 pitch.  As the pitch increases, so does the DL, which may result in an increase in lumber (grade and/or size), and larger plates than what is necessary.  This example demonstrates the importance of determining an accurate DL value, such as 8.6 psf as in this example, which may result in your design having a competitive advantage.

Does the dead load get adjusted on all of the chords?

The following points explain what is subject to an automatic slope adjustment. Dead Load slope adjustments occur for:

A.     Each sloping Top and Bottom Chord segment when applicable.

B.   Flat chords in trusses which also have sloping chords (HIPS).  The flat chord DL is generated based on the assumption the flat chord is part of a perpendicular plane of the same pitch as the sloping chords.  In cases where the sloping chords are different pitches, the load adjustment is based on the longest, lowest non-flat pitch.  For Example, in the case of a piggyback base truss, designers need to be aware the load on the least pitched chord member, such as the Flat TC, is increased.

C.  Girder trusses.  The nominal dead load of supported spans, generally applied as PLF, is calculated based on the manual input for the Pitch of Supported Trusses, as shown in Figure-V.03.  Additionally, the Girder DL is adjusted for each sloping TC segment.

Because the VIEW automatic TPI 1-2002 DL adjustment may be easily overlooked, we included reminders in the following screens:

v   The Loading screen of the New Truss Dialog Window (Figure-V.04):

v   The Common tab of the Add Loads Dialog Window in TrusCAD (Figure-V.05):

Here is what you can expect:

When a New Truss is generated and the Job Setting Analysis tab option (Figure-V.01) is set for:

A.     TPI 1-1995: VIEW does not adjust the DL for slope; whatever you input is what VIEW uses.

B.   TPI 1-2002: VIEW automatically adjusts the DL for slope when necessary.  Adjusted dead loads are reflected in the Loading Block on Shopdrawings.

What happens to existing trusses when they are rerun using TPI 1-2002 design criteria?

If at some point it becomes necessary to re-analyze trusses for TPI 1-2002 design criteria which have already been run and saved using TPI 1-1995 design criteria, there are a couple of points you should be aware of:

1.    Individual Truss Designs: Changing the design criteria for an existing truss is done in the MISC. tab of the Engineering Option settings.  When the design criteria is changed from TPI 1-1995 to TPI 1-2002, then the truss is re-analyzed and saved; NO adjustment of the DL for the slope is made.  VIEW assumes the consideration of the DL for slope has already been taken into account when the TPI 1-1995 design was analyzed and saved; therefore it does not need to be adjusted twice.  A note is added to the ShopDrawing which indicates the dead loads are stated on projected horizontal area basis.

When a New Truss is entered, analyzed, and saved with the design criteria set to TPI 1-2002, and you attempt to change the design criteria to TPI 1-1995, VIEW displays the following warning message (Figure-V.06):

2.   Multiple Truss Designs: There are two options available for rerunning TPI 1-1995 design criteria trusses in an entire job for the TPI 1-2002 design criteria.  These options are Job Rerun and Job Rerun Loading, located in the Analysis menu of TrusCAD (Figure-V.07):

v   Job Rerun: May be used to Rerun trusses for TPI 1-2002 design criteria in an entire job without adjusting the DL for slope.

1.    Change the design criteria in the Analysis tab of Job Settings to TPI 1-2002 (Figure-V.01). 

2.      Select the Configuration tab in the User Preferences dialog window.

3.      Set the Load Engineered Truss Options setting to either:

A.     Always prompt with Options

or

B.     Always use the Analysis and Plating settings from Job Settings

4.   Now select the Analysis option from the TrusCAD toolbar, and then select Job Rerun.  VIEW reruns the job for TPI 1-2002 design criteria. However, NO adjustment of the DL for the slope is made.  A note is added to the Shopdrawings which indicate the dead loads are stated on projected horizontal area basis.

v   Job Rerun Loading: May be used to rerun trusses for TPI 1-2002 design criteria in an entire job and make adjustments to the DL based on the slope:

1.       Follow steps 1 through 3 in Job Rerun above.

2.   Now select the Analysis option from the TrusCAD toolbar, and then select Job Rerun Loading.  VIEW reruns the job for TPI 1-2002 design criteria, AND adjusts the DL for the slope where necessary.

!                       Note: If you wish to reduce the original DL entered for the truss designs to something which more accurately reflects the true DL:

A.     Click Settings from the main toolbar, followed by Job Settings, and then the Loading tab.

B.     Change the DL to the new value you wish to use.  Click OK.

C.     Continue with Job Rerun Loading step 2 above.

&             Refer to the Job Rerun Loading topic for detailed information regarding the Job Rerun Loading tool.

ê  Warning: Designs which have been analyzed and saved using the TPI 1-2002 design criteria may NOT be rerun using TPI 1-1995 design criteria.  If you encounter this situation, trusses requiring TPI 1-1995 design criteria must be re-entered from New Truss.  Please be sure when inputting a new job, and/or Layout, that you’re Design Criteria setting (Figure-V.01) is correct. 

F  If you are unsure about how the TPI 1-2002 design changes are going to affect existing TPI 1-1995 truss designs which are rerun using the Job Rerun Loading tool, you may want to use the Copy Job tool to create a duplicate job which you may then analyze for TPI 1-2002 criteria.

Ø  Dead Load Deflection Creep Increase Factor

TPI 1-2002 requires creep (defined as time-dependant deformation under long term loading), to be accounted for in the total deflection calculations of wood trusses.  The Dead Load Deflection Creep Increase Factor (Figure-V.08) input option is located in the Analysis tab of Job Settings and to the Misc. tab of Engineering Options in TrusCAD to account for this requirement. 

Increase factors are applied to gravity load cases for panel-point and mid-panel deflections, as well as deflections from tail bearing Tray conditions.  This option permits you to increase the dead load deflection by a magnitude equal to or greater than 1.0.  The greater the increase factor, the greater the reported dead load and total load deflections.  This may help identify trusses that, by their profile, may exhibit excessive vertical deflection over time.  The recommended creep factor for lumber is 1.5. However, trusses have been shown to undergo greater deflection over time due to the combination of lumber and connector plates.  A creep factor of 2.0 increases the reported dead load and total load deflections, which may result in a truss that no longer meets the user-defined total load deflection criteria, causing the designer to modify the truss design to create a stiffer truss.

An additional benefit is this option may be used with either the TPI 1-1995 or the TPI 1-2002 design criteria setting.  Creep Increase Factor information is reported on Shopdrawings as deflection criteria, as shown in Figure-V.09:

!     To determine the Dead Load Deflection Creep Increase Factor used on existing trusses, go to the Configuration tab in User Preferences, and select Always Prompt with Options in the Load Engineered Truss Options section (Figure-V.10).

This causes the Load Truss Options dialog window to appear when you select a truss from the TrusCAD truss list.  If the Truss Setting and Job Setting information differ, VIEW displays information about the difference (Figure-V.11).  If the Truss and Job Settings are both set the same, then the Load Truss Options dialog window does not appear.