User Deflection Limits:
 (Available for use with VIEW, VIEW-Plus and ProVIEW)

Where are the Deflection Limit Settings Located?

Dead Load Deflection Creep Increase Factor

Deflection Button

Composite Floor Option

Settings in the "Limits" Section

 Lumber Grade Bumping

 

The User Deflection Limits section provides you with the ability to select individual option fields to set Top Chord, Bottom Chord, Cantilever, and Overhang deflection criteria based on either the Live Load or Total Load (Figure-V.01).  You may also designate the deflection limits to be applicable to a Roof or Floor condition and input deflection check criteria based on the maximum allowable vertical displacement specified in inches.

Figure-V.01

Where are the Deflection Limit settings located?

This setting criterion is available for an entire job by selecting either the Job Settings, Analysis tab (Figure-V.02), or for an individual truss by selecting the Engineering Options , Misc tab in TrusCAD (Figure-V.03).  Click the button labeled to activate the Deflection Limits dialog window (Figure-V.01).

Figure-V.02

Figure-V.03

 

Deflection Limits Settings Explanation for the Deflection Button:

   Live Load / Total Load: Once the Deflection Limits dialog window is open (Figure-V.01), click the toggle arrow in the first field to set your deflection limits based on either Live Load or Total Load.

   Roof / Floor Button: Click the button labeled either or to toggle to the desired setting.  Whichever name is displayed in the button are the values used (Figure-V.01).

!          Note: If a truss is a SY42 or a Flat truss, and the load duration factor is less than 1.01, Floor truss deflection limits apply.  If the load duration factor is greater than 1.01, Roof truss deflection limit applies.

   Limits: In this section (Figure-V.01) of the Deflection Limits dialog window you may specify two types of individual deflection settings for:

v   Span/Deflection: The TC, BC, Cantilever, and Overhang fields contain 4 standard deflection values in the toggle drop-down menu: 180, 240, 360, and 480.  You may also type in your own desired value manually into the field.  VIEW understands that each of these fields requires a minimum setting, therefore if you specify less than the minimum value required, VIEW automatically defaults to the minimum. See Figure-V.04 for the Minimum L/D Panel Point Deflection Limits:

Figure-V.04

v   Absolute (in.): You may also specify the Absolute minimum vertical deflection limit you wish to maintain.  This value is entered as inches.

!                 Note: VIEW checks both the Span/Deflection ratio and the Absolute maximum vertical deflection limit and displays a deflection warning message if either is exceeded.

   Save Button: Click the button (Figure-V.01) to save the Deflection Limit settings to the Job.Ini file in the USR01 folder.

        Remember: If you have set a Span/Deflection field to less than the minimum required value, your setting is ignored and VIEW automatically saves the setting as the correct minimum value.

   Close Button: Click the button (Figure-V.01) to close the Deflection Limits dialog window.

!           Note: If you make changes to the Limits section standard default field settings, make sure you click the button prior to clicking the button or your setting values are not saved to the Job.Ini file.

   Cancel Button: Click the button (Figure-V.01) to close the Deflection Limits dialog window.

!     Note: Prior to version 7.40, VIEW only allowed you to set deflection limits for the bottom chord and overhangs (Figure-V.05).

Figure-V.05

Additional User Deflection Limits Settings: (Figure-V.06)

Figure-V.06

   Dead Load Deflection Creep Increase Factor User Input Allowed (1.0 Min)VIEW multiples the calculated dead load deflection by the creep increase factor indicated in this field.  For truss fabricators, based on past experiences with certain truss design types, designing a truss using a creep factor may be helpful.

In general, floor trusses with high dead loads are a good candidate for applying a creep factor.  This would include trusses supporting light-weight concrete and tile, or perhaps heavy dead loads due to ceramic and granite countertops in kitchen and bathroom areas, for example.

Typical creep factor candidates for roof trusses are cambered or scissor trusses having minimal clear distance between the TC and BC members causing a lower overall truss stiffness.

Research has shown that a creep increase factor of 1.5 has been a recommended value for lumber, while a creep increase factor of 2.0 has been a recommended value for metal plate connected wood trusses.  The higher value of 2.0 accounts for joint slippage in metal plate connected trusses.  To select the value you wish to use, click the toggle arrow to open the pull-down menu and make your selection, or simply type the value you wish to use into the field.

   Composite Floor Option:  When selected , the Composite Floor option takes into consideration that there is floor sheathing glued and mechanically fastened (nails or screws) to the floor truss top chord.  This results in an increased section modulus for the truss and therefore an increased stiffness, lessening the deflection.  This means the truss must be analyzed using Rigid Sheathing for the TC Purlin option (Figure-V.07) when you select the Composite Floor option.

Figure-V.07

!           Note: If you select the Composite Floor option and then try and analyze the truss using Purlins, VIEW looks at the Purlin settings first and then turns off the Composite Floor factor if Purlins are selected.  The Composite Floor factor is turned off because Purlins are selected to be attached to the TC, therefore there is no justification for allowing an increase in stiffness when Structural Panels (sheathing) is not selected to be attached to the TC.

   Panel Options Button: Click the button (Figure-V.06) to open the Panel Options dialog window (Figure-V.08).  The settings in this dialog window define the makeup of the Structural Panels Applied to TC/Rafters option displayed in Figure-V.07.  Click the Panel Thickness, Panel Construction, and/or the Panel Grade fields or toggle arrow to activate the pull-down menu, then click the option you desire.

Figure-V.08

!           Note: Unless the Structural Panels Applied to TC/Rafters option (Figure-V.07) is selected , there is no reason to open this dialog window.

Lumber Grade Bumping

VIEW checks the deflection of the TC and BC separately based on the minimum internal deflection limits, or the deflection limit changes you have made per the instructions above.  If you find the truss analysis determines your truss design does not meet the deflection limit requirements, you may select the Lumber Grade Bumping for Deflection option in the Misc Options section of the Analysis tab (Figure-V.09).

Figure-V.09

When the Lumber Grade Bumping for Deflection option is selected , VIEW upgrades the chord lumber when the truss fails a deflection check. VIEW adheres to the logic defined below when increasing the lumber for the following components of a truss:

1.       If a TC and/or BC fail to meet the required deflection check criteria, VIEW upgrades all lumber simultaneously until the chord member(s) meet the required deflection criteria or the available lumber grade selections are exhausted.  For example, if you are using a lumber file consisting of MSR grades of 1650, 1800, 2100, and 2400 and the stress check portion of the truss analysis determines the TC requires 1650 and the BC requires 2100, yet the truss does not meet the deflection criteria requirements, VIEW bumps the TC to 1800 and the BC to 2400.  If the deflection criteria are still not met, the TC is bumped to 2100 and the BC remains at 2400 under the assumption 2400 is the last choice in your inventory list.

2.      If an Overhang fails to meet the required deflection check criteria, VIEW upgrades the lumber of the chord segment which has the overhang.  If there is no lumber within your inventory which allows the overhang to meet the required deflection criteria, a deflection warning message is displayed.

3.      If a Cantilever fails to meet the required deflection check criteria, VIEW upgrades the lumber of the chord segments which contain the cantilever.  VIEW upgrades the lumber of each chord segment simultaneously until the cantilever meets the required deflection criteria or the available lumber grades are exhausted.

See Also: