Timber beam deflection checks - US
When designing to US codes, timber beam deflection checks can be requested for:
- Dead, Live, Snow, Wind, and Total loads for instantaneous deflections
- Total load deflections can optionally consider the effects of creep deformation in design
Specify deflection checks
By default the checks are applied as
follows:
- Dead - relative span/factor limit of 360 applied
- Live - relative span/factor limit of 360 applied
- Wind - not applied
- Snow - not applied
- Total loads - relative span/factor limit of 240 applied
You can modify the deflection checks you require in either the beam properties window or dialog. Limits can be defined as a span over factor limit, an absolute value limit or both.
The images below show the beam properties window and the dialog respectively after an
additional Dead load absolute limit of 1in has been applied.
Deflection design cases
Deflection design cases are added to the
strength design cases for every Tedds design that is created. The deflection load
type and the load combination number (-C#XX) with the controlling deflection is
identified in each deflection design case name.
For each deflection design case the controlling
deflection is determined from the current analysis results for active combinations
only, using service load factors.
-
Individual loadcase deflections are taken at the location of maximum deflection for the combination.
- The deflection values are taken as the absolute value of the maximum positive and negative major axis relative deflections.
- The deflection for each load type for a single combination is the sum of all related loadcase deflections factored by the associated load combination factor. The maximum deflection across all load combinations is taken as the controlling value.
All deflection design cases and properties can
be viewed and managed from the Deflection criteria dialog within the Tedds
interface.
Long term deflections due to creep effects
Tekla Structural Designer automatically determines the effects of creep and considers the long term deflection in the total load deflection check.
You can choose to exclude these long
term effects by unselecting the 'Include' option on the Timber page in Design
Settings.
Tekla Structural Designer automatically determines the long term load ratio.
Final deflections are determined in Tedds and
displayed in the preview results as pictured below.
When long term effects are included, only gravity combinations are considered when determining the critical combination, whereas for the instantaneous total load deflection check both gravity and lateral combinations would be considered.
Automatic adjustment of design deflection values
You can choose to apply Modification Factors to the modulus of elasticity and moment of inertia values in order to create the correct distribution of forces in a model.
The relative deflection values reported in the Load Analysis View, (which are determined using Eanalysis), take these modification factors into account.
When deflection checks are created in the Tedds calculation Tekla Structural Designer automatically adjusts the values so that they are based on unfactored modulus of elasticity and moment of inertia values. They are also further adjusted to use Emean rather than Eanalysis as the Emean value is typically recommended for serviceability checks under normal conditions.
Limiting the number of deflection cases sent to Tedds
When the calculated deflections are very small, you might decide that checks are not required. In this situation you can choose to specify a lower limit beneath which checks are not performed. This is achieved by specifying an Ignore deflections for reporting value on the Design Forces page in Design Settings.