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Wood Design

Requires: Design Level

Introduction

VisualAnalysis designs beam-columns for solid sawn, post/pile, glued laminated shapes, and some structural composite lumber (LVL, PSL, etc.). Code checks performed by the program adhere to the 2012 or 2005 National Design Specification (NDS) for wood construction, either ASD or LRFD.

When using the older 2005 specification, VisualAnalysis still uses the 2012 wood shape and material design tables because it is not practical to try to implement two distinct shape and material databases for wood shapes.

Wood members designed by VisualAnalysis must be placed into design groups as described in general documentation. Each group has its own set of design parameters defined for it. These parameters include attributes such as species, grade, moisture conditions, etc., which are needed in order to determine the proper provisions to use for design. A common shape will be chosen for all members in a given group.

Assumptions

VisualAnalysis makes the following assumptions or has the limitations described below:

Capabilities

Wood Limitations

Load Duration Factor CD

Load duration factors for wood design are specified in the NDS as a function of the longevity of applied loads. Dead loads are assumed to be permanent, and live loads are assumed to last longer than construction loads. The different types of loads are associated with the duration. VisualAnalysis automatically determines what the load duration factor should be based on the "load sources" as utilized in the design load combinations, based on the shortest duration source.  The following factors are used for load sources:

Load Source

CD

Impact (untreated) 1.6
Wind or Seismic 1.6
Roof Live Loads 1.25
Snow 1.15
Live 1.0
Dead, or other loads 0.9

Impact loads are allowed a duration factor of 2.0 in the NDS, but VisualAnalysis does not use load factors higher than 1.6. 

Combined Checks

Bending and Axial Tension checks are performed per NDS  equation C3.9.2-1.  Bending and axial compression are checked per NDS section 3.9.2.

Wood Design Parameters

The design parameters represent additional information that VisualAnalysis requires in order to correctly perform code checks. Each time you create a new wood design group is created (automatic by default) you need to go to the Modify tab and make sure the design parameters are set up correctly for your new group.  Keep in mind that the parameter information you enter applies to all members of the group, so it may be wise to choose the most conservative condition that applies to any member in the group.  Note:  All of the parameters described below are edited/selected on the Modify Tab of the Project Manager  while in the Design View.

Specification:   Choose between ASD and LRFD code provisions.

Member Type:  Selects general member type, changing the selection of user input parameters.  By default, these are set based on member orientation in the model (i.e. Beam-horizontal, Column-Vertical)

Overstrength:   Designs member for higher seismic forces based on user specified Overstrength factors and IBC Seismic Design Category.

Shape Category:   Chooses shape category from which the VisualAnalysis "Design" feature selects shapes to optimize design.

Disable Checks:   Prevents design checks from being performed on the selected design group.  Allows you to speed up design checks and focus on targeted areas of larger models.

Bracing Parameters: 

Member Bracing - By default, member unbraced length is the equal to the member length between nodes.  See Bracing in VisualAnalysis for a complete description of bracing member elements for design checks.

Axial Parameters:

See Design Parameters.

Constraints Parameters:

Options under this heading allow you to specify depth and width limitations on the members chosen when the VisualAnalysis "Design" feature selects shapes to optimize design.

Wood Configuration Parameters

The following attributes are part of the "Configuration" category of parameters. Which configuration parameters are available fro a given design group depends on which Design As Shape Category (dimension lumber, timber, etc) you have chosen. 

Has Bolt Holes: This parameter is available no matter what type of wood design you are running. This parameter allows you to specify the number and size of bolt holes in the member. Note that the bolt holes only affect axial checks (pure axial and combined axial + bending). It is assumed that the bolts go through the width of the member ('b' dimension) and not the depth. It is also assumed that all bolt holes are at the same section. A third assumption is that the section is rectangular. This option should not be used for shape groups with circular or other non-rectangular cross sections.

Glu-Lam Member With Multiple Piece Laminations:

This parameter is only available for groups with the softwood gleam design type and when the selected combination is from Table 5B. It allows the program to determine the correct allowable shear stress to use, since this is dependent upon whether or not the member has multiple-piece laminations (not edge glued).

C-Factors Parameters:

Repetitive Member?

This parameter is available for Dimension Lumber, Timber, and Structurally Composite Lumber design groups. The parameter will determine whether or not the repetitive member factor, Cr, is used.

Use Incising Factor?

Indicates whether the member has been incised (for the purposes of allowing chemical treatments to penetrate better). The software uses this parameter to determine whether or not to apply the incising factor (Ci). The software assumes that the size and density of the incisions comply with the limits set forth in section 4.3.8 of the NDS.

Pressure Treated?

This item indicates whether this group's members have been chemically treated. Members that have been treated can have different load duration factor (CD) values than non-treated members.  the NDS limits duration factors to 1.6 for pressure treated members.

High Moisture Content

Indicates whether the member has sustained exposure to high moisture (19% for solid sawn, 16% for glu-lams). The software uses this parameter to determine the wet service factor (CM).

Use Buckling Stiffness Factor?

Indicates whether the buckling stiffness factor (CT) should be applied to the members of this group. NDS gives several criteria for situations in which this factor applies. Because it would be difficult for the program to try and determine this automatically, it is left for you to make this determination. The material KM value must also be entered. This factor applies only to sawn lumber members. Other conditions that determine its applicability, as well as guidelines for determining KM, are discussed in section 4.4.2 of the NDS.

Use Single Pile Factor?

 This parameter is only available for Post and Pile design groups. This option allows the Single Pile Factor (NDS 6.3.12) to be applied affecting the value of Csp.

Temperature Range

Allows you to specify if the group members will experience sustained exposure to unusually high temperatures. The software uses this parameter to determine the temperature factor (Ct).

Wood Beam Parameters

Beam Type

This is a function of the end conditions of the beam. The group members must be classified as either simply supported, cantilever, or continuous. These choices correspond to categories in NDS table 3.3.3 and are used in conjunction with the Loading Pattern to determine the effective length (le) for members in bending. This is also used to calculate the loading condition coefficient (KL) as a part of volume factor (CV) calculations for glu-lams.

Loading Pattern

Specifies the manner in which the member is loaded. The choices correspond to the categories in NDS table 3.3.3. These are used in conjunction with the Beam Type to determine the effective length (le) for members in bending. This is also used to calculate the loading condition coefficient (KL) as a part of volume factor (CV) calculations for glu-lams.

Use Custom Length Between Inflection Points (Lbi)

This allows you to override the automatically calculated length between inflection points for the member. This length is used in calculating the volume factor (CV). It might be desirable to override this quantity if the true length between inflection points extends beyond the end of the member. In this case the length calculated will be too short (it only looks up until the end of the member). Also, if a value is manually specified it will apply for all locations on the member, whereas if the value is calculated automatically it is calculated separately for each point along the member length at which checks are made.

End Notch Type

Specifies the type of notches that are assumed to exist at critical shear location. Notches may be specified as either on the tension or compression side of the beam. Refer to NDS 3.4.3.2 for the reductions in shear capacity at notch locations. When notches are specified you must also specify the notch depth and possibly the offset "e" shown in Figure 3E of the NDS. Note that the nominal depth of the notch, "dn", is calculated by taking the beam depth minus the notch depth entered.

 

Deflections Parameters:

Deflection criteria are only available when a Member Type is specified as  "Beam" or "Other".  Deflections maybe specified in a variety of ways.  Specified Deflections criteria does affect unity checks.  You must also include "deflection" load combinations in the Load Case Manager to obtain deflection checks. More information is available on the Design Concepts.

Overrides Parameters

Nearly all of the C-Factors can be manually overridden in VisualAnalysis.  The load duration factor is an exception (see above).  There are situations where a particular factor can be difficult to apply in the design module.  In these instances, factors are conservatively assumed as 1.0 (or as specified in the NDS).  Certain factors may have no effect because they do not apply to the currently selected Design Member Type (from the 'General' parameters group). Note that by overriding an adjustment factor you cause the overridden value to apply for checks made at every location on every member for every load case. Some factors might otherwise have various different values calculated, so think carefully before deciding to override a factor and double-check the results.  If factors are overridden, it is up to you to verify that the design stress values and member capacities meet code requirements.  Overridden factors are applied to all members in the Design Group.

Override COV?:  Allows a custom value for the material COVE. Normally the program will use the values specified in Table F1, Appendix F of the NDS. This value is used in calculation of the beam stability factor (CL) and the column stability factor (CP).

Fb Weak:  The allowable bending stress for flexure (for Structural Composite Lumber) causing moment about the member's section y-axis.

Fv Weak:  The allowable shear stress for shear (for Structural Composite Lumber) acting in the member's section z-axis.

Ft:  The allowable stress for axial tension (Poles).

The allowable stresses for a given structural composite lumber section or group of sections are dependent on manufacturer specific data. With this in mind, IES has created several Structural Composite Lumber material category located within the main Wood material category. The materials contained in each category each have varying properties.  If you find yourself using a particular material frequently, and the database values differ from what you prefer to use (some data base values are set at zero because no manufacturer values exist) you can go to the database and edit one of the existing materials or add a new material with customized (non-zero) allowable stress values for reuse on future projects. 

Mat Override: Overide the NDS 4.1.3.3 limitation on using 'Beam and Stringer' materials with 'Post and Timber' sized member shapes. Use this option if your materials are properly graded.

Wood Reports

You can create a design report by double-clicking on a member in the Design View, or through the Report menu. VisualAnalysis reports show the design parameters as well as 'controlling' checks. You can expand the report to show all of the checks that have been performed by double-clicking on the report and choosing the Complete Details option. 

The notation used is similar to that used in the NDS code. Subscripts are not used in the design reports so Ft is written simply as Ft. Similarly, coefficients like CD are written as CD. In general, allowable stresses use an upper case F, such as Fc for compression, Ft for tension, etc., while actual stresses use a lower case f. When you see "Ft/Fc" or "ft/fc" that means the value in that column is either tension OR compression, not the ratio. The design reports have the same format whether the member is in tension or compression.

Report Details

The report begins with a Member Unity Checks table. This table basically provides a design summary listing which member is being reported and its extreme unity value.

Next, the report provides a table of load cases that were considered when doing member checks. This table associates a load case number, referenced in the tables below, with the actual name of the load case. Note that the strength cases and serviceability cases are not numbered separately. Strength load cases are assigned numbers first and then serviceability load cases receive numbers. 

The report goes on to give a summary of the group's design parameters, including: size constraints, bracing information, deflection limits, and special wood parameters.

The last part of the report provides detailed information about the numbers involved in the code checks. The default report (created by double-clicking on a member in the design view) lists only the extreme checks for the group. If desired, there is an option to have the report display all of the information for every offset along each member of the group--simply double-click on the report and choose the Complete Details option. Note that a report for a three-dimensional structure will include weak axis checks for flexure, shear, and deflection. Also note that some columns in tables (specifically in the combined check table) represent two possible values. These values are separated by a '/' symbol, which should not be interpreted as 'divide by' but rather as 'or'. The following summarizes the information displayed in these tables:

All Tables

Member Name: The name of the member, as assigned in Model View.

Load Case #: The load case used for this particular check. The actual name of this load case can be determined by consulting the load case table above. Note that if the table you're looking at is a serviceability check (deflection) you should be checking the service number column in the design load case table.

Offset: The point along the member at which this check was made, measured in length units (feet, inches etc.; whatever you've specified under Design | Visual Design Units) from the end of the member.

CD: Load duration factor.

CM: Wet service factor.

Ct: Temperature factor.

Greek Lambda (l): Time-effect factor (LRFD only)

Greek Phi (f): resistance factor (LRFD only)

Axial Check Table

State: Tension or compression

Ft/Fc: The unadjusted allowable stress value: Ft or Fc', depending on whether the member is in tension or compression.

CF: Size factor. This factor does not apply to glued laminated timber members, so reports for members of that type (like the example report above) will not show the CF column.

CP: Column stability factor; only applies to compression checks.

Csp: Single pile factor per NDS section 6.3.12. For piles and poles only.

Cu: Untreated factor per NDS Table 6.3.5. For piles and poles only.

Ccs: Critical section factor per NDS equation 6.3-1. For piles and poles only.

Ft'/Fc': Adjusted allowable stress value; Ft' or Fc' depending on whether the member is in tension or compression.

ft/fc: Actual stress from analysis results. This number will always appear positive; the State column will indicate tension or compression.

Unity Check: Ratio of actual stress (ft or fc) to allowable stress (Ft' or Fc').

Flexure Check Table

Fb: The unadjusted allowable bending stress.

CL: Beam stability factor. For glued laminated timber members, this value will read '> CV' if it's greater than the volume factor, since only the lesser of these is to be used.

CF: Size factor. This factor does not apply to glued laminated timber members, so reports for members of that type (like the example report above) will not show the CF column.

CV: Volume factor. For glued laminated timber members only. The value will read '>=CL' if it's greater than or equal to the beam stability factor, since only the lesser of these is to be used.  For SCL Lumber, the Cv factor used is found in the shape database.

Cfu: Flat use factor. This column is only displayed for weak axis flexure checks.

Cr: Repetitive member factor. This column is only displayed if the 'Repeat member' box was checked in the group parameters dialog.

Cf: Form factor.

Csp: See Axial Check Table above.

Cu: See Axial Check Table above.

Fb': Adjusted allowable bending stress.

fb: Actual bending stress from analysis results.

Unity Check: Ratio of actual bending stress (fb) to allowable bending stress (Fb').

Shear Check Table

Fv: The unadjusted allowable shear stress.

CH: Shear stress factor. This column is only displayed if a shear stress factor was specified in the group parameters dialog.

Cu: See Axial Check Table above.

Fv': Adjusted allowable shear stress.

fv: Actual stress from analysis results. This value may include an increase due to notches.

Unity Check: Ratio of actual stress (fv) to allowable stress (Fv').

Deflection Check Table (will not appear if no deflection limits were specified)

dy: The deflection of the member with respect to the section y axis ('dz' for weak axis deflection checks).

Limit: The deflection limit specified in the group parameters dialog.

Unity Check: Ratio of the actual deflection to the deflection limit.

Combined Stresses Check Table

State: Tension or compression.

Ft'/Fc': Unadjusted allowable stress value: Ft' or Fc', depending on whether the member is in tension or compression.

Fb1*/FcE1: For members in tension, this column represents Fb1*. For members in compression, this column represents FcE1.

Fb2*/FcE2: For members in tension, this column represents Fb2*. For members in compression, this column represents FcE2.

Fb1**/Fb1': For members in tension, this column represents Fb1**; for members in compression, this column represents the adjusted allowable bending stress in the strong direction.

Fb2**/Fb2': For members in tension, this column represents Fb2**; for members in compression, this column represents the adjusted allowable bending stress in the strong direction.

FbE: For members in tension, this column is meaningless and will read '-NA-'; for members in compression, this column represents FbE.

ft/fc: For members in tension, this column represents the actual tensile stress from the analysis (ft). For members in compression, this column represents the actual compressive stress from the analysis (fc).

fb1: For members in tension, this column is meaningless and will read '-NA-'; for members in compression, this column represents the actual strong axis bending stress from the analysis results.

fb2: For members in tension, this column is meaningless and will read '-NA-'; for members in compression, this column represents the actual weak axis bending stress from the analysis results.

Notes and Messages

Note that sometimes a table heading may be followed by the message "Nothing to check, or check was disabled". This will happen when a check is meaningless for all members of a group. For example, if a group is made up of simply supported beams with no axial force, the report will not include axial or combined stresses checks. The idea behind this is to save calculation time and eliminate unneeded information in reports. This message can also appear when one or more members of a group have a different member type than the group member type. For example if a design group has been designated as glued laminated timber but includes some dimension lumber members, it will not perform unity checks on the dimension lumber members (it will still design them however, as glu-lams).

There are several messages that may occur indicating that a "Check was not performed". The "***Note:" section at the end of the report will provide specific information on why the check could not be performed. The most common note is that the "shape and material are not consistent" along with a list of the material and shape currently being used. In order to get a unity value, the shape type must match the material. An example of an incorrect match would be to have a dimension lumber Design As shape category and a Design As material from the "Visually Graded Timber" category. 

Wood References

  1. American Forest and Paper Association, National Design Specification for Wood Construction. 2012 Edition.
  2. American Forest and Paper Association, National Design Specification for Wood Construction. 2005 Edition.
  3. Breyer, Donald E., Design of Wood Structures. Third Edition, McGraw Hill, Inc., 1993.