Timber Beam-Column Design per NDS 2018 Using RF-/TIMBER AWC Module

Technical Article on the Topic Structural Analysis Using Dlubal Software

  • Knowledge Base

Technical Article

In this article, the adequacy of a 2x4 dimension lumber subject to combined bi-axial bending and axial compression is verified using RF-/TIMBER AWC add-on module. The beam-column properties and loading are based on example E1.8 of AWC Structural Wood Design Examples 2015/2018.

The member is a no. 2 Southern Pine, 2x4 nominal, 3 feet long and used as a truss member. Lateral support is provided only at the member ends and they are considered pinned. The dead (DL), snow (SL), and wind (WL) loads are applied at the top and mid-point of the beam-column, as shown below.

The member properties are shown after selecting the appropriate cross-section and material in the program.

Adjustment Factors Listed in Table 4.3.1 of NDS 2018 for ASD Design

The reference design values (Fb, Fc, and Emin) are multiplied by the applicable adjustment factors to determine the adjusted design values. For sawn lumber, these factors are found in Table 4.3.1 [1]. There are eleven different adjustment factors for the ASD design. Many of these factors are equal to 1.0 in the NDS example [2]. However, brief description and how RF-/TIMBER AWC accounts for each factor is given below.

Factors Calculated by the Program

CL ... Beam Stability Factor. It depends on the geometry and lateral support of the member as described in section 3.3.3 [1]. This factor is automatically calculated in RF-/TIMBER. (Note: The effective length, le used to calculate CL is defined by the user in the 'Effective Length' section of RF-/TIMBER AWC. The option "Acc. to Table 3.3.3" with the appropriate loading case must be selected). The image below shows the applicable loading case for this example.

CF ... Size Factor. It depends on the depth and thickness of the member as specified in section 4.3.6 [1]. This factor is automatically determined in RF-/TIMBER AWC.

Cfu ... Flat Use Factor. It accounts for weak axis bending of the member as specified in section 4.3.7 [1]. This factor is automatically calculated in RF-/TIMBER AWC.

CP ... Column Stability Factor. It depends on the geometry, end-fixity conditions, and lateral support of the member as described in section 3.7.1 [1]. When a compression member is fully supported throughout its length, CP = 1.0. This factor is automatically calculated in RF-/TIMBER AWC for both strong and weak axis directions.

Factors Defined by User-Input

CD ... Load Duration Factor. It accounts for various loading periods based on the load case such as dead, snow, and wind based on section 4.3.2 [1]. Selecting "ASCE 7-16 NDS (Wood)" as standard in RFEM activates the load duration option in the Load Cases dialogue box. The load duration class (Permanent, Ten Years, etc.) default setting is based on the 'Action Category' of the load case. This setting can be adjusted by the user in RFEM or RF-/TIMBER AWC. The value selected by the program is based on Table 2.3.2 [1].

CM ... Wet Service Factor. It accounts for the moisture service conditions of the member as specified in section 4.1.4 [1]. The user can select "wet" or "dry" in the 'In-Service Conditions' section of RF-/TIMBER AWC.

Ct ... Temperature Factor. It accounts for exposure to elevated temperatures of up to 100 degrees F, 100 to 125, and 125 to 150 as described in section 2.3.3 [1]. The user can select between the three temperature ranges in the 'In-Service Conditions' section of RF-/TIMBER AWC. The value selected by the program is based on Table 2.3.3 of [1].

Ci ... Incising Factor. It accounts for the loss of the area from the small incisions made in the member to receive preservative treatment for decay prevention as described in section 4.3.8 [1]. The user can select "Not Incised" or "Incised" in the 'Additional Design Parameters' section of RF-/TIMBER AWC.

Cr ... Repetitive Member Factor. It is used when multiple members act compositely to properly distribute a load amongst themselves as described in section 4.3.9 [1]. Ct = 1.15 for members that meet the criteria of being closely spaced and connected by a sheathing or equivalent. The user can select "Not Repetitive" or "Repetitive" in the 'Additional Design Parameters' section of RF-/TIMBER AWC.

Note: If necessary, code-based values of the user-input adjustment factors can be changed in the 'Standard' option.

Factors excluded in the Program

CT ... Buckling Stiffness Factor. It accounts for the contribution of plywood sheathing to the buckling resistance of compression truss chords as specified in section 4.4.2 [1]. This factor is used to increase Emin of the member. CT can be manually calculated per equation 4.4-1 [1] or conservatively taken as 1.0.

Cb ... Bearing Area Factor. It is used to increase the compression design values (Fcp) for concentrated loads applied perpendicular to grain as specified in section 3.10.4 [1]. Cb can be manually calculated per equation 3.10-2 [1] or conservatively taken as 1.0.

Actual Stress in the Beam-Column

In this example, the load combination has been simplified to CO1: DL + SL + WL.

Compression stress from dead and snow load, fc = 171 psi

Strong-axis bending stress from wind load, fbx = fb1 = 353 psi

Weak-axis bending stress from dead and snow load, fby = fb2 = 1,029 psi

Determine the Adjusted Design Values per NDS 2018 Table 4.3.1 ASD Method

Critical Buckling Design Value for Compression Member in Strong Axis, FcEx

Critical Buckling Design Value for Compression Member in Strong Axis, FcEx

FcEx = FcE1 = 0.822 · Emin'le1d12FcEx = FcE1 = 0.822 · 510,000 psi36.0 in3.5 in2FcEx = FcE1 = 3,963 psi

FcEx Critical buckling design value for compression member in strong axis, psi
Emin' = Emin ⋅ CM ⋅ CT ⋅ Ci = 510,000 psi
le1 Effective length = 36.0 in
d1 Depth of member = 3.5 in

Critical Buckling Design Value for Compression Member in Weak Axis, FcEy

Critical Buckling Design Value for Compression Member in Weak Axis, FcEy

FcEy = FcE2 = 0.822 · Emin'le2d22FcEy = FcE2 = 0.822 · 510,000 psi36.0 in1.5 in2FcEy = FcE2 = 728 psi

FcEy Critical buckling design value for compression member in weak axis, psi
Emin' = Emin ⋅ CM ⋅ CT ⋅ Ci = 510,000 psi
le2 Effective length = 36.0 in
d2 Thickness of member = 1.5 in

Adjusted Compressive Design Value Parallel to Grain, Fc'

Adjusted Compressive Design Value Parallel to Grain, Fc'

Fc' = Fcy' = Fc · CD · CM · Ct · CF · Ci · CPFc' = Fcy' = 1,450 psi · 1.6 · 1.0 · 1.0 · 1.0 · 1.0 · 0.29Fc' = Fcy' = 673 psi

Fc' Adjusted compressive design value parallel to grain, psi
Fc Reference compressive design values parallel to grain, psi
CD Load duration factor
CM Wet service factor
Ct Temperature factor
CF Size factor
Ci Incising factor
CP Column stability factor

Critical Buckling Design Value for Bending Member, FbE

Critical Buckling Design Value for Bending Member, FbE

FbE = 1.20 · Emin'RB2FbE = 1.20 · 510,000 psi9.652FbE = 6,577 psi

FbE Critical buckling design value for bending member, psi
Emin' = Emin ⋅ CM ⋅ CT ⋅ Ci = 510,000 psi
RB Slenderness ratio = 9.65 < 50 (NDS equation 3.3-5)

Adjusted Strong Axis Bending Design Value, Fbx'

Adjusted Strong Axis Bending Design Value, Fbx'

Fbx'= Fb1 = Fb · CD · CM · CL · Ct · CF · Ci · CrFbx'= Fb1 = 1,100 psi · 1.6 · 1.0 · 0.982 · 1.0 · 1.0 · 1.0 · 1.0Fbx'= Fb1 = 1,729 psi

Fbx' Adjusted strong axis bending design value, psi
Fb Reference bending design value, psi
CD Load duration factor
CM Wet service factor
CL Beam stability factor
Ct Temperature factor
CF Size factor
Ci Incising factor
Cr Repetitive member factor

Adjusted Weak Axis Bending Design Value, Fby'

Adjusted Weak Axis Bending Design Value, Fby'

Fby' = Fb2 = Fb · CD · CM · CL · Ct · Cfu · CF · Ci · CrFby' = Fb2 = 1,100 psi · 1.6 · 1.0 · 1.0 · 1.0 · 1.1 · 1.0 · 1.0 · 1.0Fby' = Fb2 = 1,936 psi

Fby' Adjusted weak axis bending design value, psi
Fb Reference bending design value, psi
CD Load duration factor
CM Wet service factor
CL Beam stability factor
Ct Temperature factor
Cfu Flat use factor
CF Size factor
Ci Incising factor
Cr Repetitive member factor

Combined Bi-axial Bending and Axial Compression Design Ratio

Inserting the actual stresses and limiting design values presented above into NDS equation 3.9-3 [1], the final design ratio is shown below.

Combined Bi-axial Bending and Axial Compression Design Ratio

fcFc'2 + fbxFbx' · 1 - fcFcEx + fbyFby' · 1 - fcFcEy - fbxFbE2  1.01716732 + 3531,729 · 1 - 1713,965 + 1,0291,936 · 1 - 171728 - 3536,5772 = 0.98

fc Compression stress from dead and snow load
Fc' Adjusted compressive design value parallel to grain
fbx Strong-axis bending stress from wind load
Fbx' Adjusted strong axis bending design value
FcEx Critical buckling design value for compression member in strong axis
fby Weak-axis bending stress from dead and snow load
Fby' Adjusted weak axis bending design value
FcEy Critical buckling design value for compression member in weak axis
FbE Critical buckling design value for bending member

And NDS equation 3.9-4 [1],

Combined Bi-axial Bending and Axial Compression Design Ratio

fcFcEy + fbxFbE2  1.0171728 + 3536,5772 = 0.24

fc Compression stress from dead and snow load
FcEy Critical buckling design value for compression member in weak axis
fbx Strong-axis bending stress from wind load
FbE Critical buckling design value for bending member

Result in RF-/TIMBER AWC

The user can compare each adjustment factors and adjusted design values from the analytical hand calculation method to the result summary in RF-/TIMBER AWC. As shown, the results are identical. The controlling final design ratio = 0.98 is based on the geometrically linear analysis (1st degree) calculation method. Keep in mind that the default setting in RFEM for the load combination is set to the second-order analysis. This will result in a slightly larger design ratio = 1.03. The user has the option to choose which method listed in the 'Calculation Parameters' is best for the structure.

Author

Cisca Tjoa, PE

Cisca Tjoa, PE

Customer Support & Marketing

Cisca provides technical support for Dlubal Software customers and contributes to the marketing outreach throughout North America.

Keywords

Design Timber Column NDS AWC Beam-Column

Reference

[1]   National Design Specification (NDS) for Wood Construction 2018 Edition
[2]   Structural Wood Design Examples

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  • Updated 06/17/2021

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RFEM
Package RFEM Timber EC5 5.xx

RFEM US |Timber Package 5.xx

Special software package for structural analysis and design of timber structures | Software package consisting of the main program RFEM and the add-on modules RF-TIMBER AWC, RF-TIMBER CSA, RF-DYNAM Pro - Natural Vibrations, RF-DYNAM Pro - Forced Vibrations, and RF-LAMINATE

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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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Add-on Module

Design of timber members according to the American standard ANSI/AWC NDS

Price of First License
1,120.00 USD
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The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions

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Add-on Module

Design of timber members according to the American standard ANSI/AWC NDS

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