Timber Design

Chapter

Timber Design

The timber design module can be used to design timber members in frames and trusses.

Timber Design

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Quick Reference Timber Design using PROKON

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Timber Member Design

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Timber Design

Timber Design using PROKON The PROKON suite includes a module that is suitable for design of timber members in frames and trusses. A suite of timber connection design modules is planned.

Timber Design using PROKON

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Timber Design using PROKON

Timber Member Design The timber member design module, Timsec, is used to check and optimise timber members subjected to a combination of axial and biaxial bending stresses, e.g. beams, frames and trusses. The program primarily acts as a post-processor for the frame analysis modules. It also has an interactive mode for the quick design or checking of individual members without needing to perform a frame analysis.

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Theory and application A brief background is given below regarding the application of the design codes.

Design scope The timber member design module can design timber and glued laminated timber load bearing members. Timsec currently has the following limitations: •

Only rectangular sections bent about their major or minor axes can be designed.

•

Design of tapered and haunched sections is not supported.

Design codes The program designs timber members according to the following allowable stress design codes: •

BS 5268 - 1991.

•

SABS 0163 - 1989.

Units of measurement Timsec supports Metric units of measurement only.

Symbols Where possible, the same symbols are used as in the design codes: Dimensions B : Section breadth (mm). D : Section depth (mm). L/r : Slenderness ratio. Leff : Effective length (m). Design parameters Ke : Factor with which the member length is multiplied to obtain the effective length for lateral torsional buckling. Refer to page 8 for detail. Kx : Factor with which the member length is multiplied to obtain the effective length for buckling about the x-x axis of the member. Refer to page 9 for more detail. 7-6

Timber Member Design

Ky : Factor with which the member length must be multiplied to obtain the effective length for buckling about the local y-y axis of the member. Modification factors k1 to k5 : Stress modification factors for SAB 1063 - 1989. K1 to K14 : Stress and dimensional modification factors for BS 5268 - 1991. Refer to page 11 for detail. Stresses fb : Allowable bending stress (MPa). fc : Allowable compression stress (MPa). ft : Allowable tension stress (MPa). sb : Actual bending stress (MPa) sc : Actual compression stress (MPa) st : Actual tension stress (MPa)

Sign conventions Member design is done in the local element axes. Bending about the x-x axis corresponds to strong axis bending and bending about the y-y axis to weak axis bending. Axial force and moment The local axes system and force directions are defined as follows: •

Axial force: The local z-axis and axial force is chosen in the direction from the smaller node number to the larger node number. A positive axial force indicates compression and a negative force tension.

•

Bending: Moments about the x and y-axes represent bending about the section’s strong and weak axes respectively. Positive moments are taken anticlockwise in all diagrams.

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P-delta effects Trusses are normally not sensitive to sway. However, in any structure, if you judge P-delta effects to be an important part of the analysis, you should perform a second order frame analysis.

Design parameters Different design parameters can be set for each group of elements designed: Effective length factors beams The lateral torsional stability of a beam depends on the degree of restraint to be expected at each end of the beam and of the compression edge along the length of the beam. The codes treat lateral buckling by limiting section dimensions and specifying effective length factor, Ke: •

BS 5268 - 1991: To ensure there is no risk of lateral buckling of beams, limiting depth to breadth rations are given in clause 14.8, Table 19. Degree of lateral support

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Maximum D:B ratio

•

No lateral support

•

2

•

Ends held in position

•

3

• Ends held in position and members held in line at centres not more than 30 times the breadth of the member, e.g. by purlins or tie rods

•

4

• Ends held in position and compression edge held in line, e.g. direct connection of sheathing, deck or joists

•

5

• Ends held in position and compression edge held in line, e.g. direct connection of sheathing, deck or joists, together with adequate bridging or blocking spaced at intervals not exceeding 6 times the depth

•

6

Timber Member Design

• •

•

Ends held in position and both edges held firmly in line

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SABS 0163 - 1989: Lateral stability of beams is treated in clause 6.2.3.2. The laterally unsupported should be multiplied with the effective length factor given in Table 11:

Type of beam span

• Single span beam

• Cantilever beam

Effective length factor, Ke

Position of applied load •

Concentrated at centre

•

1.61

•

Uniformly distributed

•

1.92

•

Equal end moments

•

1.84

•

1.69

•

1.06

• Concentrated unsupported end •

Uniformly distributed

at

The effective length factor may conservatively be taken as 1.92 for all situations. Effective length factors for struts and ties The effective length factors depend on the degree of restraint to be expected at each end of compression members. Guidelines are given in the codes: •

BS 5268 - 1991: Refer to clause 15.3, Table 21.

•

SABS 0163 - 1989: Compression members are discussed in clause 6.4.3, Table 12

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Effective length factors of compression members are summarised below:

End condition

Effective length factor

• Fully restrained at both ends in position and direction

•

0.7

• Restrained at both ends in position and one end in direction

•

0.85

•

Restrained at both ends in position only

•

1.0

• Restrained at one end and in position and direction and at the other end in direction only

•

1.5

• Restrained at one end in position and direction and free at the other end

•

2.0

Considering a typical plane timber truss, the effective length Lx relates to in-plane buckling. For struts where rotational fixity is provided by the connection, e.g. two or more fasteners, a value between 0.70 and 0.85 is usually appropriate. Where rotation at the joints are possible, e.g. single bolted connection, a value of 1.0 would normally be applicable. For a typical plane truss, the effective length Ly relates to buckling out of the vertical plane. This phenomenon can often govern the design of the top and bottom chords of a truss that can buckle in a snakelike ’S’ pattern, giving an effective length equal to unrestrained length. Lateral restraints are normally provided to reduce this effective length. For example, with braced purlins connected to the top chord of the truss, the effective length could be taken equal to the purlin spacing. The effective length Le relates to lateral torsional buckling of a member about its weak axis. The length depends on the spacing and type of restraint of the member’s compression edge. Using an effective length factor Ke of 1.92 would be conservative for all cases.

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Timber Member Design

Stress modification factors The codes list several stress and other modification factors, not all of which are applicable to Timsec. Some factors are not covered by scope of the program and other are supported indirectly only by modification of other factors or design parameters. BS 5268 – 1991: K1 : Modification factor by which the geometrical properties of timber in the dry condition should be multiplied to obtain values for the wet exposure condition. If applicable, you should manually adjust section sizes for the wet exposure condition. K2 : Modification factor to be applied to dry stresses and moduli (Tables 9 through 13 and 15 of the code) to obtain values for the wet exposure condition. The same K2 factor is applicable to bending and tension while a different factor is applicable to compression. K3 : Modification factor for duration of loading. Values from Table 17 of the code are summarised below: Duration of load • Long term, e.g. permanent imposed loads

K3 and

•

1.00

• Medium term, e.g. snow and temporary imposed loads

•

1.25

• Short term, imposed loads

•

1.50

•

1.75

•

e.g.

dead

temporary

Very short term, e.g. wind loads

Note: Since load duration factor may differ for different loads on the structure, you should divide the relevant loads with this factor at the analysis stage. K4 : Modification factor for bearing stress. Not applicable. K5 : Shear strength factor to allow for notches. Not applicable. K6 : Form factor for solid non-rectangular sections. Not applicable.

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K7 : Multiplication factor for grade bending stresses for members graded to BS 4978, BS 5756 or "NGLA and NGRDL Joist and Plank rules". Likewise grade tension stresses can be multiplied with K14. The factors K7 and K14 are depended on the section dimensions and are automatically calculated during the design process if required. K8 : Factor for load sharing by members connected in parallel. All grade stresses are multiplied by this factor. Tip: You may use the factor for load sharing to include any other modification factors that are not applicable to standard timber sections, e.g. factors applicable to glued laminated timber. K9 : Load sharing factor for calculating deflections. Not applicable. K10, K11 : Size factor for modification of grade compression stresses and moduli of elasticity for members graded in accordance with North American NLGA and NGRDL rules. If applicable, the K10 and K11 modification factors can be included by adjusting the grade stresses. K12 : Factor for allowable compression stress due to slenderness. This factor is automatically calculated during the design process. K13 : Modification factor for the effective length of spaced columns. Instead of using this factor, you should adjust the effective length factors Kx, Ky and Ke if required. K14 : See K7. SABS 0163 – 1980: k1 : Load duration factor. Since load duration factor may differ for different loads on the structure, you should divide the relevant loads with the Cr factor at the analysis stage. Load division coefficients are given in Table 9 of the code and summarised below:

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Duration of load

Cf

• Longer than three months, e.g. dead and permanent imposed loads

1.0

• Medium term (one day to three months), e.g. snow and temporary imposed loads

0.8

•

0.66

Timber Member Design

Short term (less than one day), e.g.

wind loads and infrequently imposed loads k2 : Factor for load sharing by members connected in parallel. All grade stresses are multiplied by this factor. Tip: You may use the factor for load sharing to include any other modification factors that are not applicable to standard timber sections, e.g. factors applicable to glued laminated timber. k3 : Stress modification factor for the type of structure. The value may be taken as 1.10 where the consequences of failure are small. For other structures a value of unity should be used. k4 : Modification factor for quality of fabrication. If the fabricated member complies with an SABS specification, the value may be taken as 1.05. k5 : Stress modification factor for moisture content. If the moisture content in a compression member may occasionally exceed 20%, use a value of 0.75. Slenderness limits BS 5269 - 1991 (clause 15.4) and SABS 0163 - 1989 (clause 6.4.4) specify similar slenderness ratios for members in compression. The slenderness limit for compression is taken as 180 in most cases. For tension members, a maximum slenderness ratio of 250, as specified by BS 5268 - 1991, is generally used. When launching Timsec, the slenderness limits given by the selected design code will be used by default. You are free to alter the maximum slenderness ratio for each individual load case or combination if required. For example, in the case where a member is carrying self-weight and wind load only, the codes allow the maximum slenderness ratio for compression members to be increased to 250.

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Member design techniques The programs have two basic modes of operation: •

Read and post-process the frame analysis results.

•

Alternatively, you can do an independent interactive design of one or more members.

The following text gives details of the design techniques and also explains how the database of timber grades and sections sizes can be customised.

Limitations of the timber member design module Timsec can be used to design timber members subjected to any combination of axial force, uni-axial and biaxial bending moment. The program cannot design non-rectangular sections or members of varying section.

Reading and post-processing frame analysis results Working through the input and design pages, the frame design procedure can be broken up into the following steps: •

The Input page: Defining design tasks by choosing a design approach, selecting members to be designed, setting the design parameters and selecting load cases and slenderness limits. The concept of tasks is described in detail on page 18.

•

The Members page: Define internal nodes and enter effective lengths. Refer to page 24 for detail.

•

The Design page: Evaluating the design results. See page 26 for detail.

•

The Calcsheet page: Accumulate design results. See page 28 for detail.

Re-analysis of the frame Having evaluated the various member sizes, you may find it necessary to return to the original frame analysis and make some changes to section sizes. Before exiting the member design module, first save the task list using the Save command on the File menu. After re-analysing the frame, you can return to the member design module and recall the task list to have the modified structure re-checked without delay. Note: For a task list to be re-used with a modified frame, a reasonable degree of compatibility is required. Tasks that reference specific laterally supported nodes, for example, will require modification if relevant node numbers have changed.

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Timber Member Design

Interactive design of members As an alternative to the above procedure, individual members can be designed without needing to perform a frame analysis. To enable the interactive design mode, select ’Interactive input of data’ on the Input page. Design steps Working through the input and design pages, the interactive design procedure can be broken up into the following steps: •

The Input page: Choose a design approach, set the design parameters and enter the element loads.

•

The Design page: Evaluate the design results. More detail is given on page 26.

•

The Calcsheet page: Accumulate design results to print or send to Calcpad. See page 28 or detail.

Modifying timber grades and sections Depending on the selected design codes, the program uses the relevant timber grades and nominal rough-sawn dimensions, i.e. as typically available in the United Kingdom or South Africa. You can customise the default grades and sections to include grades and sections readily available in your country.

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To add, delete or modify grade properties or section sizes: •

Use the Edit Timber Grades (F5) function on the Input page to display the database of grades and sections. Refer to page 15 for details.

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Edit the properties on the Timber Grades page as required. Note that each grade requires a size number.

•

On the Section Sizes page, enter available section dimensions for each size number used on the Timber Grades page.

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Press OK to permanently save your changes.

•

Use Save as Default and Load Defaults to record your preferred grades and sections independent from the selected design code.

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Timber Member Design

Tasks input On entering Timsec, it defaults to reading the last compatible frame analysis for post-processing. You can then choose to: •

Read and post-process the frame analysis results: Define one or more design tasks by grouping members with relevant design parameters.

•

Interactive design: Ignore the frame analysis and interactively input and design members.

The text that follows describe the use of the programs for reading and post-processing frame analysis results. Information regarding interactive design is given on page 21.

Choosing the data input and design mode The appearance of the Input page determined by your selection of the mode of operation: •

If you choose to read and post-process the results of the frame analysis modules, you will use the Input page to define design tasks.

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•

However, if you opt for interactive design of members, the Input page displays a table for entering member geometry and loading.

Reading frame analysis output files You can select another frame output file or view the current file: •

Read data from: Use this option to load the output of a different frame module than the one displayed. Click the box and select the relevant file from the list or enter a file name.

•

View output: To display the current frame analysis output file.

Defining design tasks Central to the process of post-processing frame analysis results, are design tasks. By grouping selective members with their relevant design parameters into one or more design tasks, you should find it easy to manage the vast amount of frame analysis data generated for larger frames. The design of a frame should be simplified by breaking it into one or more manageable tasks. Each task then defines a group of members to be designed together with the relevant design parameters to be used, e.g. timber grade, section sizes and load cases considered. Once you have defined one or more design tasks, the Design page is enabled – viewing that page automatically performs all design tasks. After having carefully defined a number of tasks, you can save the task list to disk for later re-use. This means that you can return to the relevant frame analysis module, make some changes to the structure, re-analyse it and then repeat the previous design tasks by simply reloading the task list. Defining tasks To define design tasks, you have to select or enter the following information: 1.

Select the timber grade to use

2.

Select the members to be designed.

3.

Enter the design parameters and select the section dimensions to use.

4.

Select the load cases to be considered and enter the maximum slenderness ratios.

To save a task, enter a Task title and click Add task. Once added to the task list, a task will be automatically performed when you go to the Calcsheet page. Define as many tasks as necessary to design the frame in the required detail.

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Timber Member Design

Modifying design tasks To modify an exiting task: 1.

Click Task title to display a list of defined tasks.

2.

Select the task you want to modify.

3.

Make the necessary changes to the selected members, design parameters etc.

4.

Click Update task to save the changes.

Deleting tasks To remove a task from the list, first select the task and then click Delete task. To save the complete task list to disk, use the Save commands on the File menu. Note: Saving the task list with File | Save also saves the intermediate nodes and effective lengths entered in the Members page.

Selecting a design code The current selected design code is displayed in the status bar. To select a different design code, use the Code of Practice command on the File menu or click the design code on the status bar.

Choosing a design approach Depending on what you would like to achieve, e.g. preliminary sizing or final design checks, you can choose between the following design approaches: •

Select lightest sections: Elements can be optimised for economy using mass as the criterion. You can optimise the section breadth and height separately or simultaneously by setting the respective values to ’Auto’.

•

Evaluate specific sections: To check specific section sizes, select the required sized for breadth and depth.

Selecting the timber grade Select the required timber grade from the list. To modify the grade properties, add a new grade or delete existing grades, use Edit Timber Grades (F5). Refer to page 15 for details.

Selecting members for design Use the Element groups (F6) function to select one or more element groups from the list or by clicking members in the picture. A lateral supports is assumed at each node. If certain internal Timber Member Design

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nodes are not laterally supported, you can indicate them on the Members page. Refer to page 24 for detail. Note: To modify the available section sizes for the selected timber grade, click Edit Timber Grades (F5). Refer to page 15 for details.

Setting the design parameters Use the Design parameters (F8) function to enter appropriate design parameters and material properties. You can select a different set of design parameters with each task. Refer to page 7 for a discussion of the K-factors for modifying stress and other parameters. Note: Effective length factors are entered on the Members page.

Selecting load cases and limiting slenderness ratios When loading the last frame analysis results, the program automatically displays a list of all load cases and combinations that can be designed and also the default slenderness limits for struts and ties. In the Maximum L/r ratios (F9) table, you can exclude any load case or combination from the design by clicking its right-most column. Tip: In the frame analysis modules you can also select to analyse load combinations only. The analysis output will then be more compact due to the omission of individual load case results. You are free to modify the slenderness limit for each individual load case or combination as required. In the case where uplift due to wind is dominant, for example, you may be able to set a higher slenderness limit. Refer to page 11 for more detail.

Controlling design output The amount of information that will be added to the Calcsheet page can be controlled using the Settings function on the Input page. You can choose between showing detailed calculation with or without diagrams or a tabular summary of results. The option to add the Timsec Data File to the output on the Calcsheet page, allows you to later recall the design tasks by double-clicking the data file object in Calcpad.

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Timber Member Design

Interactive input The interactive design mode offers an alternative method of designing members. Instead of performing a frame analysis and then and post-processing the results, you can enter member length and forces and design them interactively. To enable the interactive design mode, select ’Interactive input of data’ on the Input page. The pages that follow describe the use of the programs for interactive member design. The procedure to reading and post-processing frame analysis results is explained on page 14.

Selecting a design code The current selected design code is displayed in the status bar. To select a different design code, use the Code of Practice command on the File menu or click the design code on the status bar.

Choosing a design approach Depending on what you would like to achieve, e.g. preliminary sizing or final design checks, you can choose between the following design approaches: •

Select lightest sections: Elements can be optimised for economy using mass as the criterion. You can optimise the section breadth and height separately or simultaneously by setting the respective values to ’Auto’.

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•

Evaluate specific sections: To check specific section sizes, select the required sized for breadth and depth.

Setting the design parameters Use the Effective lengths (F6) function to enter effective length factors. Use Design parameters (F8) to enter appropriate design parameters. All members designed in a particular interactive session use the same set of design parameters. Refer to page 6 for a discussion of the K-factors for modifying stress and other parameters.

Effective length factors Specify the effective length factors to be used for bending about the major and minor axes and for lateral torsional buckling. For more detail on the code requirements regarding effective length factors, refer to page 8.

Specifying slenderness limits Use the Maximum L/r ratios (F9) function to enter appropriate maximum allowable slenderness ratios for compression and tension.

Entering member lengths and forces One or more lines of information can be entered for each member. The program automatically accumulates multiple lines of loads for the same member. The following input data is required: •

Name: A descriptive name for each member.

•

L: Length of the member (m).

•

F: Axial force with compression being positive (kN).

•

X/Y: Axis of bending relating to the values that follow next. Use as many lines as necessary to define the loading on the member about the x-x and y-y axes.

•

M1: Moment applied at the left end (anti-clockwise positive) about the X or Y-axis (kNm).

•

M2: Moment at the right end (anti-clockwise positive) (kNm).

•

W1: Distributed load at the left end. The load works over the whole length of the member load and varies linearly between the left and right ends (downward positive) (kN/m).

•

W2: Value of distributed load on right side (kN/m).

•

P: Point load applied on the member (downward positive) (kN).

•

A: Position of the point load, measured from the left end (m).

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Timber Member Design

Note: For allowable stress design with BS 5268 - 1991 or SABS 0163 - 1989, you should enter working loads. The profile of the members to evaluate is chosen using the Profile (F5) function. On opening the Design page, the lightest section will be chosen for each member. Lighter or heavier sections of the same profile can then be browsed as required.

Viewing design results The design results are presented on the Design page. Refer to page 26 for detail.

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Member definition Internal nodes and effective lengths are defined on the Members page. The data entered on the Members page are applicable to all design tasks defined on the Input page.

Defining internal nodes An internal node is defined as a node in-between the end nodes of a member. When you add internal nodes, the program joins relevant members to allow for easy input of effective lengths Adding an internal node You can add internal as follows: •

Enter internal node numbers in the table or click them with the mouse.

•

Use the Auto Select function to let the program detect all internal nodes.

Removing an internal node

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Timber Member Design

You can remove an internal node by deleting it form the list or by clicking it again in the picture. Consolidation of members With the addition of each internal node, the relevant node is ’removed’ by joining the two adjacent members into a single member. The table of members is continuously updated to show the new member layout. The program uses the following guidelines to when joining members at an internal node: •

For the automatic selection of internal nodes, adjoining members must have the same section.

•

Only members with an included angle greater than 100° (where 180° corresponds to a perfectly straight member) are joined.

•

Where members of different sections intersect, the larger section defines the main member that should be joined.

•

Where two or more members intersect, the internal node is taken to belong to one of the intersecting members only. The chosen member will be the straightest member or, if the same, the first in the table of members.

Entering effective lengths Enter effective length factors as follows: •

Apply the same value of Kx, Ky or Ke to all members by clicking the Kx, Ky and Ke buttons in the table heading.

•

Enter the effective length factors for individual elements. Note: The list of internal nodes and effective length factors are automatically saved when you save the task list. See page 18 for detail. Tip: You can quickly find a member in the table by pressing Ctrl+F. Enter the member name by referring to one or both of its end node numbers.

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Design results Select the Design page to perform all design tasks and display the design results. All specified load cases and combinations are considered for each member designed. Unless a very large number of elements and load cases are involved, the design procedure will normally be completed almost instantaneously. By default, the results for the design task active on the Input page are displayed. The results of any other design task can be displayed by selecting the task from the list (see description below). If an interactive member design was performed, the displayed results will be for the interactive design task instead.

The design criteria The following criteria are used in the design: •

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The interaction formulae given by the relevant design code are used to evaluate the combined effect of axial stress and bending stress. In calculating the allowable stresses, the program takes account of the member slenderness.

Timber Member Design

•

The slenderness ratio checked against the specified maximum allowable slenderness ratio for compression and tension.

Viewing results The complete interaction formulae are displayed for the critical load case of the first member of the first design task. Individual calculations have ’OK’ and ’FAIL’ remarks to indicate success or failure. To view the results of another task, member, section or load case: •

Use the Up and Down buttons to move up or down the list of available options. Tasks and load cases are listed in the order of definition. Sections are ordered by mass. Alternatively click the item, i.e. sections, and use the Up and Down arrow keys.

•

Alternatively click the relevant input box and select an item from the list that drops down.

Adding results to the Calcsheet page The following options are available when adding design results to the Calcsheet page: •

Member to Calcsheet: Add the current displayed member only. This option is not available when the design results are set to include only a tabular summary.

•

Task to Calcsheet: Add the design results of all members in the current task, including those members not currently displayed.

•

All tasks to Calcsheet: Add all members of all tasks. This option is not available in the interactive design mode because only a single design task, i.e. the interactive design task, is involved. Note: The level of detail of the information added to the Calcsheet can be set using the Settings function on the Input page. Refer to page 20 for detail.

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Calcsheet The design results of all tasks are grouped on the Calcsheet page for sending to Calcpad or immediate printing. Use the Output settings function on the Calcsheet page and Settings function on the Input page for the following: •

Embed the Data File in the calcsheet for easy recalling from Calcpad.

•

Clear the Calcsheet page.

Recalling a data file If you enable the Data File option (Settings function on the Input page) before sending a calcsheet to Calcpad, you can later recall the design tasks by double-clicking the relevant object in Calcpad. A data file embedded in Calcpad is saved as part of a project and therefore does not need to be saved in the member design module as well.

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Timber Member Design