Masterclass NDS: Dive Deep into Timber Design with RFEM 6!

Welcomes learners to the NDS timber structures course in RFEM 6, outlining modules from basic calculations to design, serviceability and fire resistance. Notes free slides and RFEM models, a completion certificate, and a 90 day free trial or student version.

Outlines the course's five chapters, from NDS and other timber standards through material theory to hands on modeling of loads, results and design in RFEM 6. States that NDS design needs the Timber Design add-on (RSTAB and RFEM) and the Multilayer Surfaces add-on for CLT (RFEM only).

Covers timber's orthotropic behavior, where rolling shear leaves GYZ at only about 10% of the shear modulus expected in an isotropic material. Applies the Poisson's ratio formula E over 2G minus one to E of 1700 ksi and G of 106.25 ksi, far above the 0.5 limit, so RFEM 6 keeps it at zero for 1D members.

Creates a new 3D model named Example 1 in RFEM 6, then deactivates the Timber Design add-on and combination wizard, not needed here. Imports Steel A36 (AISC) into a new thickness, makes it a user-defined isotropic material with E of 1400 ksi and Poisson's ratio 0.1, and sets the surface thickness from 8 to 2 inches.

Models a 13 by 7 foot surface in RFEM 6 with three line supports: one fixed only in Z, two others with a small spring stiffness in X or Y to avoid instability without a statically indeterminate model. Applies a 0.05 KSF downward surface load and deactivates self-weight for a fair timber comparison.

Copies the model twice at a 10 foot displacement with copy including loading, then gives the two new plates a new 2 inch thickness using Douglas Fir-South, visually graded decking (AWC NDS 2024) set to orthotropic linear elastic. Rotates the third surface's axes by 90 degrees to reorient its stiffness.

Runs the calculation and compares deformation: nodal displacements show 1.426 inches for the isotropic plate, about 2.77 inches for the orthotropic surface, and almost 15 inches for the rotated one. Reviews local axis moments (MX), noting higher stiffness gives higher internal forces.

Creates a new RFEM model, activates the Timber Design add-on, and sets the standards group to ASCE 7 wood with NDS 2024. Models a two-span, 30 foot beam (13 and 17 feet) with a 7 by 10 inch 24F-V4 DF glulam section, adds nodal supports, and applies a -0.1 kip per foot self weight load.

Creates two live load cases with a -0.35 kip per foot load on each span, then groups both members into a member set. Adds an effective length using the analytical method. Reviews the ultimate limit state, serviceability and fire resistance configurations and the KCR factor in the base data.

Creates a design support with a 9.5 inch support length for the compression perpendicular to grain check of NDS section 3.10.2, then copies it as an inner support for the middle node. Checks deflection in the local Z and Y directions and sets up three serviceability design situations, one with the creep factor.

Reviews the 5 design situations, switching the second ASD case to fire limit state, then shows why individual members are deactivated in favor of the member set, selectable by its outer dotted line. Runs the calculation with RFEM 6's multi-core processors and checks the overview tab for messages.

Reviews 2 fire resistance errors for bending about the Y-axis, adds a 0.5 kip wind load, and turns on fire design checks for the supports, lowering shear utilization once shear reduction is enabled. Compares the analytical NDS and eigenvalue methods for buckling, then reads a 0.232 inch Y direction deflection.

Runs the optimization tool to find a section with utilization ratios below 1.0, testing 7 to 12 inch widths and 10 to 15 inch depths in half inch steps and choosing an 8 by 11 inch section. Applies that section to the model in place of the original 7 by 10 inch one and reruns the design, turning every ratio green.

Slides explain the curvature factor from Section 5.3.8 and radial tension checks per 5.2.8 and 5.4.1 for curved glulam beams. RFEM 6 then builds Example 3 with the Timber Design add-on, ASCE 7 Wood and NDS standards, an Arc via 3 nodes, and a member with an 8 by 24 inch section in 24F-1.8E glulam.

Nodal supports restrain rotation about the X axis and a 0.15 Kip per foot load is applied to the arc, keeping only the ASD design situation after deleting serviceability and LRFD. Calculation gives a 0.02 inch max deformation, and low bending moments about the local Y axis mean lateral torsional buckling matters less.

An effective length definition disables lateral torsional buckling for the curved member and sets the length to the full member length. Timber Design then flags several nondesignable stability checks because the slenderness ratio exceeds its limit, with a warning on the effective length method for curved members.

An intermediate on member node braced in the local Y direction halves the effective lengths, clearing the slenderness ratio error with no design ratios exceeded. Details show the radial tension check per 5.4.1, then LTB is re-enabled by the eigenvalue method, giving MCrit from AlphaCrit and a mode shape in wire frame.

Example 4 models a 60 foot tapered member: a saddle distribution runs from a 7 by 14 inch start section to a 7 by 80 inch section at point K, with pinned and roller supports and a 0.350 Kip per foot load along the true length. Switching the load case to live load changes the load duration to 10 years.

Deformation reaches about 1.05 inches and bending about the local Y axis nearly 201.2 kip feet. The governing lateral torsional buckling check per NDS 3.9.2 fails at about 238% with the default 3.7-3 reference depth, and bracing node 3 in the local Y direction cuts the ratio to a 15% failure.

Optimize for current row sweeps the 7 by 14 inch section width from 8 to 12 inches in 0.125 inch steps, and the optimized 8.375 inch width still fails slightly, so 8.5 inches passes. Numbered stress points give CI of 0.35 at a 10 degree taper at point 1 against CI of 1 at point 9 in tension.

Model Example 5 turns on the Timber Design and Multilayer Surfaces add-ons with ASCE 7 wood and the NDS standard, using an 8 by 8 column and 8 by 12 girder in 24F-1.8E. Two eccentricity definitions set the girder flush with the CLT floor and column via rigid links.

A layers thickness builds a CLT wall layup from three 1.5 inch PRG 320 southern pine plies, with the middle ply rotated 90 degrees, set to orthotropic linear elastic surfaces. The import layers from library button then pulls in an ANSI/APA PRG 320 grade E4, 4 1/8 inch, 3 layer product for the walls instead.

The stiffness reduction tab covers K33 torsional and K88 in-plane factors, which FAQ 4119 and Eurocode 5's Austrian annex supply, while K44 and K55 stay at 1.0. The shear failure mode options stay unchecked, and RFEM builds the panel's full stiffness matrix by laminate theory, not the shear analogy method.

The wall local axes are set parallel to line 9, then a rectangular opening is placed from coordinate 8, 0, 3 to 3, 0, 7. A quarter inch A36 round bar set to tension forms the X bracing, and member hinges release phi Y and phi Z at the column and phi X at the girder start.

A line hinge releases the moment and adds a 150 kips per square foot UX spring for slip, applied on surface 5 at line 5 and on both walls, with the opposite side left fixed. A line release then frees line 15 on surface 5 with definition node 6, releasing the moment so the girder takes no torsion from the floor.

Nodal and hinged line supports are added to the timber column and walls, then dead, live (half the dead load), and wind area loads of 0.02 and 0.03 KSF are applied. The FE mesh is generated at one foot, static results show 0.7 inches of deflection under wind, and CLT layer stresses and grid points are reviewed.

Design situations are edited to two ASD ones, strength and fire, plus three serviceability, and the two cross bracing rods, members 6 and 7, are removed from design. Effective lengths and CLT design configurations follow, with the C con factor from PRG 320 and the nominal charring rate set to 0.25.

Show Results runs the design, and ratios are reviewed on members and surfaces: serviceability reaches 1.6 at the wall opening, object four, location 607, against a 0.433 inch limit. YZ shear covers rolling shear, and a section cut through surface 4 plots the design ratios across the wall.

A new printout report is created from the toolbar with the input data and reduced results template, while print setup and cover page are left as default, and Save and Show opens it in a separate window. Multiprint places X, Y and Z view graphics in the report, which can export to PDF, HTML or RTF.