Very often our clients ask us to proceed with the optimization of platform-frame timber construction systems, especially for large-scale projects.
In the field of platform-frame timber structures, given the base dimensions of the module and sheathing panels, optimization necessarily focuses on connection systems.
The first step is to analyze the individual module to obtain information on its deformability and resistance as the connection system varies.
Description of a “typical” analyzed module
The following presents results for a “typical” frame measuring 2800x1250mm constructed with 80x200mm profiles in glulam C24.
On both sides, 10mm thick OSB3 sheets are applied (analyses were performed with different thicknesses and using single panels, which are not reported herein).
The sheet/frame connection was designed using the following fasteners:
- staples G 1.6x32mm;
- Ring nails strip P34° 2.8x64mm;
- Anker nails strip P34° 4x40mm;
- screws HZB 4x40mm.
FEM Modeling of Platform-Frame Timber Frame
Frame elements are used to model the frame members, while shell elements are used for the sheets.
The connections were modeled with linear elastic link elements having stiffness equal to that calculable using the formulas in section 7.1 of UNI EN 1995-1-1.
The module is constrained to the ground at the two lower extremities with a roller and a hinge.
A force of 10 kN was applied at the upper vertex to enable displacement verification.
The material behavior was assumed to be linear elastic.
Results and Comparative Numerical Analyses
Numerical analyses were performed to evaluate the maximum displacement of the timber frame wall as both the connection type and spacing varied.
The numerical result was compared with the analytical result obtained using the following formulation*:
In this formula, the displacement of the control point is given by the sum of the shear deformation contribution of the sheathing panel, the panel/frame connection system, the shear slip of the angle brackets, the slip of the hold-downs, and finally the compression deformation of the frame.
In this case, only the first two contributions were considered.
The results obtained are presented below.
Subsequently, we calculate the mechanical resistance associated with the different connection types using the analytical formulation of Method A in section 9.2.4.2 of UNI EN 1995-1-1.
Finally, a cost analysis of the individual fasteners was performed using the price list of a well-known hardware supplier.
Based on these results, it is possible to decide with the client the most advantageous connection system from both a performance and economic standpoint.
The choice of connection system must be made on a case-by-case basis because it is influenced by the stresses acting on the walls, their geometry, and the inter-story deformations to be ensured (considering that the deformation contributions of the ground connection systems must be added).
Once the connection system is defined, the equivalent stiffness of the individual wall module is obtained to enable modeling through links.
With the computational burden for wall modeling reduced, it is possible to proceed with the modeling and optimization of module-to-module connections.
* Structural behavior of timber framed buildings, Tiziano Sartori, 2012
Assignment: Economic Optimization of Platform-Frame Timber Structures Through Numerical Modeling