CLT-to-CLT joints with hardwood dowels and screws - monotonic and cyclic single-shear tests with varying fastener spacing

Sep 15, 2026

Mass timber offers practical, cost-efficient connections using large-diameter wood screws and hardwood dowels, yet research on hardwood dowels has mainly focused on single-fastener joints under monotonic loading. This study reports 96 monotonic and cyclic single-shear plane CLT-to-CLT joint tests using Red Oak and Yellow Birch dowels and 8 mm self-tapping screws in Grand-Fir CLT. Fastener spacing was varied and mechanical properties, group action, and a nonlinear force-displacement model were assessed, enabling prediction and optimization of behavior.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages15

Reading Time38 min

AccessFree to download

Introduction

Background on dowel-type fasteners and timber joints

Motivation for hardwood dowels in CLT-to-CLT connections

Experimental study overview

Joint tests: monotonic and cyclic single-shear

Fastener types, installation angle, and spacing variables

Mechanical evaluation and modeling

Strength, stiffness, displacement, ductility

Comparison with analytical equations and group action factors

Nonlinear force-displacement model

Testing and modeling of CLT-to-CLT joints made using hardwood dowels and screws with varying spacing Aivars Vilgutsa,*, Adam R. Phillips b, Chrissi Antonopoulos c , Darrin Griechend a Department of Civil and Environmental Engineering, Virginia Tech, 750 Drillfield Drive, Blacksburg, VA 24061, United States b Department of Civil and Environmental Engineering, Virginia Tech, 102 C Patton Hall, 750 Drillfield Drive, Blacksburg, VA 24061, United States c Pacific Northwest National Laboratory, Building Systems Group, 902 Battelle Blvd., Richland, WA 99354, United States d Green Canopy NODE, WA 98134, United States A R T I C L E I N F O Keywords: Cross-laminated timber Hardwood dowels Self-tapping screws Wooden dowels Single-shear connection Timber materials Timber connections Mass timber A B S T R A C T With the increasing popularity of mass timber, utilizing large-diameter wood screws and hardwood dowels can offer an easy and cost-efficient way to join structural elements. However, the existing research on hardwood dowels is limited to single fastener joints under monotonic loading. No research is available on the group effects of joints with multiple hardwood dowels. This paper presents the results of 96 monotonic and cyclic single-shear plane CLT-to-CLT joint tests with hardwood dowels (Red Oak and Yellow Birch, 25.4 mm diameter) and selftapping wood screws (8 mm diameter) in Grand-Fir CLT, where screws were inserted both perpendicular and at a 45-degree angle to the grain for comparison to hardwood dowels. The study evaluated the mechanical properties of the joints by changing fastener spacing. Mechanical properties such as yielding and peak strength, displacement, elastic and yielding stiffness, and ductility were evaluated. The results showed equal strength and stiffness properties for hardwood dowels comparable with large-diameter screws. Additionally, the hardwood dowel joints demonstrated moderate ductility properties. The strength and stiffness of the tested joints were compared with analytical equations found in the literature by considering the group action factor of the fasteners. Lastly, a nonlinear force-displacement model was presented and compared to the experimental results and analytical calculations, which gives the possibility to predict and optimize the mechanical behavior of hardwood dowel and screw joints with varying numbers and spacing between fasteners. 1. Introduction In timber engineering, joints are crucial for sufficient load transfer between members. Additionally, as wood is naturally a brittle material, the joints need to be able to provide ductility between structural elements. To achieve high strength and ductility in timber structures, both laterally and axially loaded dowel-type fasteners have been widely used in timber-to-timber, timber-to-steel, and timber-to-concrete joints [1–7]. The most common dowel-type fasteners used in timber structures are nails, screws, and bolts, which are all made of steel. Another alternative that has received attention in only very few previous studies is hardwood dowels, which can expand the variety of available fastener options for mass timber construction. One of these few previous studies was conducted by Sebastian et al. [7], who used hardwood Iroko studs to produce and test lime concrete-to-timber floor systems. Lime concrete, or limecrete, further reduces embodied carbon. Using inclined iroko studs increased the hybrid member’s stiffness and strength by 138 % and 87 %, respectively. Moreover, hardwood dowels demonstrate high strength and moderate ductility [8–11], are easy to work with, and can be cut through during deconstruction without special tools or damage to the material. Hardwoods also have lower embodied carbon than steel fasteners, potentially reducing the environmental impact of the fasteners [12–14]. One potential application for dowel-type fasteners in mass timber construction is cross-laminated timber (CLT)-to-Glu

FAQs

Why are hardwood dowels of interest for mass timber connections?

Hardwood dowels can expand the fastener options for mass timber, with high strength and moderate ductility and potential environmental benefits due to lower embodied carbon than steel fasteners.

What variables were changed in the CLT-to-CLT joint experiments?

The study varied fastener spacing and compared hardwood dowels with self-tapping wood screws inserted both perpendicular and at a 45-degree angle to the grain.

How were the joints evaluated mechanically?

Yielding and peak strength, displacement, elastic and yielding stiffness, and ductility were assessed for both monotonic and cyclic single-shear plane joints.

What modeling approach was used to predict joint behavior?

A nonlinear force-displacement model was presented and compared with experimental results and analytical calculations, incorporating group action through an effective group factor for fasteners.

Sep 15, 2026

Uploaded by Felix Montgomery