Limit state design of timber I-joists with web openings and flange notches
Creators: name: Ahmadipour, Moein
Publisher: name: University of British Columbia
Year: 2023

description: Wood has been a key construction material in North America and around the world for decades for the principal residence of the public. It is gaining constant attention due to its renewability nature, cost-effectiveness and lightweight compared to other major construction materials. I-joists are engineered timber beams widely used in the building industry as main structural components. They have gained popularity over conventional sawn lumber beams, thanks to their effectiveness and convenience of use. These beams are mainly meant to carry the loads in roof and floor systems. During construction, however, these beams are often drilled in their web or notched through their flange to pass service, electrical or mechanical facilities through. Even though this makes them more favorable for construction, cutting the beam through can reduce its capacity to a significant extent. This raises serious concerns for the safety of the structure and the public. It is vital to notify the responsible sectors of the importance of this fact and have guidelines to evaluate the extent of the decadence and design the joists accordingly. The current wood design code in Canada does not have direct instructions for designing I-joists with openings. In the absence of verified test results, it leaves the allowability of web openings with I-joist manufacturers and does not allow flange cuts. In practice, however, it has been seen that these limits are at times contravened, and joists are retrofitted improperly using unsupported methods to compensate for their reduced capacity. Having a guideline for openings in I-joists will lead to safer and more efficient building designs. Such guidelines will provide structural engineers with directions to determine the sufficiency of I-joists considering their opening’s shape, location and size. It may also help designers find remedies to reinforce the joists by retrofitting them using approved means. In this PhD dissertation, the stiffness of simply-supported timber I-joists with a web hole and those with a flange notch is examined. The susceptibility of these joists to openings is studied through experimental and numerical study. As a first step, wood specimens are tested in order to obtain the material properties of the timber used in the I-joists. Three-dimensional large-scale finite element models of the beams are then developed. In the next step, a sensitivity analysis is conducted to study the effects of the shape, size, and location of the opening, and find the important factors affecting the performance of the joist. Using the response surface method, expressions are then developed to predict the response characteristics of the joists, including the stiffness of the joist for given serviceability criteria. Furthermore, an additional study is performed to assess the effectiveness of two retrofit types in reinforcing joists with an opening. Characteristic parameters of the retrofits are also studied, such as the type and length of the retrofit used. Finally, guidelines are proposed for designing I-joists with a web hole and those with a flange notch, assimilating the routines of the current Canadian wood design standard, CSA O86-19. In addition, guidelines are also proposed to suggest retrofits to bring the joists back to their expected serviceability criteria.