141
Instructors: Cory Zurell
Team: Brian Chan
Materials: Bamboo Sticks, Stir Sticks, Wood Glue
Project Status: Seismic Tested
Winter 2022
Overall Construction
The structural model labelled “141” is a three-storey building structure specifically designed to resist lateral loading during seismic events. The model stands at 450 mm tall and sits on a building footprint of 150x150 mm. The built model utilizes Chevron Bracing (V Bracing) and Eccentric Bracing on all four sides of the building to maximize lateral stability in every direction. Eccentric bracing was chosen for 2 of the 4 sides to accommodate the passage of a 130x130x19mm steel weight and a door on the first floor. The other two elevations of the building use Chevron bracing since we didn’t need to create a passage for the steel weight. The Eccentric bracing elevations on the building are known as our “strong” laterally supporting direction, this is because the connection of the bracing and beam elements to the built-up columns are more rigid (due to the layering sandwich of materials) than the lateral supporting elements in the opposite direction (elevations using chevron bracing).
The model was carefully crafted to ensure the connections between the beams, built-up columns, and bracing elements are kept rigid. For example, each column was made with a “sandwich” of materials (both bamboo and stir sticks) that lined up with the bracing elements on that face, meaning the stir sticks that were used within the column provided room for the bracing members to then sit within the column creating one cohesive member throughout the whole elevation. Throughout the entire model, there are common layers that act as “spacers” to ensure all stir and bamboo sticks become aligned with one another. The floor system is a series of stir sticks glued side by side to one another, creating a rigid diaphragm to transfer any lateral loading to the beams and columns on the outside perimeter. The structural model utilizes 40 bamboo sticks and 224 stir sticks, and the entire model uses wood glue to integrate all members.
Predicted Seismic Testing Failure
The predicted failure mechanism is a localized connection failure on either end of a beam in our “weak” direction elevation. The beam members on our weak elevations hold the chevron bracing components and the floor diaphragm. The beam then transfers all of this lateral loading to the ends where they sit on the beams in our strong direction (eccentric bracing direction). The notch that was made to join the members has been glued using wood glue, although under extreme lateral loading could be a source of structural failure if the notch connection between the two beams were to suddenly detach. Another reason we might think this is the source of failure is that the notch that was made on either end of the beams are half of its depth, which can lead to a sheared beam end if the glue were to keep the members connected.
Seismic Testing
The structural model underwent a shake test to determine how it would fail under significant lateral loading. The model was loaded with 2.5kg steel weights on each of its floors and subjected to the shake test. The model experienced three different rounds of loading, each round increasing the number of steel weights on the models’ roof by two. Photo (right) shows the first-round-weighting attached to model. The model did not experience much deformation or structural movement for the first two rounds and the majority of the third round.
Nearing the end of the third round, beam connections failed and multiple bracing members buckled, reducing the model’s rigidity and ability to resist the lateral loads. It is believed that the excessive rigidity of the model played a large role in causing these components to fail. These failures caused the upper floors of the model to move out of phase with the foundation. This movement continued for a short period (about 30 seconds) until the model experienced a slight torsional effect followed by an overturning moment. This led to the columns detaching from the foundations and the model’s eventual collapse.
After testing was complete, it could be seen that the model, although detached from the foundation, had all its floors intact and was still stable under gravity loads. Therefore, it is concluded that the main cause of failure was a large P-delta effect causing an overturning moment, with buckling and joint failure being what induced this failure mode. The photos (below) show where bracing members failed under significant compression and where beam connections have sheared under the large loading.
Intricate Connections
Predicted Failure Connection
Weight on Model (First Round)
Connection Shear Failure
Bracing Failure
Comparison to Other Models
Comparing our model to other groups showed how craftsmanship and detail in planning influence the performance during testing. Many of the models that appeared to have been carefully constructed were observed to survive the longest during the shake tests. The models that survived the least time seemed to have been crafted hastily and with less care – these models were asymmetric or had inherent deflections/twisting before being loaded. Our model was able to consistently handle the lateral loading and increased dead loading between rounds, while other models often suffered extreme torsional effects and bracing compression failure much earlier in the rounds. Therefore, we believe our planning and good craftmanship in making the model played a major role in its increased seismic performance compared to other models.
A comparison of models also showed that the amount of material used can impact seismic performance. The longest lasting models during the shake test were also the heaviest in terms of mass. Our model weighed the most of all the models, at 840g, but was able to survive the longest. Therefore, we believe that the use of more material can increase seismic performance. However, it should be noted that another comparison showed that rigidity and flexibility can play a large role in the seismic performance of the models. The heaviest models also appeared to be the most rigid. While these models performed longer under loading, it was observed that they collapsed almost immediately after differential movement was induced between the upper floors and the foundation. Meanwhile, many of the lighter-weight models which were also much more flexible were observed to survive very long periods of time under the same type of movement. If our model were to be more flexible while still using the same amount of material, it is possible that its seismic performance would become better.
Suggestions to Improve Performance
One suggestion that could increase our performance would be to continue the use of eccentric bracing on our model’s “weak” sides. Our weak sides use typical chevron bracing and were also the sides where buckling failure occurred in the bracing elements. Using eccentric bracing on these sides of the model would allow for more ductility/flexibility of the structure by distributing the high compressive force from the bracing to the beam, which is expected to have high bending capacity due to it being made of bamboo. This would not increase our material use but may increase the efficiency of the overall structure. An additional suggestion would be to increase the depth of the foundation by drilling completely through the wooden plate. This would reduce the likelihood of the columns tearing out from the plate (which occurred with our model), increasing the moment required for overturning.
Group of Models on Shake Table
(141 - Rightmost Model)
Standing Model (Post-testing)