Stylized illustration of a concrete knee joint under stress.

Unveiling the Weak Spot: How Seismic Activity Tests Concrete Joints

Jordan Keane in Tech & Innovation September 2025 • 3 min read.

"New Research Reveals Critical Insights for Safer Building Design"

In earthquake-prone regions, the integrity of buildings is paramount. A building's joints, particularly beam-column joints, are critical for overall structural stability. When these joints fail under seismic stress, the consequences can be devastating.

Traditional construction practices often treat knee joints—common in structures like bridges and building frames—the same as other joints. However, new research indicates this approach overlooks key differences in how these joints respond to stress, leading to potential vulnerabilities.

A groundbreaking study from the Hong Kong University of Science and Technology has investigated the behavior of reinforced concrete knee joints under conditions simulating seismic activity. This article breaks down the study's findings, offering insights that could reshape how we design and construct buildings in earthquake-prone areas.

Why Knee Joints Behave Differently Under Seismic Stress

Stylized illustration of a concrete knee joint under stress.

Unlike typical beam-column joints, knee joints experience unique forces during seismic events. These forces include combined closing and opening actions, which can weaken the joint and lead to brittle failure. Think of it like bending a paperclip back and forth; eventually, it snaps. The same principle applies to concrete joints, especially when the forces are not properly accounted for in the design.

The Hong Kong University of Science and Technology study focused on understanding the cyclic behavior of these joints by testing six reinforced concrete specimens. These specimens were designed to mimic various stress conditions that knee joints might experience during an earthquake.

Closing Actions: Forces that compress the joint.
Opening Actions: Forces that pull the joint apart.
Cyclic Loading: Repeated stress simulating earthquake tremors.

By analyzing how these specimens behaved, researchers uncovered critical insights into how closing and opening actions affect the joint's overall strength and durability. The specimens were divided into three groups, each designed with different angles to represent variations in structural design.

Toward Safer Structures: Key Recommendations

The study's findings suggest that current design codes may overestimate the shear capacity of knee joints, particularly those subjected to high opening stresses. To mitigate this risk, researchers recommend a more nuanced approach that considers the combined effects of closing and opening actions. They propose reducing the ACI352R-02 recommended joint shear capacity by 15% for closing actions and 30% for opening actions. By implementing these recommendations, engineers can design structures that are more resilient to seismic activity, ultimately saving lives and protecting property.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1016/j.engstruct.2018.10.025, Alternate LINK

Title: Reversed Cyclic Performance Of Reinforced Concrete Knee Joints Under Variable Closing And Opening Stresses

Subject: Civil and Structural Engineering

Journal: Engineering Structures

Publisher: Elsevier BV

Authors: Srinivas Mogili, J.S. Kuang

Published: 2019-01-01

Everything You Need To Know

What makes reinforced concrete knee joints vulnerable during seismic events?

Reinforced concrete knee joints are particularly vulnerable because they experience a combination of closing and opening actions during seismic events. These actions exert unique forces that can weaken the joint, potentially leading to brittle failure. Unlike other types of joints, knee joints are subjected to forces that compress and pull them apart repeatedly, similar to bending a paperclip until it breaks. This cyclic loading, if not properly accounted for in the design, can compromise the joint's integrity and structural stability.

How did the study at the Hong Kong University of Science and Technology investigate the behavior of knee joints?

The study from the Hong Kong University of Science and Technology investigated the behavior of reinforced concrete knee joints by testing six reinforced concrete specimens under conditions simulating seismic activity. The researchers focused on analyzing how these specimens responded to cyclic loading, which mimics the repeated stress of earthquake tremors. The specimens were designed to mimic various stress conditions, including closing actions, forces that compress the joint, and opening actions, forces that pull the joint apart. Each specimen was designed with different angles to represent variations in structural design. By observing how the specimens behaved under these simulated conditions, researchers gathered critical insights into the joint's overall strength and durability.

What are the key findings of the study regarding the shear capacity of knee joints?

The study's key findings suggest that current design codes may overestimate the shear capacity of knee joints, particularly those subjected to high opening stresses. This means that existing building designs might underestimate the forces these joints can experience during an earthquake. The researchers found that the combined effects of closing and opening actions needed to be more accurately considered in design calculations. The study emphasized that opening actions could significantly reduce a joint's shear capacity and lead to unexpected failures.

What recommendations did the researchers make to improve the design of structures in earthquake-prone areas?

To improve the design of structures in earthquake-prone areas, the researchers recommended a more nuanced approach that considers the combined effects of closing and opening actions on reinforced concrete knee joints. They specifically proposed reducing the ACI352R-02 recommended joint shear capacity by 15% for closing actions and 30% for opening actions. By implementing these recommendations, engineers can design structures that are more resilient to seismic activity. This approach is critical for ensuring that buildings can withstand the stresses of an earthquake and protect lives and property.

How do closing and opening actions affect reinforced concrete knee joints differently, and why is this important?

Closing actions compress the reinforced concrete knee joint, while opening actions pull it apart. The study found that both types of forces, especially opening actions, could significantly impact a joint's shear capacity. This is crucial because current design codes may not fully account for the effects of both actions simultaneously, potentially leading to structural failures during earthquakes. Understanding and correctly accounting for these forces allows engineers to design joints that can withstand the stresses of seismic events more effectively, improving structural integrity and safety. Without this consideration, buildings risk premature failure, which could result in significant damage and loss of life.