Introduction
Moment frames are a vital part of structural engineering, offering a robust and flexible solution for lateral load resistance in buildings and other structures. These systems allow engineers to design efficient frameworks that balance structural integrity, cost-effectiveness, and aesthetic considerations. In this post, we’ll walk through the key aspects of moment frame design, covering the design of steel members, connections, and foundations, with an illustrative example problem using CalcBook to demonstrate a streamlined workflow.
What Are Moment Frames?
Moment frames are structural systems designed to resist lateral forces—such as those caused by wind or seismic activity—by allowing controlled flexural deformation in beams and columns. Unlike braced frames or shear walls, moment frames rely on the rigid connections between members to transfer loads and resist rotation, giving them a high degree of flexibility in design and application.
Moment frames are widely used in mid-rise and high-rise buildings where open floor plans and architectural freedom are priorities. Their primary benefits include the ability to accommodate large openings and the potential for significant energy dissipation during seismic events. However, these advantages come with trade-offs: moment frames can be more expensive to construct due to the complexity of their connections and may require careful attention to deflection limits and stability.
Key Considerations
Here are the critical factors to keep in mind throughout the design process for a moment frame:
Geometry: The arrangement of beams, columns, and connections affects overall stability, stiffness, and load transfer. Evaluate the height, width, and spacing of members to optimize the frame’s performance.
Loads: Consider all relevant loads, including gravity, wind, seismic, and live loads, and their combinations. Moment frames are particularly sensitive to lateral forces and overturning moments.
Material Properties: Understand the properties of the steel and concrete used in the design. High-strength materials can reduce member sizes but may lead to increased connection demands.
Member Design: Check beams and columns for flexural, shear, and axial capacity, including combined loading per AISC 360 requirements. Ensure members meet strength and serviceability criteria.
Connection Design: Rigid connections are the cornerstone of moment frames. Analyze point loads on beam flanges and web panel-zone shear per AISC 360 Chapter J to ensure adequate strength and stiffness.
Foundation Design: Spread footings must resist both axial and lateral loads. Ensure they are designed for bearing pressure, sliding, overturning, one-way shear, two-way shear, and flexure per ACI 318, considering load combinations and eccentricity.
Example Problem
The example problem below is divided into three sections: Member Design, Connection Design, and Foundation Design (Solutions Provided Using CalcBook) Part 1: Member Design Problem Statement:
Applied Loading:
Analysis Results:
Column Design Summary:
Beam Design Summary:
Part 2: Connection Design
Problem Statement:
Calculate Loading:
Input Summary:
Flange Local Bending:
Web Local Yielding:
Web Local Crippling:
Web Buckling:
Panel-Zone Shear:
Controlling Demand/Capacity Ratio:
Part 3: Foundation Design
Problem Statement:
Loads and Design Inputs:
Input Summary:
Controlling Bearing Pressure:
Nominal Sliding Resistance:
Overturning Moment:
One-Way Shear:
Two-Way Shear:
Controlling Flexure:
Controlling Demand/Capacity Ratio:
Conclusion:
Moment frames are a cornerstone of structural engineering, offering versatility and resilience in complex loading scenarios. By carefully considering factors such as geometry, loading, and material properties, engineers can achieve designs that are both safe and efficient. Download a free trial of CalcBook to explore the example problem above in full detail. Whether you're applying it to your own design needs or experimenting with the step-by-step workflow, CalcBook simplifies tedious manual calculations, allowing you to focus on what matters most in your projects.