Steel Frame Structures: Types, Connections, Span Capabilities, and Building Applications

How to evaluate and select steel framing systems for buildings based on structural type (moment frames, braced frames, open web joists), connection behavior (shear vs. moment), span-to-depth ratios, fire resistance requirements, and building function. Covers W shapes, HSS, and long-span options with emphasis on design trade-offs.

ARE · Project Planning & Design

Overview

Steel is one of the most versatile structural materials available to architects, and selecting the right steel framing system shapes everything from the floor plan to the building envelope to the fire protection strategy.

The choice between a moment frame, a braced frame, and a simple beam-and-column system changes how lateral loads travel through the structure. That choice ripples outward. Moment frames allow open floor plates but demand heavier connections. Braced frames are stiffer and lighter but place diagonal members that constrain circulation and window placement. Open web steel joists span long distances economically for roofs and floors, yet they cannot serve as part of the lateral force-resisting system without special engineering.

Steel sections come in a range of shapes: wide-flange W shapes carry most beams and columns, HSS tubes handle bracing and architectural columns, channels work for light framing, and angles serve as lintels and connections. Each has different section properties that affect load capacity, weight, and constructability.

Fire protection is an ever-present consideration. Unprotected structural steel loses roughly half its strength at approximately 550 to 650 degrees Celsius. The type and thickness of fire protection material, whether spray-applied fireproofing, intumescent coatings, or gypsum board enclosures, depends on the required fire-resistance rating and the member's section factor.

For the ARE, you need to evaluate steel systems against cost, span capability, lateral stiffness, and design impact. This topic covers how to make those evaluations and what trade-offs come with each decision.

Essential

Steel Framing System Types Steel framing systems fall into three broad categories based on how they resist lateral forces: moment frames, braced frames, and simple framing with separate lateral systems. Moment frames (also called rigid frames) resist lateral loads through the bending stiffness of beam-to-column connections. The connections are designed to transfer both shear and moment, making the frame act as a portal that deforms by bending rather than racking. Moment frames give architects maximum flexibility for open floor plans because there are no diagonal braces blocking views or circulation. The trade-off: larger steel sections and more expensive fabricated connections. Moment frames also drift more under lateral loads compared to braced frames of similar weight, which affects cladding selection. A building with brittle cladding like brick veneer may need stiffer framing. The Direct Analysis Method is the current AISC approach for evaluating frame stability.

Professional

When Moment Frames Win Consider a 12-story office building in a moderate seismic zone. The client wants column-free floor plates of 40 feet from core to perimeter, with a full curtain wall envelope. Braced frames along the perimeter would interrupt the glazing. Concrete shear walls at the core could work, but the developer prefers an all-steel structure for speed of erection. A Special Moment Frame (SMF) along the perimeter and core becomes the likely solution. The perimeter moment frames sit behind the curtain wall mullions, keeping the facade unobstructed. The penalty: connection costs run 2 to 3 times higher than shear connections, and the column sizes increase to handle combined axial and bending demands. The design team must verify that story drift satisfies the cladding manufacturer's warranty requirements, which for a unitized curtain wall might be more restrictive than code minimums.

TLDR

Moment frames allow open plans but need heavier connections and larger columns; they drift more than braced frames
Braced frames (X, chevron, V) are stiffer and cheaper but limit architectural openings in braced bays
The SER must design all lateral force-resisting connections; simple shear connections can be delegated
W shapes are the standard for beams and columns; HSS tubes handle bracing and architectural columns
Span-to-depth ratio for steel beams: roughly L/20 to L/24 for floor loads, shallower for roofs
Open web joists span up to 144 feet and allow ducts through the web, but they carry gravity loads only
Fire protection critical temperatures: approximately 600 degrees Celsius for columns, 550-650 degrees Celsius for beams
Drift limits vary by cladding: brick veneer L/600, EIFS L/240, cement board L/360, metal siding L/120
SFRM is cheapest fire protection; intumescent coatings cost 3-5x more but allow exposed steel finishes
The Direct Analysis Method uses 80% nominal stiffness and K = 1.0 for all members

ELI5

Think of a steel building like a giant construction toy. You have vertical pieces (columns), horizontal pieces (beams), and sometimes diagonal pieces (braces). How you connect them determines how the building stands up to wind and earthquakes. There are two main connection styles. A loose connection (called a shear connection) is like a hinge. The beam can rotate a tiny bit at the ends. It holds weight going straight down, but it won't stop the building from swaying sideways. A tight connection (called a moment connection) is like welding two pieces of the toy together so they can't rotate. It resists both downward loads and sideways sway. Tight connections cost more because they need extra welding and bigger pieces. So how does a steel building resist wind? Two main ways. First way: moment frames. Every connection between beam and column is tight.

AREProject Planning & Design

Steel Frame Structures: Types, Connections, Span Capabilities, and Building Applications

2 min read241 words

Steel Framing: Picking the Right System for the Building

Want to track your progress and access more study tools?

Create a free account