Introduction
Composite beams (steel beams with an integrated concrete deck) are used extensively in modern construction due to their superior strength, stiffness, and efficiency. The flexural design of composite beams involves analyzing the behavior of the beam under bending loads to ensure its structural integrity. In this blog post, we will explore the fundamental concepts of composite beams, discuss different design methods, explain partially and fully composite sections, and delve into the calculation of moment capacity. Additionally, we will highlight how CalcBook Software simplifies the design process for composite beams while providing step-by-step calculations.
Plastic Stress Distribution Method vs. Superposition of Elastic Stress Method:
When analyzing composite beams, two primary methods are commonly employed: the plastic stress distribution method and the superposition of elastic stress method. The plastic stress distribution method assumes a linear-elastic behavior of steel and concrete, considering the redistribution of stress at the ultimate limit state. On the other hand, the superposition of elastic stress method involves summing the stresses in the steel and concrete components, considering their individual material properties.
Determination of which method to utilize is based on the height-to-width ratio of the steel beam web. Steel beams with a compact web shall be designed based on the plastic distribution, while shapes with non-compact or slender webs shall be determined based on superposition of elastic stresses. This blog post focuses on steel beams with a compact web and the plastic distribution method.
Partially Composite vs. Fully Composite Sections:
The behavior of a composite beam depends on the degree of interaction (shear transfer) between the steel and concrete components. Steel headed stud anchors or steel channel anchors are typically used to transfer shear between the steel beam and concrete deck. Steel headed stud anchors are most commonly used in practice. If the capacity of the steel anchors exceeds the minimum capacity of the concrete section and steel section, then the section is fully composite. Conversely, if the capacity of the anchors is less than the minimum capacity of the concrete section and steel section, then the section is partially composite.
ΣQn > Min (Cc,max, Ts,max) Fully Composite
ΣQn < Min (Cc,max, Ts,max) Partially Composite
*Cc,max = maximum possible compressive force carried by effective slab area
*Ts,max = maximum possible tensile force carried by steel beam
*ΣQn = Stud Anchor Capacity
The moment capacity of a partially composite section is reduced based on the limited capacity of the stud anchors.
Locating the Plastic Neutral Axis:
The plastic neutral axis(PNA) is a critical parameter in composite beam design. It represents the axis where the steel and concrete components of the beam resist bending moments equally. The location of the PNA determines the extent of composite action and influences the beam's overall behavior.
The location of the PNA can be in three different locations:
The concrete slab
The steel beam flange
The steel beam web
The location of the PNA is determined based on the compression force in the slab, the tension force in the steel beam (flange, web, and entire beam), and the capacity of the anchors. A simple guide for finding the location of the PNA is shown below:
Concrete Slab or Steel Beam:
Cc > Ts,max PNA is in the concrete slab
Cc < Ts,max PNA is in the steel beam
Steel Flange or Steel Web:
Cf,max > Cs PNA is in the flange
Cf,max < Cs PNA is in the web
*Cc = minimum of: Cc,max, Ts,max, and ΣQn
*Cf,max = maximum possible compressive force carried by top flange of steel beam
*Cs = actual compressive force in the steel beam
Example Problem (Solutions Provided Using CalcBook): Problem Statement:
Step 1: Check Web Compactness
Step 2: Determine Stud Anchor Capacity
Step 3: Determine if Fully Composite
Step 4: Determine Location of Plastic Neutral Axis (PNA)
Step 5: Calculate Locations of Forces
Step 6: Check Design Flexural Strength
Conclusion:
Designing composite beams in flexure involves considering the interaction between steel and concrete components, locating the plastic neutral axis, and determining the moment capacity of the composite section. The AISC Steel Design Manual and Specifications provide valuable guidelines and equations for this purpose. However, to expedite the design process and ensure accuracy, engineers can leverage powerful software tools like CalcBook. By simplifying calculations and providing a clear visualization of the design process, CalcBook enables efficient and effective design of composite beams, enhancing the structural integrity and performance of modern construction projects.