Post-Installed Concrete Anchors Expert Article

This article defines post-installed concrete anchors in terms of their composition, use, and the engineering considerations for their use. Failed anchors can result in damage to property, equipment and building infrastructure, as well as personal injury.

Post-Installed Concrete Anchors
Structural Engineering Overview

Post-installed concrete anchors are fasteners used to attach structural and non-structural elements to hardened concrete. The elements can vary from mechanical, electrical, plumbing equipment to structural members. Attached elements can be generators, industrial machinery, façade panels, HVAC equipment, etc.

Unlike cast-in-place anchors, which are embedded/placed during the concrete pouring process, post-installed anchors are installed after the concrete has been poured and has cured. Post-installed concrete fasteners are common in construction, renovation, and retrofit projects across nearly all industries.

Post-installed anchors can be categorized into four main types:

1. Adhesive anchors
2. Mechanical expansion anchors
3. Undercut anchors
4. Concrete screw anchors

Each type of anchor has its unique installation requirements and load-resisting mechanism, making them suitable for different applications and different loading requirements.

Failure Modes of Post-Installed Concrete Anchors

Each type of anchor has its own unique failure mechanism. Anchors can fail due to improper installation, improper anchor selection, and unanticipated loading. In legal disputes, this may manifest as:

• Falling mechanical equipment or facades
• Equipment vibrating loose
• Falling or damaged canopies and balconies/decks
• Construction repairs and/or delays

Understanding the potential failure modes of post-installed anchors is crucial for ensuring safe and reliable installations. There are several ways in which these anchors can fail during use.

Steel Failure

Steel failure occurs when the tensile stress and/or shear stress in the anchor exceeds the steel capacity. This type of failure is characterized by the physical fracture or elongation of the anchor shaft. Steel tensile failure is ductile and can provide some warning before complete failure, making it a preferred failure mode in many design situations.

Concrete Breakout Failure

Concrete breakout failures arise when the concrete surrounding the anchor fails before the anchor itself. This failure mode is often represented by an inverted cone of concrete being pulled out along with the anchor (see Figures 1 and 2).

The anchor’s depth of embedment, spacing, edge distance, the strength of the concrete all have significant roles in determining the likelihood of concrete breakout failure.

Pull-out Failure

Pull-out failure is when the anchor slips out of the hole without significant damage to the surrounding concrete.

Pull-out can happen if the mechanical expansion mechanism doesn't provide sufficient friction against the hole. It can also happen if the bond between the adhesive and the concrete or the bond between the adhesive and the anchor rod fails.

Factors influencing bond strength include hole cleaning, adhesive properties, water content in concrete, and concrete curing conditions.

Concrete Splitting

Concrete splitting failure can occur when anchors are installed too close to an edge or to each other. The expansive forces created during installation or loading can cause the concrete to split, significantly reducing the anchor's capacity.

Design Requirements

Designing post-installed anchors requires careful consideration of various factors to ensure safety, reliability, and compliance with building codes.

Engineers must accurately determine the types and magnitudes of loads that the anchors will be subjected to, including:

• Static loads (dead loads, live loads)
• Dynamic loads (wind, seismic, equipment vibration)
• Combined loading scenarios (tension + shear)

Design calculations incorporate safety factors and load reduction factors as specified by Building Code Requirements for Structural Concrete, ACI 318. Safety and load reduction factors help account for uncertainties in loading, material properties, and installation.

Anchor Selection

Proper anchor selection considers:

• Base material properties: concrete strength, thickness, cracked or uncracked
• Environmental conditions: (temperature, humidity, chemical exposure
• Loading conditions: static, dynamic, fatigue, direction of load
• Installation constraints: edge distance, spacing, embedment depth

Engineers calculate the capacity of the anchors for various failure modes, including:

• Steel strength
• Concrete breakout strength
• Pull-out strength
• Bond strength (for adhesive anchors)

The controlling capacity is determined by the lowest value among these failure modes.

Proper edge distance and anchor spacing are crucial for preventing premature failure and ensuring optimal load distribution. These requirements vary based on anchor type, size, and loading conditions.

Certain applications require additional design considerations:

• Seismic design requirements
• Fire resistance ratings
• Fatigue loading scenarios
• Corrosion resistance in harsh environments

Installation Requirements

Proper installation is critical for the performance and reliability of post-installed anchors. Anchor installation instructions are provided by the anchor manufacturer and ensure that the published and calculated values can be achieved.

Rigorous testing, as specified in Qualification of Post Installed Anchors in Concrete, ACI 355, is required to ensure reliability and durability.  The International Building Code and local jurisdictions specify the required special inspections for anchor installation.

Structural Failure Expert Witness Investigations 

Post-installed concrete anchors are essential components in modern construction and renovation projects. Their proper selection, design, and installation are crucial for ensuring structural integrity and safety.

At Robson Forensic, our experts are frequently tasked with investigating the causation and factors contributing to anchor failures. We provide analysis of the failure, accounting for the applicable standards of care, the specific loading to the anchors, the type of failure, the factors that led to the failure, and the parties that would have or should have known about the condition that led to incident.

For more information, submit an inquiry or call us at 800.813.6736.

Featured Expert

David Beh, P.E., MSCE

Civil Engineer & Construction Expert

David Beh is an expert in structural engineering with nearly two decades of professional experience in the design of buildings and a variety of other structures. He has specialized knowledge in wind,… read more.