New CMHA Manual Advances Masonry Storm Shelter Design

Words: Kristen Garman

A new manual from the Concrete Masonry & Hardscapes Association (CMHA) provides structural guidance to help engineers, architects, and mason contractors design and build ICC 500-compliant tornado shelters using reinforced concrete masonry.

Funded by the National Concrete Masonry Association Education and Research Foundation, commonly known as the NCMA Foundation, the Tornado Storm Shelter Prototypes Using Reinforced Concrete Masonry manual was developed through work led by the Masonry Institute of St. Louis. The institute brought together leading masonry engineers, code experts, mason contractors, field personnel, and suppliers to address both structural performance and constructability.

“The caliber of the people who contributed to this manual is one of its greatest strengths,” said Mark Wilhelms, chair of the NCMA Foundation Program Review Committee. “These are highly respected professionals who understand masonry engineering, help shape the codes and know what it takes to build the work in the field. They spent years examining the details from every angle, not only whether an engineer could design it, but whether a mason contractor could build it as efficiently as possible.”

The manual arrives as severe storms cause mounting losses, and more schools and communities focus on protecting occupants during extreme wind events. Severe convective storms, including tornadoes, hail and straight-line winds, caused $51 billion in insured losses in 2025. It was the third consecutive year those losses exceeded $50 billion, according to an April 2026 Insurance Information Institute report.

Codes are also driving the need for storm shelters. The International Building Code requires ICC 500-compliant tornado shelters in critical emergency operations facilities and most Group E educational occupancies within the 250-mph tornado wind-speed zone.

Three Prototypes Address the Structural Questions
To help project teams meet those requirements, the report examines three prototype shelters based on educational facilities, ranging from a smaller room to a large gymnasium. Each prototype uses fully grouted, single-wythe, load-bearing concrete masonry walls, but each features a different roof system.

“We looked at three shelter sizes and roof systems that included hollow-core plank, double tees and steel joists,” said Dave Gillick, chairman of the Masonry Institute of St. Louis and who served as the lead on the research and report. “The prototypes provide detailed information for the reinforced masonry walls and show how they can interface with various roof and foundation systems. The most critical issue is maintaining the load path through the roof, wall connections, and foundation so the structure continues to act as a box under extreme wind pressure and uplift.”

Each prototype is treated as a stand-alone structure without an adjoining building transferring wind or seismic loads into it. The structural criteria for the prototypes were based on the 2018 International Building Code, 2014 ICC 500, ASCE 7-16, TMS 402-16 and TMS 602-16. The masonry was designed using the TMS 402 strength design method.

In the end, the manual provides detailed masonry wall information that designers can use to coordinate with several possible foundations, including reinforced concrete foundation walls and strip footings; fully grouted, reinforced CMU foundation walls with concrete strip footings; or reinforced concrete foundation walls supported by piles, caissons, or piers.

The manual also includes preliminary cost models for all three prototypes, demonstrating that reinforced concrete masonry remains a cost-effective storm shelter solution. Even with the additional reinforcement required by ICC 500, installed masonry wall costs range from approximately $37 to $46 per square foot, giving project teams reliable budgeting benchmarks early in design.

“Because every project is different, these prototype details must still be evaluated and adapted by the project’s registered design professional,” says Wilhelms. “These designs are not an ‘easy button,’ but they do give design professionals a technically sound starting point for developing structures that can withstand extreme wind pressures and be built efficiently in the field.”

Roof Connections Are Critical
While the prototypes establish the wall and foundation criteria, the report also addresses one of the most critical design challenges: connecting the roof to the masonry.

The prototypes pair the walls with three roof systems commonly used in educational facilities: hollow-core plank, double tees and steel joists. Other options may include precast box beams, composite steel beams, trusses or deep long-span joists.

Regardless of the system selected, the roof, walls and foundation must function together as a structural box. The connections maintain the continuous load path needed to resist extreme pressure, suction and uplift. If a connection fails, the performance of the entire shelter can be compromised.

A concrete topping slab is often used to provide impact resistance, form a structural diaphragm and add weight to counter uplift. Each prototype assumes the roof acts as a rigid diaphragm under ASCE 7.

Engineers, contractors, and suppliers worked together to develop connections that could meet the structural demands while remaining practical to construct. For the double-tee system, the team developed a sequence that allows the tees to be set at their bearing points before the masonry crew returns to complete an enclosure around them. This eliminates the need to lay units between the double-tee legs, resulting in a cleaner and more buildable detail.

What Contractors Need to Know
The manual also highlights responsibilities that directly affect mason contractors.

ICC 500 requires the registered design professional to prepare a quality assurance plan covering the main wind force-resisting system and its components. Before beginning work, the contractor responsible for constructing or installing any listed system or component must submit a written statement of responsibility to the authority having jurisdiction, the registered design professional, and the owner.

The prototypes also require ICC 500 inspections and structural observations, TMS 402 Level 3 special inspections, and applicable IBC special inspections and tests. An independent peer review of the shelter design is required before construction.

Understanding these requirements during estimating and pre-construction can help contractors plan for documentation, coordination, inspections, and quality control before work begins.

A Starting Point, Not a Standard Design
This new manual focuses on structural masonry concepts, with some architectural considerations. It does not address every requirement for essential facilities or replace project-specific architectural and engineering analysis, but it is a solid starting point for architects, engineers and contractors.

It supplements the Storm Shelter Design Guide released in 2015 by the National Concrete Masonry Association (now CMHA). That earlier resource provides a broader discussion of architectural requirements for a classroom-wing shelter. Together, the two documents give project teams a stronger foundation for evaluating masonry storm shelters.

Put the Report to Work
The value of industry research like this comes from putting it to work. By bringing together leading technical expertise and real-world contractor input, CMHA has turned NCMA Foundation-funded research into practical guidance that can help project teams confidently design, specify, and build with masonry.

To download the full report or access additional technical resources, visit CMHA.org.

New CMHA Manual Advances Masonry Storm Shelter Design

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