How to Correctly Assess Corrugated Bolted Steel Water Storage Tanks with a Flexible Membrane Liner for Fire Protection
SYNOPSIS
When it comes to fire protection, the safety and reliability of a functional water storage tank are of utmost importance. These fire protection tanks serve as a crucial resource for safeguarding lives and property in the event of a fire-related emergency. To ensure that these water tanks meet the rigorous standards necessary for such a task, the National Fire Protection Association (NFPA) has established NFPA 22: Standard for Water Tanks for Private Fire Protection. This standard defines various water-tank styles that have been in service for several generations and allows for ‘new technology’ fire protection tanks to be utilized under Section 1.4 Equivalency.
This article explores the importance of NFPA 22 and the requirements for demonstrating equivalency in water-tank manufacturing. We reference the latest standards essential for water storage and fire protection tanks: 2023 NFPA 22, ASCE 7-22, AWWA D103-19, 2021 IBC, ACI 318-19(22), AWWA D10221, AWWA D130-11, and FM 4020. By focusing on these standards and requirements, this article aids engineers and Authorities Having Jurisdiction (AHJs) in ensuring compliance and safety in fire protection water tank design.
THE SIGNIFICANCE OF NFPA 22
NFPA 22 is the industry standard that defines the requirements for the design, construction, installation and maintenance of water storage tanks used for fire protection. The primary objective of this standard is to ensure that these water tanks are structurally sound and equipped with the necessary fire protection appurtenances. Section 1.4 allows for an equivalency process for technologies not explicitly covered by the standard.
UNDERSTANDING NFPA 22 EQUIVALENCY
Section 1.4 of NFPA 22 states that technical documentation shall be submitted to the AHJ to demonstrate equivalency. The concept of equivalency, as it pertains explicitly to corrugated bolted steel fire tanks complying with NFPA 22, has been a topic of significant discussion within the NFPA 22 Technical Committee. To date, there has not been a summarized guideline that defines the metrics a corrugated bolted steel fire tank must meet to be equivalent in quality, strength and effectiveness. Because of this, fire protection tank manufacturers and installers struggle to provide the necessary documentation to prove equivalency. Meanwhile, local governmental and non-governmental entities responsible for enforcing building and fire codes, or AHJs, such as Fire Marshals and engineers, find it challenging to determine what constitutes NFPA 22 equivalency and may overlook critical design elements.
For one to demonstrate NFPA 22 equivalency, a water-tank manufacturer must provide key pieces of information, with the most essential being:
1. Proof of Structural Compliance: Structural calculations of live loads, dead loads, wind loads, snow loads, seismic loads and more must be performed by a structural or civil engineer and are essential to demonstrate that the water tank meets the structural requirements outlined in NFPA 22 Chapter 4, Section 4.12. Structural reviews ensure the tank’s safety during installation, inspection, testing, maintenance and operation, even during natural emergencies.
2. Proof of Compliance with Fire Protection Appurtenances: The fire protection tank must accommodate the appurtenances defined in the NFPA 22 standards and meet the usable volume requirements. A proper tank design with fire protection appurtenances may be carried out by a fire protection engineer or a professional specializing in fire protection tank design.
NFPA 22 WATER TANK STRUCTURAL REQUIREMENTS
To validate structural compliance, the following aspects must be reviewed by an AHJ.
Load Requirements
The fire protection tank must conform to NFPA 22 and ASCE 7 standards for snow, wind and seismic load calculations. AWWA D103 provides formulas for allowable hoop shell tension. It must also be engineered to Risk Category IV under IBC Chapter 1604.5 to ensure it maintains water pressure for fire suppression. Beware of suppliers claiming NFPA 22 compliance but only meeting Risk Category II. Verify that the tank is engineered to Risk Category IV to withstand increased loads and prevent failure during extreme climatic events, reducing hazard risk to the community.
Roof Design
According to NFPA 22, roofs with slopes less than 30 degrees must support a uniform weight of 25 lb/ft² (122 kg/m2) on the horizontal projection, regardless of snow or ice loads. This requirement is especially important for flat-roof water tanks and ensures a safe, functional and serviceable roof for personnel access.
Geographical Considerations
Different geographical areas produce varying wind, snow and seismic load requirements. Adjusting structural designs based on site-specific climatic loads is crucial for water-tank design. Unfortunately, some suppliers use generic designs for all locations, increasing the risk of failure. The absence of the site address or coordinates on the engineering cover sheet often indicates improper design.
To verify the correct load requirements for a specific location, AHJs can use tools like the ASCE Hazard Tool website (ascehazardtool.org). This resource uses ASCE 7 standards to determine the precise design loads needed for a given project address, considering regional factors.
The below chart, based on the ASCE Hazard Tool, shows the Risk Category IV design loads for three different parts of the United States.
| CITY | WIND (mph) | SNOW (lb/ft2) | SEISMIC (SDC) |
| New York, NY | 130 | 62 | C |
| Houston, TX | 145 | 11 | A |
| Portland, OR | 107 | 69 | D |
Wind Load
For high-wind areas, the roof and sidewall of the water tank must be rigid and durable enough to withstand horizontal and updraft wind forces. Section 4.12.3.1 of NFPA 22 requires these loads to withstand 30 lb/ft² (147 kg/m2) on vertical surfaces, 18 lb/ft² (88 kg/m2) on cylindrical surfaces, and 15 lb/ft² (73 kg/m2) on conical and double-curved surfaces. ASCE 7 calculates wind load factors such as wind directionality, topography, velocity pressure, gust effects and internal and external pressure. This load’s importance factor is 1.15 for Risk Category IV.
Seismic Load
In high seismic areas, water tanks must be securely affixed to prevent toppling or lifting. Guidelines in ACI 318 govern this factor. Tank sidewalls must withstand seismic compressive forces, possibly requiring vertical stiffeners. Seismic anchors or braces should be installed, with anchor rods stretching eight times their diameter. The load’s importance factor is 1.50 for Risk Category IV.
While NFPA 22 does not specify seismic loads, it mandates compliance with local building codes. Specific design criteria should be detailed in the relevant chapter or local codes, whichever is stricter, to ensure NFPA 22 compliance.
Concrete Foundation
NFPA 22 Chapter 12.2 requires tank foundations to be set on a concrete slab, concrete ring wall with a sand cushion, compacted crushed stone, granular base or self-anchoring system supported by granular berms. Additionally, NFPA 22 Chapter 12.6 mandates soil testing by a lab to ensure foundational support.
Given these options, the best foundation for a fire tank is a concrete-rebar slab, offering the highest structural integrity and safety. While some suppliers use a sand or gravel base, this can risk foundational shifts and compromise the tank’s structural and liner integrity, especially in areas with soft soil or prone to flooding. We recommend installing a concrete-rebar slab in accordance with relevant codes. Corrugated bolted steel fire protection tanks must comply with the equivalence and intent of the relevant sections to meet NFPA 22 standards.
Full Water Load Capacity
The water tank must be designed to withstand the live load of water when it is full. This includes both the weight of water (vertical projection) and the hoop stresses on the walls (horizontal projection). Freeboard calculations are necessary to demonstrate that the water tank can withstand the sloshing effect of water. If these load capacities are not met, the water tank may be at risk of failure, thereby setting the liner and tank up to burst out through the bottom.
FIRE PROTECTION DESIGN OF NFPA 22
NFPA 22 specifies the essential appurtenances for compliant water storage tanks, and the design is typically handled by a licensed professional engineer or a fire protection expert.
Submission requirements for corrugated galvanized steel water tanks are similar to other tank styles. To prove equivalency, the designer must show the tank supports essential appurtenances and meets the required water capacity as dictated in NFPA 22 Section 4.1. This includes the net capacity between the inlet of the overflow and the discharge outlet, or for suction tanks, the level of the vortex plate. NFPA 22 Section 4.6 details what must be included in a proper submittal:
• Size and arrangement of all pipes
• Size, location and type of all valves, tank heater, and accessories
• Frost-proof casing construction details
• Heat-loss calculations (if heating is required)
• Structural drawings and calculations
• Seismic bracing details and calculations
• Underground details, including foundations, compaction and backfill.
The AHJ will then review the submittal to ensure compliance, just as with any other water tank style.
FLEXIBLE MEMBRANE LINERS
While NFPA 22 plays a central role in ensuring the safety and reliability of fire protection tanks, the quality of flexible membrane liners paired with the tank system must also be considered.
Flexible membrane liners are effective for sealing water in fire protection tanks. They are easy to install and repair, and they extend the tank’s lifespan while being cost-effective. The liner must support the water’s pressure and weight without damage.
There are no specific standards for flexible PVC membrane liners in steel, above-ground fire protection tanks, but three resources provide guidance:
• AWWA D102: Minimum dry film thickness of 25 mils
• AWWA D130: Minimum thickness of 30 mils, though this may be excessive for water tanks
• FM 4020: Unreinforced liners should be at least 30 mils thick; scrim-reinforced liners should be at least 24 mils thick.
T he authors suggest a minimum thickness of 25 mils for equivalency in corrugated fire tanks. Geotextile pre-liners are recommended to protect against potential tears in the liner caused by friction or wear from the steel walls and bolts. It additionally allows condensation to wick between the liner and steel walls by preventing the two surfaces from creating a seal due to water capacity pressure or heat expansion. The selection of the flexible liner is determined by the water storage tank designer with supporting documentation provided to the AHJ for equivalency review.
CONCLUSION
When there is no specific industry standard for corrugated bolted steel water tanks, use of these tanks can be permitted by the AHJ when shown to be equivalent to the requirements of NFPA 22. This includes focusing on the tank’s structural integrity and its ability to function as a fire protection storage vessel under emergency conditions. Simply adding NFPA 22 specified appurtenances does not provide equivalency; these must be thoroughly reviewed and approved by an AHJ and potentially a licensed fire protection engineer.
Confirming a water tank’s equivalency is a multitiered process. Experienced engineers and designers must provide detailed calculations and compliance evidence with submittal packages. Due to the unique disciplines of structural engineering versus fire protection design, it is likely that two PE stamps be provided before the AHJ’s review and approval: one for structural design calculations and another for fire-tank design. These two documents can then be combined for submittal.
The structural safety and functionality of fire protection water tanks are paramount for safeguarding lives and property during fire emergencies. Consulting with experienced, licensed professional engineers or AHJs ensures adherence to safety standards.
For additional guidance, consider reaching out to trusted manufacturers committed to high compliance standards.
For more information www.waterstoragetanksinc.com
Peer Review: Justin Billodeau, P.E.
