INSPECTION GUIDEBOOK
For firestopping installations
May, 2024
1. FIRE PROTECTION BASICS
Fire protection refers to measures taken to prevent fires, reduce their impact, or extinguish them. Three primary strategies exist in construction: Detection (active systems like alarms), Suppression (active systems like sprinklers), and Containment (passive barriers like compartments). Fire detection and suppression are active, while containment is passive, designed to slow or prevent fire and smoke spread.
1.1 Active and Passive Fire Protection
Active fire protection requires action to stop or combat a fire after it starts, such as sprinkler activation or manual firefighting. Passive fire protection involves building components that slow or impede fire and smoke spread without activation, like fire-rated walls, floors, glass, and intumescent coatings.
1.2 Compartmentation
Compartmentation uses physical (passive) barriers to restrict fire and smoke movement, containing fires within sections to allow safe evacuation and protect assets. Effective compartmentation requires collaboration among architects, planners, engineers, and owners. Firestop systems in fire-rated assemblies must meet or exceed the compartment's fire rating.
Figure 2: Diagram illustrating horizontal compartments along a building's height.
Figure 3: Diagram showing vertical compartments dividing a floor plan.
Compartments limit fire spread by addressing heat, fuel, and oxygen. They protect escape routes like corridors and stairs, aiding occupant evacuation and firefighter access. Compartment subdivision depends on building type, occupancy, fire load, height, and sprinkler system availability, as guided by local regulations.
1.3 Firestop Systems
Firestop systems seal openings in fire-rated assemblies (walls, floors) to prevent the passage of fire, smoke, water, and sound. They are crucial for restoring the integrity of assemblies breached by pipes, conduits, cables, and other building systems, as well as for closing gaps that allow for assembly movement.
- Through-penetration firestop systems restore integrity when items pass through a fire-rated floor or wall.
- Fire-resistive joint systems protect joints or spaces within or between fire-rated assemblies.
- Membrane penetration firestop systems restore integrity when a fire-rated wall is penetrated on only one side.
- Perimeter fire containment systems seal gaps between fire-rated floors and non-fire-rated exterior curtain walls to prevent vertical fire spread.
2. DIFFERENCE BETWEEN FIRESTOPPING AND FIREBLOCKING
Firestopping products and fireblocking materials are often confused but serve different purposes and applications.
- Firestopping uses specialized materials to resist fire and byproduct spread through openings in fire-rated assemblies for penetrations.
- Fireblocking uses generic materials like lumber, wood panels, gypsum board, or mineral wool in concealed spaces to block fire and hot gas migration. It subdivides stud cavities, soffits, and spaces between finishes.
Fireblocking: Uses approved building material to resist fire and hot gas migration in concealed spaces.
Firestopping: Seals around penetrations (openings) in fire-rated walls or floors.
Figure 4: Diagram illustrating the difference between firestopping and fireblocking.
3. FIRESTOP CODES AND STANDARDS
3.1 Building Code Requirements
Firestopping is defined in model building and safety codes, which specify requirements, testing standards, and life safety measures. Relevant codes include the International Building Code (IBC), International Fire Code (IFC), NFPA 101, UAE FIRE and Life Safety Code of Practice, and SAUDI BUILDING CODE-GENERAL SBC 201-CR.
3.2 Applicable Standards
Test standards relevant to firestop systems include:
- ASTM E 1399 "Cyclic Movement and Measuring the Minimum and Maximum Joint Widths of Architectural Joint Systems"
- ASTM E 1966 (ANSI/UL 2079) "Standards Test Method for Fire-Resistive Joint Systems"
- ASTM E 2174 "Standard Practice for On-Site Inspection of Installed Firestops"
- ASTM E 2307 "Standard Test Method for Determining the Fire Resistance of Perimeter Fire Barrier Systems Using the Intermediate Scale, Multi-Story Test Apparatus"
- ASTM E 2336 "Standard Test Methods for Fire Resistive Grease Duct Enclosure Systems"
- ASTM E 2393 "Standard Practice for On-Site Inspection of Installed Fire Resistive Joint System and Perimeter Fire Barriers"
- ASTM E 2750 "Standard Guide for Extension of Data from Firestop Penetration System Tests Conducted in Accordance with ASTM E814"
- ASTM E 814 (ANSI/UL 1479) "Standard Test Method for Fire Tests of Through-Penetration Firestops"
- ICC ES AC179 "Acceptance Criteria for Metallic HVAC Duct Enclosure Assemblies"
- ISO 6944 "Fire Resistance Tests Ventilation Ducts"
4. THIRD-PARTY TESTING AGENCIES
Independent testing laboratories, or third-party agencies, conduct fire tests on firestop products according to specific standards. Successful tests result in design listings published in directories, which are crucial for plan review and inspection.
Recognized independent laboratories include:
- Underwriters Laboratories Inc. (UL), Northbrook, IL (847) 272-8800, www.ul.com
- Southwest Research Institute San Antonio, TX (210) 522-2311, www.fire.swri.org
- Factory Mutual Norwood, MA (781) 762-4300, www.fmglobal.com
- Intertek Testing Services, San Antonio, TX (210) 625-8100, www.intertek.com
4.1 Identification Guide (UL)
To find the correct UL firestop system for a specific site requirement, understanding UL's nomenclature is essential.
UL uses Category Control Numbers (CCNs) for different firestop system types:
- XHEZ (Through-penetration firestop systems)
- XHBN (Joint systems)
- XHHW (Fill, void, or cavity materials)
- XHDG (Perimeter fire containment systems)
- CLIV (Wall opening protective materials)
UL employs nomenclature rules to further categorize systems within each CCN.
4.2 Nomenclature for Penetrations
Firestop systems for penetrations use an alphanumeric system: the first part denotes the assembly type (walls, floors, or both), the second part the assembly material, and a number indicates the penetrating item type.
Example: C-AJ-1194
Combination: F = Floors, W = Walls, C = Walls and Floors
Construction Type of Floor or Wall
Individual System Number / Penetrant Type
Second letters specify wall/floor construction:
- A: Concrete floors 5 inches thick
- B: Concrete floors > 5 inches
- C: Framed floors / floor-ceiling assemblies
- D: Steel deck construction
- J: Concrete or masonry walls 8 inches thick
- K: Concrete or masonry walls > 8 inches thick
- L: Framed walls / gypsum wallboard assemblies
First digit describes the penetrating item(s):
- 0: Blank openings
- 1: Metal Pipe, conduit, or tubing
- 2: Non-metallic pipe, conduit or tubing
- 3: Cables
- 4: Cable trays
- 5: Insulated pipes
- 6: Miscellaneous electrical (busways)
- 7: Miscellaneous mechanical (ductwork)
- 8: Groupings of penetrations, including any combination of items listed above
4.3 Nomenclature for Joints
Joint systems use an alphanumeric system: the first two letters denote the joint type, the third letter indicates movement capability (S for static, D for dynamic), and a number specifies the nominal joint width.
Example: HW-D-0034
Joint Type: FF = Floor-to-Floor, WW = Wall-to-Wall, FW = Floor-to-Wall, HW = Head of Wall
Movement: D = Dynamic, S = Static
Individual System Number
Joint Width: 0000-0999 = up to 2 inches, 1000-1999 = up to 6 inches, 2000-2999 = up to 12 inches, 3000-3999 = up to 24 inches, 4000-4999 = over 24 inches.
5. LISTED FIRESTOP SYSTEMS
A UL/CUL listed firestop system provides comprehensive details of a tested system, including components, parameters, fire ratings, variations, penetrant sizes, and applicable products. It serves as a credible document for installations following UL/ASTM standards.
Figure 5: Illustration of a sample UL listing for a through-penetration system, detailing fire rating, assembly thickness, penetrant size, sealant thickness, backing material, and applicable products.
UL systems offer variations for specific assemblies. Always check for an available tested system before considering a manufacturer's Engineering Judgment (EJ).
Manufacturers may offer multiple approved systems for the same assembly/penetrant; installation guidelines and materials are not interchangeable between them.
Fire Rating (ASTM E814)
Min. Floor/Wall Assembly Thickness
Max. Penetration Size & Material
Min. Sealant/Firestop Product Thickness
Valid Components & Annular Space
Min. Packing Material Thickness & Density (e.g., Mineral Wool)
Applicable Hilti Products
6. ENGINEERING JUDGMENTS (EJ)
When a firestopping application is not covered by a tested system, manufacturers provide custom Engineering Judgments (EJs) for specific jobsite conditions.
Project Name
Customer/Company Name
Engineering Judgment Firestop Detail
Ratings: F-RATING = 2-HR.
This EJ represents a firestop system expected to pass stated ratings if tested.
Cross-Sectional View
1. Concrete Floor Assembly (Min. 4-1/2" Thick, 2-HR. Fire-Rating).
2. Max. 1" Nominal Diameter Steel Conduit/EMT with Max. 3/4" Thick AB/PVC Insulation.
3. Min. 4" Thick Mineral Wool (Min. 4 PCF Density), tightly packed and recessed for sealant.
4. Min. 1/2" Depth Hilti FS-ONE MAX Intumescent Firestop Sealant.
Notes: Max. Opening Area = 40 sq. in. Annular Space = Min. 1/2", Max. 12".
Referenced Tested System
Referenced Tested Systems: UL/cUL System No. C-AJ-8099 & C-AJ-8143
Project Application Details: CS0182799
Applicable Test Method: UL 1479
Hilti Firestop Systems
Hilti, Inc. Plano, Texas USA. Designed by Hilti FPE: Travis Pearce.
Saving Lives through Innovation and Education
Sheet 1 of 1. Scale: 3/16" = 1". Drafter: AN. Date: Aug. 17, 2023.
Drawing No.: 602367b
Figure 6: Example of an EJ provided by Hilti.
According to IFC guidelines, EJs are firestop designs by qualified personnel based on tested systems. IBC (Sections 703.2, 703.3) supports EJs, ensuring systems maintain or exceed their tested fire ratings.
IFC guidelines for evaluating EJs include:
- Use tested systems instead of EJs when available.
- Issued only by qualified technical personnel.
- Issued for a single project; not transferable without review.
- Issued only where local code enforcement permits their use.
- Clearly indicate the system is an EJ, not a listed system.
- Must identify project name, contractor, non-standard conditions, and required hourly rating.
- Must show issue date, signatures, and issuer's contact information.
- Must reference the tested system(s) the design is based on.
Key points to remember:
- Qualified personnel may include manufacturer-trained engineers.
- UAE fire code requires manufacturer, consultant, and fire contractor stamps on site-specific EJs; consultants verify authenticity and accuracy.
- Using multiple suppliers can reduce EJ needs, but materials from different manufacturers cannot be mixed within a single system.
Project submittals must clearly identify EJ-based details and provide them to field inspectors.
6.1 UL Technical Evaluation Developer Program (TEDP)
In 2023, UL Solutions launched the Technical Evaluation Developer Program (TEDP) to enhance the quality of manufacturer-developed firestop Engineering Judgments (now called Technical Evaluations). Qualification involves technical personnel exams, UL staff audits, and testing of selected EJs.
The TEDP aims to reduce low-quality EJs and promote testing/certification, giving code officials and professionals confidence in TEDP-qualified manufacturers' EJ solutions for life safety.
7. INSPECTION PROCESS
Passive fire protection system performance depends on installation quality. Firestop inspections ensure installed systems meet building code fire resistance requirements.
IBC (Section 1705, 2018) mandates inspections for high-rise and high-risk category projects by licensed third-party special inspectors, following ASTM E2174 and E2393 for visual and destructive testing.
General inspection is vital for life safety even outside special inspection requirements. ASTM standards offer guidelines for inspecting a percentage of work, as inspecting every penetration or joint is impractical.
The inspection process starts with approved documentation. Contractors must maintain records of listed systems and EJs used. These documents guide installers and inspectors for proper selection and verification.
Recommended inspection guidelines and best practices:
- Coordinate inspections during installation and at final walk-through.
- Do not conceal firestop systems before inspection and approval.
- Review construction documents to locate fire-rated assemblies.
- Obtain approved firestop system submittal package.
- Compare installed systems with approved submittal details.
- Check product packaging to verify installed materials match submittals.
Subsequent sections detail inspection guidelines for specific firestop system types.
Figure 7: Illustration of measuring sealant thickness while wet.
8. INSPECTION GUIDELINES FOR THROUGH-PENETRATION FIRESTOP SYSTEMS
When building system components penetrate fire-rated floors or walls, their integrity is breached. Appropriate firestopping materials, installed per a tested system matching the application, are required to restore integrity.
Common firestopping materials for through penetrations include:
- Preformed firestop products (plugs, collars, wrap strips) for single pipe penetrations.
- Firestop blocks for multiple pipe penetrations in large openings.
- Firestop devices (modular sleeves, speed sleeves) for cable bundles.
- Cast-in-place firestop floor devices.
- Acrylic, Silicone, and Intumescent based firestop sealants.
8.1 Steps for Inspection
Checkpoints for Through-Penetrations
| Checkpoint | Observation | Comments |
|---|---|---|
| 1) Is the hourly fire rating of the referenced firestop system or EJ greater than or equal to the fire rating of the floor/wall assembly being penetrated? | Yes/No/N/A | |
| 2) Does the floor or wall construction match the floor/wall assembly listed in the firestop system or EJ? | Yes/No/N/A | |
| 3) Does the penetrating Item(s) match the acceptable list of penetrants (material, size, diameter, insulation type and thickness, etc.) shown in the firestop system or EJ? | Yes/No/N/A | |
| 4) Does the field installation comply with the following requirements of the referenced firestop system or EJ? | Yes/No/N/A | |
| a) Size of opening | Yes/No/N/A | |
| b) Minimum and maximum annular space requirements | Yes/No/N/A | |
| c) Proper backing material (when required) | Yes/No/N/A | |
| d) Proper firestopping product, including type, amount, thickness, orientation, etc. | Yes/No/N/A | |
| e) Proper accessories installed, including anchor/fasteners, mounting or cover plates, frames, etc. | Yes/No/N/A | |
| f) For multiple penetrants: proper distance between penetrating items | Yes/No/N/A | |
| g) For cables: allowable cable size, spacing, bundle size, and percent fill of opening | Yes/No/N/A | |
| 5) For wall assemblies, verify that the field installation was installed properly on both sides of the wall | Yes/No/N/A |
8.2 Example Comparison
8.2.1 Correct Installation
Figure 8: Hilti CFS-BL Firestop Block used for large openings with multiple through penetrations, showing stacked blocks with staggered seams.
Figure 9: Hilti CP-643 N Firestop Collar sealing large metallic pipes.
Figure 10: Hilti CFS-SL GA Speed Sleeve with gang plate for small cable bundles.
Figure 11: Hilti CFS-MSL GPA Modular Sleeves with gang plate for multiple cable bundles of varying sizes.
Figure 12: Hilti CFS-D Firestop Cable Disc sealing a cable bundle under 1 inch.
Figure 13: Hilti CFS-DID Device Flange secured to the top surface as per listed system.
8.2.2 Incorrect Installation
Figure 14: Sealant improperly tooled, leaving gaps.
Figure 15: Multiple Hilti CP-643 N cable collars used for a large opening, leaving gaps.
Figure 16: A blank opening shown below two sealed small openings.
Figure 17: Hilti CP 680 sleeve not cast into concrete.
Figure 18: Gaps present in the opening.
Figure 19: Incorrect collar installation on the top side of a floor.
Figure 20: Incorrect installation (left) with uneven intumescent strip; correct installation (right).
9. INSPECTION GUIDELINES FOR MEMBRANE PENETRATION FIRESTOP SYSTEMS
A membrane penetration occurs when a component penetrates only one side of a fire-rated assembly, such as electrical boxes or single-sided pipe penetrations.
Membrane penetrations can be sealed by following through-penetration firestop system requirements that match the application.
Common firestopping materials for membrane penetrations include:
- Preformed firestop products (plugs, collars, wrap strips) for single pipe penetrations.
- Firestop putty discs for cable penetrations.
- Firestop putty pads for protecting outlet boxes and junction boxes.
- Flexible, silicone based firestop sealants.
- Elastomeric firestop sealants.
- Intumescent firestop sealants.
9.1 Steps for Inspection
Checkpoints for Membrane Penetrations
| Checkpoint | Observation | Comments |
|---|---|---|
| 1) Is the hourly fire rating of the referenced firestop system or EJ greater than or equal to the fire rating of the wall assembly being penetrated? | Yes/No/N/A | |
| 2) Does the wall construction match the wall assembly listed in the firestop system or EJ? | Yes/No/N/A | |
| 3) Does the penetrating Item(s) match the acceptable list of penetrants (material, size, diameter, insulation type and thickness, etc.) shown in the firestop system or EJ? | Yes/No/N/A | |
| 4) Does the field installation comply with the following requirements of the referenced firestop system or EJ? | Yes/No/N/A | |
| a) Size of opening | Yes/No/N/A | |
| b) Minimum and maximum annular space requirements | Yes/No/N/A | |
| c) Proper backing material (when required) | Yes/No/N/A | |
| d) Proper firestopping product, including type, amount, thickness, orientation, etc. | Yes/No/N/A | |
| e) Proper accessories installed, including anchor/fasteners, mounting or cover plates, frames, etc. | Yes/No/N/A | |
| f) For multiple penetrants: proper distance between penetrating items | Yes/No/N/A | |
| g) For putty pads: is it installed on all proper sides of the outlet box | Yes/No/N/A |
9.2 Correct Examples
Figure 21: Hilti CP 617 Putty Pad sealing an electrical box inside a wall.
10. INSPECTION GUIDELINES FOR FIRE-RESISTIVE JOINTS
Fire-resistive joints occur where fire-rated assemblies intersect (e.g., head-of-wall, floor-to-wall) and must be sealed per a fire-resistive joint system to prevent fire spread. These systems often accommodate movement.
Fire-resistive joint systems prevent fire spread through linear openings between assemblies and typically accommodate joint movement.
Common firestopping materials for fire-resistive joints include:
- Preformed firestop products (top track seal, bottom track seal).
- Flexible, silicone based firestop sealants.
- Elastomeric firestop sealants.
- Water-based firestop sprays.
10.1 Steps for Inspection
Checkpoints for Joints
| Checkpoint | Observation | Comments |
|---|---|---|
| 1) Is the hourly fire rating of the referenced fire resistive joint system or EJ greater than or equal to the fire rating of the floor/wall assembly? | Yes/No/N/A | |
| 2) Does the floor and/or wall construction match the floor/wall assembly listed in the firestop system or EJ? | Yes/No/N/A | |
| 3) Is the fire resistive joint system tested for the amount of movement required for the assembly? Does it match the required class and type of movement? | Yes/No/N/A | |
| 4) Does the field installation comply with the following requirements of the referenced fire resistive joint system or EJ? | Yes/No/N/A | |
| a) Minimum and/or maximum width of the joint | Yes/No/N/A | |
| b) Proper backing material (when required), including proper orientation and compression of backing material | Yes/No/N/A | |
| c) Proper firestopping product, including type, amount, thickness, orientation, etc. | Yes/No/N/A | |
| d) Proper accessories installed, including anchor/fasteners, mounting or cover plates, frames, etc. | Yes/No/N/A | |
| e) For firestop spray products: does the applied firestop spray overlap the adjacent surfaces properly? | Yes/No/N/A |
10.2 Example Comparison
10.2.1 Correct Installation
Figure 22: Hilti CP606 (white) flexible firestop sealant applied to both sides of a wall with mineral wool backing.
Figure 23: Firestop sealant applied to a top-of-wall joint.
Figure 24: Hilti CFS-TTS used on a top-of-wall joint.
Figure 25: Hilti CFS-TTS used on a top-of-wall joint.
Figure 26: Hilti CFS-TTS MD installed correctly per EJ.
Figure 27: Bottom track seal installed on a floor runner.
Figure 28: Hilti CFS-TTS MD installed correctly per EJ.
10.2.2 Incorrect Installation
Figure 29: Gap in application.
Figure 30: Inadequate sealant or gap width.
Figure 31: Head-of-wall joint exceeding the listed system's width.
11. INSPECTION GUIDELINES FOR PERIMETER FIRE BARRIER SYSTEMS
Perimeter fire barrier systems prevent fire spread through linear openings between fire-rated floors and non-fire-rated exterior walls, ensuring continuity with the exterior wall assembly as required by building codes.
Common firestopping materials for perimeter fire barrier systems include:
- Preformed firestop products (e.g., Hilti CFS EOS Quick Seal).
- Water based firestop spray.
- Silicone based firestop spray.
11.1 Steps for Inspection
Checkpoints for Perimeter Fire Barrier Systems
| Checkpoint | Observation | Comments |
|---|---|---|
| 1) Is the hourly fire rating of the referenced perimeter fire barrier system or EJ greater than or equal to the fire rating of the floor assembly? | Yes/No/N/A | |
| 2) Does the floor construction and the exterior wall construction match the floor/wall construction listed in the perimeter fire barrier system or EJ? | Yes/No/N/A | |
| 3) Does the curtain wall construction comply with the following requirements of the referenced perimeter fire barrier system or EJ? | Yes/No/N/A | |
| a) Does the system include vision glass (if applicable)? | Yes/No/N/A | |
| b) Proper spacing of mullions and transoms? | Yes/No/N/A | |
| c) Proper mullion coverings (type, thickness, density, etc.) | Yes/No/N/A | |
| d) Proper curtain wall spandrel insulation (type, thickness, density, etc.) | Yes/No/N/A | |
| e) Proper spandrel panel perimeter angles or stiffeners (dimensions, gauge, fastener spacing, etc.) | Yes/No/N/A | |
| 4) Does the field installation of the firestopping materials comply with the following requirements of the referenced fire resistive joint system or EJ? | Yes/No/N/A | |
| a) Minimum and/or maximum width of the joint | Yes/No/N/A | |
| b) Proper backing material installed including proper orientation, depth and compression of backing material | Yes/No/N/A | |
| c) Proper firestopping product, including type, amount, thickness, orientation, etc. | Yes/No/N/A | |
| d) Proper accessories installed, including support clips for backing material. | Yes/No/N/A | |
| e) For firestop spray products: does the applied firestop spray overlap the adjacent surfaces properly? | Yes/No/N/A |
11.2 Example Comparison
11.2.1 Correct Installation
Figure 32: Hilti CP-672 sealant applied in an edge-of-slab assembly.
Figure 33: Correct sealant depth with Hilti CFS-SP SIL.
Figure 34: Comparison of Hilti CFS-EOS QuickSeal versus a traditional method.
11.2.2 Incorrect Installation
Figure 35: Cracked sealant coating.
12. HIGHLY RECOMMENDED GLOBAL BEST PRACTICES
Before selecting materials, contractors must prepare a site-specific Bill of Quantity (BOQ) detailing penetrants (size, material, type) and opening sizes with corresponding tested systems/EJs, submitting it to the consultant for approval.
Contractors should submit an undertaking letter confirming the BOQ's project specificity. Consultants approve suppliers based on site-specific BOQs with approved systems.
If a tested system is unavailable, check multiple suppliers before requesting an EJ. To minimize EJs, follow these steps:
EJs must include a justification letter with logic and references. Consultants or Houses of Expertise approve/reject EJs, potentially rejecting those without proper justification.
Final handover reports should include:
- Percentage/quantification of tested systems vs. EJs used.
- Comparison of initial BOQ quantities vs. final procured quantities.
- Progress inspections by qualified inspectors (consultant/HOE) at 20%, 40%, 60%, 80%, and 100% installation completion, with all reports included in the final handover.
13. PERFORMANCE OF CONSTRUCTION MATERIALS UNDER FIRE
13.1 Mineral Wool vs. Insulation
Mineral wool is a preferred backing material for firestop sealants in larger annular spaces because it is non-conductive, resists temperatures over 1,000°C, and offers better acoustic insulation than fiberglass. Materials like foam or fiberglass insulation are unsuitable as they burn easily. Proper orientation and compression of backing material are crucial during inspection.
For correct installation, Mineral Wool (MW) should be cut perpendicular to its fibers and assembled along the fibers to allow for compression.
13.2 Combustible and Non-Combustible Pipes
Combustible pipes (e.g., PVC) burn away, requiring intumescent materials (sealants, collars) that expand in fire to seal the opening. Non-combustible pipes (e.g., metal) melt; non-intumescent sealants are typically sufficient. Always consult tested systems or EJs for specific applications. Different plastic pipe types (PP, PPR) have varying compositions and fire performance.
13.3 Dependency on Tested Systems
- Material performance heavily relies on site system sealing matching tested systems.
- Check annular gaps between multiple penetrants; deviations from tested system requirements can affect fire rating.
- Systems tested for closed pipes cannot be used for open pipes, and vice-versa. Closed systems are capped/sealed on the unexposed side during testing; open pipes are unpressurized and open at one end.
- Avoid applying firestopping material on dampers, as it can impede function. Obtain damper manufacturer approval if unavoidable.
- Tested systems should specify firestopping requirements for retaining angles, which stiffen large ducts to prevent bending in fire.
