Fire Design with the Concrete-to-Concrete Module
The requirements for a structure’s fire resistance defined by the International Building Code (IBC) and the National Building Code of Canada (NBCC) are based on occupancy and classification. A building may be designed and constructed with fire-rated walls, floors, and ceilings for structural integrity as well as to prevent fire and smoke from spreading throughout the building. The requirements defined by model building codes are intended to allow the occupants to exit a structure safely and limit damage from the fire. Both above mentioned building codes have provisions for constructing safe fire barriers to limit fire spread and support safe egress in the event of a fire.
Currently, engineers can use ACI/TMS 216.1 Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies to examine how structural elements in concrete and masonry buildings will be affected when exposed to fire. ACI/TMS 216.1 is the standard describing acceptable methods for determining fire resistance of concrete and masonry building assemblies and structural elements including walls, floors, roofs, columns, etc. ACI/TMS 216.1 Section 4.3 includes provisions for utilizing concrete cover as a fire-resistant barrier for cast-in steel reinforcement.
ACI/TMS 216.1 Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies
Section 4.3 provides guidance to design steel reinforced concrete members for the fire resistance defined by the occupancy and classification of a structure. With respect to reinforced concrete design, Section 4.3 includes provisions to determine minimum concrete cover requirements for floors, roof slabs and beams. These cover requirements are derived from an hourly fire rating, which can range from 1 to 4 hours.
Limitations of ACI/TMS 216.1-14
ACI/TMS 216.1 provides provisions to determine fire ratings for steel reinforced structural assemblies. Engineers may observe that the data provided in ACI/TMS 216.1 reference tables are predicated on the assumption that there is a heat source on only one surface of the structural component. For example, the tables representing the ASTM E119 temperature distribution curves for a floor slab having a given concrete cover and fire rating assume the heat source only acts on one side of the slab.
Reinforced concrete slab heated on single surface with associated ACI/TMS 216.1 Figure 4.4.2.2.1a demonstrating temperature gradient in the slab during an ASTM E119 fire test.
ACI 216.1 includes fire rating provisions for cast-in steel reinforcing bars. There are no provisions within ACI 216.1 or other reference codes in the United States or Canada that consider the effects of fire on connections for post-installed reinforcing bars.
Updated Acceptance Criteria for post-installed reinforcing bar connections
In 2022, the International Code Council Evaluation Service (ICC-ES) updated the Acceptance Criteria for Post-installed Adhesive Anchors in Concrete Elements (AC308) with testing provisions for fire evaluation of post-installed reinforcing bars designed for development. These provisions can be used to design post-installed reinforcing bars for a specific fire rating. Hilti HIT-RE 500 V3 has been evaluated per these provisions, and the product evaluation report ESR-3814 has been updated with data to design post-installed reinforcing bars for development in conjunction with a specific fire rating. View HIT-RE 500 V3 Evaluation Service Report (ESR-3814).
Additionally, Hilti released a new product in 2023, HIT-FP 700 R, which is an injectable cementitious inorganic adhesive that offers a high resistance to heat in comparison to organic adhesives. Cementitious, inorganic adhesives are not included in the definition “adhesive” outlined by AC308, thus HIT-FP 700 R does not have an ICC-ES evaluation report (ESR). However, HIT-FP 700 R has been tested in accordance with the new AC308 fire provisions, and technical data is given in the Hilti publication titled Post-installed Reinforcing in Fire Conditions
Hilti fire tested adhesive anchor portfolio.
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Designing post-installed reinforcing bar connections for fire conditions using the PROFIS Engineering Concrete-to-Concrete module
Fire evaluation for post-installed reinforcing bar connections was introduced into the PROFIS Engineering Concrete-to-Concrete design module in 2023. Using this module, post-installed reinforcing bar connections can now be designed for a fire rating in conjunction with design per the development and splice provisions of ACI 318 or CSA A23.3.
Fire evaluation for post-installed rebar connections is available in the Extension and Structural Joint applications in the PROFIS Engineering Concrete-to-Concrete design module.
In lieu of the ACI 216.1 tables, which utilize ASTM E119 fire curves, the PROFIS Engineering Concrete-to-Concrete module utilizes heat transfer curves derived using finite element simulations. These simulations are more sophisticated compared to the ACI 216.1 tables, and map heat transfer through concrete that closely resembles actual fire conditions. For example, to assess the temperature field along the reinforcing bars in a concrete structure, a heat conduction equation was developed using a finite element code to model heat distribution on post-installed reinforcing bars for six applications.
Common post-installed reinforcing bar applications can be designed in the PROFIS Engineering Concrete-to-Concrete design module.
How heat sources are simulated in the PROFIS Engineering Concrete-to-Concrete Module
Multiple applications can be modeled for fire conditions within the PROFIS Engineering Concrete-to-Concrete module. Temperature distributions are simulated with a finite element program named COMOSOL, which uses a three-dimensional mesh. The following information explains how PROFIS Engineering utilizes these simulations for various applications within the Concrete-to-Concrete module.
Connection Type: Extension
Application: Slab-to-Slab, Wall-to-Wall
Heat Source: A heat source is modelled to act along the full length of the top and bottom of slab connection, or along the full length of the front and back of the wall connection. Since the full embedment length of the post-installed reinforcing bar is exposed to uniform heat distribution, the temperature along the length of the bar is assumed to be constant.
Connection Type: Extension
Application: Beam-to-Beam, Column-to-Column, Beam-to-Slab, Column-to-Wall
Heat Source: A heat source is modelled to act on each side of the column or beam connection along the full length each member. Since the full embedment length of the post-installed reinforcing bar is exposed to uniform heat distribution, the temperature along the length of the bar is assumed to be constant.
Connection Type: Extension
Application: Slab-to-Slab at Support / Beam-to-Beam at Support / Beam-to-Slab at Support
Heat Source: A heat source is modelled to act along the full length of the slab connection at the support, beam connection at the support, or beam-to-slab connection at the support. The heat source is applied to all four exposed sides of each member. Since the full embedment length of the post-installed reinforcing bar is exposed to uniform heat distribution, the temperature along the length of the bar is assumed to be constant.
Connection Type: Structural Joint
Application: Slab-to-wall, wall-to-slab
Heat Source: A heat source is modelled to act along the full length of the slab-to-wall connection, or along the full length of the wall-to-slab connection. The heat source is applied along the length of each member. The post-installed reinforcing bar is exposed to a temperature gradient along its embedded length which varies depending on the distance from the heat source acting at the exposed surfaces.
Connection Type: Structural Joint
Application: Beam-to-wall, column-to-slab
Heat Source: A heat source is modelled to act along the full length of the beam-to-wall connection, or along the full length of the column-to-wall connection. The heat source is applied along the length of each member. The post-installed reinforcing bar is exposed to a temperature gradient along its embedded length which varies depending on the distance from the heat source acting at the exposed surfaces.
Connection Type: Structural Joint
Application: Beam-to-column
Heat Source: A heat source is modelled to act along the full member length of the beam-to-column connection. The post-installed reinforcing bar is exposed to a temperature gradient along its embedded length which varies depending on the distance from the heat source acting at the exposed surfaces. The temperature along the embedded depth of the reinforcing bar in the middle of the column decreases since the concrete at this location is farther away from the heat source.
Summary
The finite element fire simulations in the PROFIS Engineering Concrete-to-Concrete module
can be used to model post-installed reinforcing bar connections to meet a particular fire rating. This functionality can be used in conjunction with the development and splice provisions of ACI 318 or CSA A23.3.
Resources:
Get started on a fire-rated post-installed rebar design with PROFIS Engineering
Learn more about Hilti’s fire tested adhesives in the Post-installation Reinforcing in Fire Conditions Technical Guide