Fire safe rigid concrete to concrete joints, the Hilti method.

Introduction
Fire disasters happen frequently everywhere in the world, and fires are an unfortunate reality that can occur on all types of structures, thus being a significant risk to lives and assets that might lead to grievous casualties.

During the event of a fire, structures can typically resist the heat for some time, but if the fire duration extends long enough, it impacts the stability of concrete structures with the eventual consequence of complete or partial collapse. Hence fire safety is of critical importance and an essential requirement that all structural building codes globally prescribe through mandatory fire resistance durations for structural elements and systems, including their connections. This helps ensure enough time for occupants to evacuate, for rescue activities to be completed, and for the fire to be safely extinguished.

Building codes generally provide guidance to design traditional reinforced concrete systems. Unfortunately, most codes do not provide sufficient guidance to ensure fire safety when using non-traditional solutions, such as post-installed rebar systems. In these cases, it is the responsibility of the structural engineer to ensure building structures meet the code-mandated fire resistance to meet the mandated requirements through expert considerations and engagement with specialist solution providers.

Post-installed reinforcing bars (rebars) are an effective solution on most job sites to connect concrete members cast at different times. This solution is nowadays widespread in both existing constructions, e.g., for strengthening or refurbishment needs as well as in new buildings, e.g., to optimize the construction process or correct design/construction mistakes. These structural connections shall be designed to resist various types of loading including static loading, seismic actions, and fire exposure. However, the design of rigid joints with post-installed rebars under fire exposure is not covered in state-of-the-art codes and guidelines. Because of this, Hilti being a leader in the industry toward safer buildings, provides solutions by developing hardware, design methods, and design software that help ensure a fire-safe built environment.

Background
Post-installed rebar connections in concrete can be used to extend a structural element using lap splices (e.g., slab extension, wall extension, etc.) and in some cases to ‘simply support’ an element onto another (e.g., slab onto a wall). However, end anchorages using post-installed rebars are often required to enable rigid connections of one structural member to another which are usually perpendicular in direction to each other (e.g., column/wall arising from foundations, a beam onto a column). The scope of such post-installed rigid moment-resisting joint applications is limited by the design method following provisions of the Eurocode EN 1992-1-1, where such rigid joints are usually designed and executed as lap splicing. This is not always a feasible solution since in many cases the members are already constructed, and the new members are to be post-installed.

EOTA has developed a post-installed reinforcement (rebar) design technical report (TR) 069 where the structural elements that are experiencing bending moments can be designed without the need for a splice configuration in the existing member. The design method extends the design range of applications which now includes end-anchorage connections for rigid joints in concrete structures.


EOTA TR 069 considers possible different modes of failure such as steel yielding, concrete cone failure, resistance to pull-out, and splitting failure, which is a holistic and straightforward approach towards verifications based on anchor theory (which takes into account the tensile resistance of concrete in the breakout verification) without the need for the assumption of rebars being anchored only in confined zones. Furthermore, the product-dependent bond strength of the specific post-installed rebar system is considered via assessment according to the EAD 332402, which is much higher than Eurocode limitations.

Limitations in fire design for rigid nodes
In the European code and design framework, the splice connections and anchorages in simply supported connections can be designed for fire following the provisions of EN 1992-1-2 and EN 1992-1-1. While EOTA TR 069 allows for the design of post-installed, moment-resisting rigid joints in reinforced concrete connections under static loading and seismic loading conditions, unfortunately, it does not include design provisions when exposed to fire action. This limitation of the design method has been a pain point for engineers in encompassing all the design actions while providing a complete solution.


Solution for fire safe design of rigid joints – Hilti method
To bridge the gap of non-availability of fire design of rigid joints using EOTA TR 069 investigations on the bond-splitting behavior of post-installed rebars at elevated temperatures were conducted (see test setup and specimen images below). As a result, the new Hilti method for fire design of rigid joints was formulated as a smart design approach that utilizes the design strength verification equations for failure modes from EOTA TR 069 with few meticulous modifications. A key parameter is the reduced bond strength in terms of the reduction factor expressed as 𝒌𝒇𝒊,𝒑 (θ), of the chemical mortar under fire exposure from the relevant ETA published as per EOTA EAD 330499-02-0601 which is used for bonded anchors.

Design flow
For the fire design of rigid nodes using this design method, the following steps are applied:

Design procedure
Partial factors for material strength and actions
The partial safety factors for materials as per EOTA TR 069 (for static) and as per Hilti method
(for fire), and partial factors for load actions are shown in the following table. They follow the
principles of EN 1992-1-1 and EN 1992-1-2 for static and fire loading cases, respectively.


Note: under fire exposure, the material resistances decrease, but due to the lower increasing factors on the loads and partial safety factors for material resistances equal to unity, this design case is not always decisive.

Failure modes
For a given anchorage length, the lowest design resistance among the following three failure modes must be determined. The anchorage length allowing the required design resistance for the two loading cases static and fire, will be the final design length.


Hilti’s qualified products for rigid joints design according to EOTA TR 069 can be used with this new design method as well



Quick Fire design of rigid joints using PROFIS ENGINEERING software
Designing several end-anchorage applications as per EOTA TR 069 for static load, seismic load, and Hilti method for fire-safe rigid joints, given its iterative nature, can be very time-consuming. Hilti’s cloud-based design software PROFIS Engineering helps designers to quickly create code-compliant designs, considering the static and fire load combinations in one design to ensure a safer and more efficient workflow. PROFIS Engineering helps in choosing suitable solutions using qualified products (adhesive mortar systems) for fire exposure and other installation parameters. Below is a fire design example of a rigid joint (wall to slab) using PROFIS Engineering:



Note: This design example is not representative of all design cases. Every connection design will differ depending on geometry, fire exposure conditions, duration, material properties, etc. that can be designed using the single tool PROFIS Engineering

Design fire safe rigid joints with Hilti method using PROFIS ENGINEERING Stay on the safe side.

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This article is part of an ongoing series dedicated to Rebar topics. Find more here