Design of Glued-In Rods for Mass Timber Construction
The changing of the 2021 International Building Code (IBC) to 18 story mass timber buildings has opened a door to new possibilities, but it has also created an array of new problems for engineers to solve. Naturally, as the scale of a design increases, the complexity, as well as the service loading tends to rise. Mass timber is no exception. As with all buildings, forming complete load paths is vital to the safety of the structure and its occupants. The connections bridging members together should be chosen with care. Pioneering new mass timber design can be intimidating, especially when faced with design forces otherwise unencountered by traditional wood frame building methods. Fortunately, familiar technology may be applicable for consideration in forming new connections for these mass timber buildings.
Adhesives for adhering steel elements (i.e. threaded rod or deformed reinforcing bars) to concrete structures are nothing new to the building industry; Hilti has been developing and manufacturing them for decades. Products such as HILTI HIT-RE 500 V3 have traditionally been marketed solely as concrete chemical anchors. However, with the publication of Technical Report 070 (TR070) and ETA 19/0194 it is possible to use HILTI HIT-RE 500 V3 in glued-in rod (GIR) connections for mass timber construction.
TR070 is a glued-in rod design methodology published by the European Organization for Technical Assessment (EOTA). In order to design per this methodology, an adhesive must be tested in accordance with European Assessment Document for GIR for timber connections (EAD 130006-00-0304). Once the product is tested and evaluated, a European Technical Assessment (ETA) can be published. The ETA will describe the tests that were performed and provide the required design information, like the adhesive bond strength. TR070 establishes design rules for GIR designs utilizing Eurocode 5: Design of Timber Structures (EN1995) and the adhesive’s ETA.
As noted above, ETA 19/0194 and TR070 can be used to design glued-in rod (GIR) connections. Below is an example of a simple collector beam-to-column connection using a wide flange steel fixture and glued-in rods. This example utilizes the design methodology described in TR070 to calculate the tension and shear capacity of the glued-in rods. The project Engineer of Record (EOR) will need to verify the HIT-RE 500 V3 solution is applicable for its intended use. The calculation method below is only evaluating the loads on the HIT-RE 500 V3 adhesive and the EOR is responsible for the design of the steel fixture, beam and column. 
GIR Example Calculation per TR 070
Beam to Steel Fixture check
Given Material Strengths and Properties:
Check minimum allowable distances:
Rods are installed parallel to grain
TR070 Table 4.1 (minimum distances for rods axially loaded)
TR070 Table 4.2 (minimum distance for rods laterally loaded and glued-in parallel to grain)
Summary: Spacing and edge distances are all good.
Check minimum bond length (TR070 Eq. 4.4)
Given bond length = 8”
Check axial load capacity (TR070 Section 4.1.4):
Failure of the steel rod: 
Failure of bond line:
Service Class 1 (EN 1995, for service class 1, the average moisture content of most softwoods will not exceed 12%)
Short term action (EN 1995, Load-duration class, wind loading)
Failure of wood adherent:
Service Class 1 (EN 1995, for service class 1, the average moisture content of most softwoods will not exceed 12%)
Short term action (EN 1995, Load-duration class, wind loading)
Check block shear failure (TR070 Sec. 4.1.7 & EN 1995 Annex A):
Service Class 1 (EN 1995, for service class 1, the average moisture content of most softwoods will not exceed 12%)
Short term action (EN 1995, Load-duration class, wind loading)
Summary:
Failure of steel rod = 16.4 kips > 7.5 kips
Failure of bond line = 21.7 kips > 7.5 kips
Failure of wood adherent = 23.4 kips > 7.5 kips
Block shear failure = 57.3 kips > 7.5 kips
Design is good for tension
Check lateral load capacity (EN 1995.1.1, Section 8.2 & 8.5):
Flange of the fixture is greater than or equal to d, d = 0.5” à thick steel plate in single shear
Shear planes = 2
Fasteners per shear plane = 2
Shear capacity = 2.6 kips > 2 kips à Good.
Check combined loading:
Assumed tension load = 7.5 kips
Assumed shear load = 2 kips
Tension capacity = 16.4 kips
Shear capacity = 2.6 kips
Good for combined loading
Column to Steel Fixture check
Given Material Strengths and Properties:
Check minimum allowable distances:
Rods are installed perpendicular to grain
TR070 Table 4.1 (minimum distances for rods axially loaded)
TR070 Table 4.2 (minimum distance for rods laterally loaded and glued-in parallel to grain)
Summary: Spacing and edge distances are all good.
Check minimum bond length (TR070 Eq. 4.4):
Given bond length = 9.5”
Check axial load capacity (TR070 Section 4.1.4):
Failure of the steel rod: 
Failure of bond line:
Service Class 1 (EN 1995, for service class 1, the average moisture content of most softwoods will not exceed 12%)
Short term action (EN 1995, Load-duration class, wind loading)
Failure of wood adherent:
Service Class 1 (EN 1995, for service class 1, the average moisture content of most softwoods will not exceed 12%)
Short term action (EN 1995, Load-duration class, wind loading)
Check block shear failure (TR070 Sec. 4.1.7 & EN 1995 Annex A):
Service Class 1 (EN 1995, for service class 1, the average moisture content of most softwoods will not exceed 12%)
Short term action (EN 1995, Load-duration class, wind loading)
Summary:
Failure of steel rod = 16.4 kips > 7.5 kips
Failure of bond line = 21.7 kips > 7.5 kips
Failure of wood adherent = 23.4 kips > 7.5 kips
Block shear failure = 47.3 kips > 7.5 kips
Design is good for tension
Check limit for axial load perpendicular to the grain (TR 070 Section 4.1.6):
It is the opinion of Hilti that axial load perpendicular to the grain as indicated in TR070 Section 4.1.6 may lead to a splitting failure when the ratio of connector embedment depth, la, to member thickness, h, is less than 0.7. If the ratio of la / h < 0.7 then an additional verification of the timber base material to prevent splitting must be performed.
Check lateral load capacity (EN 1995.1.1, Section 8.2 & 8.5):
Shear capacity = 7.8 kips > 2 kips à Good.
Check combined loading:
Assumed tension load = 7.5 kips
Assumed shear load = 2 kips
Tension capacity = 16.4 kips
Shear capacity = 6.8 kips
Good for combined loading
Glued in rods are not currently cited in the National Design Specification (NDS) for wood. Naturally, their absence may create some nervousness in their use for anything other than simple ornamental attachments rather than sturdy structural connection with higher withdrawal capacity. This article hopes to shed some light on an otherwise little-known topic. Using TR 070, ETA 19/0194 and EN 1995, where applicable, it is possible to bring glued-in rods into mass timber construction.
The documents referred to in this article (i.e. EAD 130006-00-0304, ETA 19/0194 & TR070) were not produced by Hilti North America, and no testing or evaluation has been performed by Hilti North America to determine their accuracy or suitability for use according to North American codes and design practices. The design methodologies presented are for informational purposes only – it is the responsibility of the Engineer of Record (EOR) to decide whether the concepts and methodologies are appropriate for their intended use.
For further reading on glued-in rods, refer to the articles linked below:
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