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By: Daniel Brandon, RISE
This article summarizes the work on engineering methods for structural assessment of Cross Laminated Timber (CLT) members with design fires. These methods involve the prevention of phenomena leading to sudden exposure of protected timber material to the fire, such as gypsum failure and delamination of CLT. For full report see [1]. This work is part of a larger project with the goal to quantify the contribution of Cross Laminated Timber (CLT) building elements (wall and/or floor‐ceiling assemblies) in compartment fires. [2]
A previous gap analysis [3] identified the need to evaluate the contribution of mass timber elements to room/compartment fires with the types of structural systems that are expected to be found in tall buildings (e.g. CLT, etc.). Subsequent research has shown that timber elements contribute to the fuel load in buildings, can increase the initial fire growth rate and possibly lead to sustained fires that do not burn out, because of failure of the base layer of gypsum boards, delamination of CLT lamellas or due to an excess of unprotected timber.
If it cannot be assumed that the fire brigade or sprinkler activation will suppress a fire, it may be necessary to design for burn-out without successful fire suppression. For this, engineering methods to limit the impact of gypsum failure, delamination and an excess of exposed timber are needed. Additionally, a method for structural design for CLT structures considering natural fires is needed to avoid collapse during the full duration of a fire. Engineering methods for predicting the structural damage and assessing gypsum board fall-off are presented in the full report [1]. Both methods were based on parametric fires, which were modified to take the contribution of timber to the fire into account. The use of parametric fires is only suitable if the compartment dimensions are limited (with floor areas up to 500m2 according to EN1991-1-2, 2002) and if delamination of CLT and failure of the gypsum protection is avoided. A possible method to prevent delamination involves the use of non-delaminating adhesives, which can be identified with furnace fire tests according to [4].
As can be seen from Figure 1 the method results in conservative predictions of the char depth if delamination and gypsum fall-off are avoided.
The method proposed to predict gypsum fall-off successfully predicted the fall-off of the exposed layer of gypsum from the ceiling of all compartments of the analysis as shown in Figure 2. It should be noted that this was only validated with tests in which more than one layer of fire protective gypsum board was applied. Concerning prediction of fall-off of the second layer, the method correctly predicted gypsum board fall-off for compartments with medium to high opening factors. Predictions of fall-off for compartments with opening factors equal to or lower than 0.04m1/2 were more conservative.
A method to avoid heat delamination of CLT involves the use of non-delaminating adhesives. Some of these adhesives were identified by Janssens [5] and Brandon and Dagenais [4].
It should be noted that delamination and fall-off of the base layer of gypsum boards do not necessarily lead to a secondary flashover in the late stages of a fire. However, the model presented in this work is not equipped to consider the effects of delamination or fall-off of the base layer. The approach taken is conservative as it does not allow delamination and fall-off of the base layer during the whole fire, but may be extended to consider the effects of delamination or fall-off of the base layer at a late stage of the fire in future studies.
References
- [1] Brandon D., “Fire Safety Challenges of Tall Wood Buildings – Phase 2: Task 4 – Engineering Method”, Report prepared for the Fire Protection Research Foundation, 2018
- [2] Fire Protection Research Foundation, “Fire Safety Challenges of Tall Wood Buildings”, https://www.nfpa.org/tallwood
- [3] Gerard R., Barber D., Wolski A. (2013) “Fire safety challenges of tall wood buildings”. Report prepared for the Fire Protection Research Foundation, 2013
- [4] Brandon D., Dagenais C., “Fire Safety Challenges of Tall Wood Buildings – Phase 2: Task 5 – Experimental Study of Delamination of Cross Laminated Timber (CLT) in Fire”, Report prepared for the Fire Protection Research Foundation, 2018
- [5] Janssens M., “Development of a fire performance assessment methodology for qualifying cross-laminated timber adhesives”, South West Research Institute, 2017