NO20200247A1

NO20200247A1 - Fire retardant CLT element

Info

Publication number: NO20200247A1
Application number: NO20200247A
Authority: NO
Inventors: Geir Jensen; Alexander Jensen
Original assignee: Fss Tre As
Priority date: 2019-12-11
Filing date: 2020-02-28
Publication date: 2021-06-14
Also published as: NO20191468A1; NO345746B1; EP4072854A1; NO345852B1; WO2021116305A1

Description

The present invention is related to a self-extinguishing timber structure comprising one or more self-extinguishing cross laminated timber (CLT) elements. More specific, the present invention is related to a self-extinguishing cross laminated timber (CLT) element, each CLT element comprising a plurality of lamellae (plural form lamellae used throughout; same meaning as lamellas) and each lamella comprises one or more boards. The lamella and boards are arranged such that during a fire exposure in an enclosure or exterior, the at least one of the plurality of lamellae chars seamlessly until self-extinction occur.

BACKGROUND

A wall element of solid timber boards with a thickness exceeding e.g. 50 mm when subjected to a room fire, may self-extinguish in the decay phase after room content is burnt away. This is due to a char layer which is always protecting fresh wood behind it and is well documented. Solid homogenous wall elements of e.g. thicknesses 50-200 mm are not available today due to high cost, less practical to manufacture without warping and susceptibility of developing cracks in use. The most common timber wall elements used today are cross-laminated ones (CLT).

Cross-laminated timber (CLT) is a wood panel product made from gluing layers of solid-sawn lumber together. Each layer of boards is usually oriented perpendicular to adjacent layers and bonded to the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. Regular timber is an anisotropic material, meaning that the physical properties change depending on the direction at which the force is applied. By gluing layers of wood at right angles, the panel is able to achieve better structural rigidity in both directions. It is similar to plywood but with distinctively thicker laminations (or lamellae).

Traditional cross laminated timber (CLT) elements, when not protected by gypsum or calcium silicate, cementitious boards or fire protective paint (i.e. on wood surfaces facing the room) are practical up to a certain point only. Further development of the fire after such protective layers delaminate becomes less predictable in terms of self-extinction (see below).

All CLT elements have layers of wood (lamellae), typically glued together. During a fire, as the protective layer or some areas of the first timber lamella is burnt through the remaining of the first lamella may be abruptly detach from the wood element and drop down simultaneously. Because the room fire is uniform at his stage, this detachment will occur about the same time for all first lamella of the CLT wall elements in the room (ceiling CLT elements may delaminate a bit earlier).

This simultaneous separation of the protective layers or the first timber lamellae will expose uncharred fresh surface of the next lamella layers of the CLT element, which will cause the fire heat release rate (HRR) to regain substantially by radiation and reradiation. The HRR is the rate of heat generated by fire. It is a measure of the heat flux that is available in every square meter of surface absorbing heat within a particular surface. The HRR may be measured in Joules per second or Watts.

By regaining the HRR, the room temperature may increase so reignition and secondary flashover occur. As a result, flame impinging on facade out from window increases as well. Inside the burning room, the fire may cause increased charring rate each time a CLT lamella delaminates. The fire eventually does not decay and do not self-extinguish. Full burn-through of a CLT element is a critical fire compartmentation failure which often leads to total loss and pose risk of loadbearing collapse.

Full scale fire tests have shown that due to the unwanted delamination of the CLT element, at least two walls and ceiling of a room need to be protected by several layers of gypsum or other protection rated for e.g. 60 min fire resistance to ensure self-extinction in case of fire.

If delamination does not occur abrupt and simultaneous, the number of exposed wood walls in a room may be increased to three or all four, depending on type of ceiling and ventilation openings of room on fire. This is what the market and architects are craving for and yet out of reach.

However, it is neither the mean HRR nor the total energy of the CLT elements that prevent fire extinction or cause secondary flashovers. It is the post-flashover peaks of HRR that define the difference of solid timber and CLT. The peaks of HRR are caused by abrupt delamination of large areas. Without the HRR peaks, or if peaks are evened out and reduced sufficiently, the CLT will self-extinguish similar to solid timber.

Furthermore, tests have shown that the effect of current CLT is delicate: even a small reduction in delaminated area can reduce the HRR peak sufficiently for the room fire to self-extinguish.

Currently, the best fire-resistant glues to effectively mitigate the delamination effect are rarely applied due to secondary disadvantages. Those products have several disadvantages such as: harmful to environment, extended hardening times, application difficulties and/or increased cost. Some have proposed to use solid timber instead of or as first lamellae of CLT or using high density wood specimens.

Neither is viable however, as the benefits of stiffness, accessibility, ease of CLT production and low cost then may be lost. Thicker boards will reduce stability, cause warping, deflection or twist and are prone to develop large or small cracks soon after installation.

A strong incentive for using CLT in buildings is to allow the wooden surfaces to be visually exposed.

In view of the above, one object of the present invention is to provide a wood structure or a wood element that resolve or at least mitigate one or more of the above-mentioned problems related to fire.

Another object of the present invention is to provide a wood structure or element that in case of a fire will have increased probability of self-extinction by application of single boards of readily available dimensions.

SUMMARY OF THE INVENTION

The present invention is related to a self-extinguishing cross laminated timber (CLT) element comprising a plurality of lamellae arranged to form the CLT element, each lamella comprises one or more boards. The boards are arranged side by side, to form one lamella.

At least one lamella of the plurality of lamellae comprises a plurality of boards, each board having a rectangular shape cross-section with a short side (B) and a long side (A).

Each board having its wide face(s) with the long side (A) bonded to the wide face of adjacent board such that during an enclosure fire the plurality of boards of the at least one lamella char seamlessly across a thickness (L) of the at least one lamella, and where the thickness (L) equals a length of the long side (A).

Thus, the at least one lamella may comprise a plurality or boards bonded together along the long side (A) of each board.

The board is arranged with its short side (B) facing the long side (A) of boards of adjacent lamella.

The terms “timber” and “wood” are used interchangeably throughout the application.

The term board is referred to a flat, rectangular piece of wood, with parallel wide faces defined by long side (A) and the length of the board. Seen in a cross section the board has a rectangular shape, with a short side (B) and a long side (A). The board may have different lengths and may comprise one or more board elements.

Each board may be constructed with short board lengths having their wide face bound to the wide face of adjacent board. The term cross section is in this application referred to a longitudinal cross section of the board, wherein the cross section of the board has a long side (A) with a length which is greater than the short side (B).

The term wide face denotes side area of a board having the largest area. The area is thus defined by a longitudinal length of the board and the long side (A).

According to the invention, the long side (A) of a board is facing, mating and/or bonded to a long side (A) of adjacent board of the at least one lamella.

The plurality of boards of the at least one lamella may have the same orientation.

The short side (B) of each board may be arranged facing the long side (A) of adjacent lamella. Thus, the short side (B) may be bonded to the long side (A) of adjacent lamella, such that the boards of the adjacent lamella are arranged with its wide face bonded to the at least one lamella.

The long side (A) may be at least 40 mm, or the long side (A) may be at least 50 mm. Thickness of the at least one lamella may need to exceed 40 mm to ensure selfextinction without delamination for the most common periods of fire endurance and safety margins.

The plurality of boards of the at least one lamella may be oriented perpendicular to adjacent lamella.

The at least one lamella may be an outer lamella of the CLT element, or the at least one lamella may be arranged below an outer lamella.

The plurality of lamellae of the CLT element may be arranged symmetric so that the outer lamellae of the CLT element have the same orientation.

The term self-extinguishing cross laminated wood element may be a CLT element or massive wood element made such that during a fire exposure the at least one lamella of perpendicular boards chars seamlessly through the long side (A) (edge to edge) of its boards.

The present invention provides a self-extinguishing cross laminated timber (CLT) element by avoiding that the lamellae delaminate and thereby expose the fresh wood of lamellae behind and causing the heat release rate (HRR) to increase. By use of an outer lamella of wide boards assembled perpendicular to the plane of CLT element, the char layer of the present invention will be attached to the CLT element for a longer time due to longer time of burn through, thus protecting the fresh wood layer behind.

Since the lamella of the present invention may be e.g. 2 or 3 times thicker than practical boards of common CLT elements, fixed or glued on their wide face to adjacent boards, no delamination of layers (lamellae) will occur for most room fire loads and the CLT element may respond to fire similar to solid timber. The protective char layer ensure self-extinguishing and prevent secondary flashovers.

By applying a lamella of perpendicular boards to the plane of CLT element, i.e. their edges fixed to flat cross-laid lamellae instead of wide face to wide face, on both outermost sides of the CLT element the present invention makes the element universal to be applied in any orientation. Said lamella may be located between lamellae or on one side only of the CLT element. CLT element may have odd or even number of lamellae.

The at least one lamella of the present invention may be an outer lamella of a CLT element. Thus, the at least one lamella provides a seamless charring through the thick outer lamellae of CLT element, while allowing crack-free and smooth surfaces by use of readily available boards.

A major function of the present invention is to enable self-extinction, prevent regrowth of fire, prevent secondary flashovers and prevent increased fire exposure to façades by using regular board dimensions and not requiring types of glue that are harmful to environment, require long curing times, are expensive or less practical.

The dimension of boards of the at least one lamella, the long side (A) of boards which constitutes the thickness of the lamella (L) and thus the seamless travel of charring are preferably calculated by adding burnout time of inventory of room or fuel package at facades to the applicable decay time and then multiply the sum (minutes) with the specific charring rate of timber used. E.g. lamella thickness (A in figure 3a) must equal or exceed "40+30 min times 0.8 mm/minute" which is 56 mm.

This is an exemplified method of calculation. Other methods of calculating seamless travel of charring may also apply.

The invention is also related to a self-extinguishing timber structure, wherein the timber structure comprising one or more cross laminated timber (CLT) element(s) as described above. The timber elements may be arranged side by side to form a wall and/or ceiling of a room in a building. Each timber element may be arranged with the at least one lamella according to the present invention facing into the room such that during an exposure to fire the plurality of boards of the at least one lamella char seamlessly across a thickness of the at least one lamella. Thus, with increased probability of self-extinction of the timber structure.

Further objects, structural embodiments and advantages of the present invention will be seen clearly from the following detailed description, the attached drawings and the claims below.

FIGURES

The invention will now be described with reference to the attached figures, wherein:

Figure 1 shows a prior art CLT element comprising five layers (lamellae).

Figure 2 shows a self-extinguishing cross laminated timber (CLT) element according one embodiment of the present invention.

Figure 3a shows self-extinguishing cross laminated timber (CLT) element according another embodiment of the present invention.

Figure 3b shows a cross section with dimensions for reference in description.

Figure 4 shows graphically the HRR rate over time during a room fire, the first in a room of solid wood panels and the second in a room of CLT panels where lamellae delaminated.

Figure 5 shows graphically the Mass Loss Rate (g/m2’*s) over time, during a room fire in a room with respective solid wood panels CLT panels. ;Figure 6 shows graphically the Mass Loss Rate (g/m2’*s) over time of figure 5, where the Mass Loss Rate of the present invention is indicated to coincide with test results of solid timber.

DETAILED DESCRIPTION OF THE FIGURES

In the following, embodiments of the invention will be discussed in more detail with reference to the appended drawings. It should be understood, however, that the drawings are not intended to limit the invention to the subject-matter depicted in the drawings.

Figure 1 shows a prior art cross-laminated timber (CLT) element 10. The CLT element 10 is a wood panel product made from layers (lamellae) 11 of solid-sawn lumber joined together. Each lamella 11 comprises boards 12 crossing adjacent lamella 11 and bonded on the wide faces of each board 12. The boards 12 of each lamella 11 are bonded side by side providing a bond line 18 between the boards. Each board 12 has a rectangular cross section with a short side (B) and long side (A).

The lamellae 11 are further arranged symmetric such that outer layers (top and bottom in figure 1) have the same orientation. An odd number of lamellae (such as 3, 5 or 7 layers) are most common, but there are configurations with even numbers as well. The figure 1 shows CLT element having 5 layers or lamellae 11. The outer lamellae are arranged symmetric about the center lamella, and all lamellae have same thickness.

The figure 2 shows an embodiment of the present invention where the CLT is a 5-ply symmetric element with the outermost lamellae in the same orientation. The at least one lamella 11 according to the present invention is arranged at the outermost layers (top and bottom in figure 2).

Each board 12 of the at least one lamella 11 has a rectangular cross section with a long side A facing and bonded to long side A of adjacent board. The mating long side A are bonded along bonding line 18. Thus, each board of the at least one lamella 11 has its wide face(s) bonded to the wide face of adjacent board (12) such that during exposure to fire, the plurality of boards 12 of the at least one lamella 11 char seamlessly across a thickness L of the at least one lamella 11, and where the thickness L equals a length of the long side A.

The figure 3A shows an embodiment of the present invention where the CLT is a 6-ply unsymmetric element with the outermost layers in opposite orientations according to the present invention. This is to strengthen loadbearing of a horizontal deck element or to prevent warping of wall elements at applications which require it.

The figure 3B show a cross section with signature dimensions for reference. The dimensions of perpendicular boards of the at least one lamella 11, the long side A of boards which constitutes the thickness L of the lamella 11 and thus the seamless travel of charring may be calculated by adding burnout time of inventory of room or fuel package at facades plus the applicable decay time and then multiply the sum (minutes) with the specific charring rate of timber used. E.g. lamella thickness (A in figure 2) must equal or exceed "25+30 min times 0.8 mm/minute" which is 44 mm. Preferred max thickness is 90-120 mm to limit overall thickness of CLT elements.

During an enclosure fire the outermost lamella chars seamlessly, meaning that the charring takes place at a fixed rate across lamella. The glue line 18 is never exposed because fire is out by then, if outermost lamella thickness is correctly dimensioned by calculation taking into account the fire load, duration, room geometry, ventilation and safety margin.

Figure 4 is from publication Wiesner: "Structural capacity in fire of laminated timber elements in compartments with exposed timber surfaces", Engineering Structures Publication 179. 2019, following graphs demonstrates the HRR over time for a wall element.

The first graph a) Beta1, in Figure 4 shows a graph of the Heat Release Rate (HRR) over time (duration) for a wall element of solid wood during a room fire. The HRR is a rate of heat produced by the fire and is available to all exposed surfaces. The accumulated heat exposing wood surfaces will ignite them at certain level of heat or radiation. HRR are measured in Joules per second or Watts (Megawatt).

The graph shows that the HRR increases rapidly in a room fire when the outer surface of the solid wood element gets involved. The HRR increases and has a peak of about 5 Megawatt at 10-20 min., during which a char layer has been built on the outer surface of the solid wood element. Due to the char layer of the wood which is always protecting the fresh wood behind, the HRR rate will drastically drop after approximately 20 min, when all the contents in the room is burnt away. The added heat to the solid wood surface will decrease and the HRR rate will drop drastically, causing the fire to self-extinguish in the decay phase.

The graph b) Beta2, of figure 4 shows a graph of the Heat Release Rate (HRR) over time (duration) for a wall element of CLT panel during a room fire. The CLT panel which comprises a plurality of layers, will have the same fire development as for the solid wood in the decay phase. The HRR increases quickly to a peak of about 5 MW in about 10-15 min. Due to the development of the char layer (first layer), the HRR drops when all the contents in the room is burnt away at about 15 min. When the first layer is burnt through the remaining of the first layer may be abruptly detach from the CLT panel and drop down simultaneously. Because the room fire is uniform at his stage, this detachment will occur about the same time for all first layers of the CLT panels in the room.

This is shown in greyscale (“char fall-off) in the graph b) Beta2. The delamination of the first layers will cause the fire HRR to regain substantially by radiation and reradiation. By regaining the HRR, the room temperature may increase, reignition and secondary flashover may occur. As a result, flame impinging on facade out from window increases as well. Inside the burning room, the fire may cause increased charring rate as layer by layer of the CLT elements delaminates as shown in the graph. The fire eventually does not decay and do not self-extinguish. Manual fire extinction must be performed in order to extinguish the fire. Final burn-through of a CLT panel is a critical fire compartmentation failure which often leads to total loss and pose risk of loadbearing collapse.

As disclosed above, it is neither the mean HRR nor the total energy of the CLT panels that prevent fire extinction or cause secondary flashovers. It is the peaks of HRR that define the difference of solid timber and CLT without adequate glue. The peaks of HRR are caused by abrupt delamination of large areas. Without the HRR peaks the CLT will self-extinguish similar to solid timber. Furthermore, tests have shown that the effect of current CLT is delicate: even a small reduction in delaminated area can reduce the HRR peak sufficiently for the room fire to self-extinguish.

Figure 5 and 6 are taken from the publication: "Analysis of cross-laminated timber charring rates upon exposure to non-standard heating conditions”, by Bartlett, University of Edinburgh.

The dotted line in figure 5 shows a traditional CLT panel with a plurality of layers, where the MLR decreases after a char layer has been built up on the outer surface of the CLT panel. Due to the delamination of the outer layer, the MLR increase again when the reignition and flashover starts on the fresh wood behind the outer layer, causing the MLR to increase until a new char layer has been established. The dotted line in figure 5 shows the time when the delamination occurs, resulting in an increase in MLR just after the delamination.

Figure 6 shows the same graphs of figure 5, but with the graph of a CLT panel according to the present invention included. This graph in thick solid line quali tatively illustrates the effect of the invention by the embodiment shown by figure 3a, 3b and 3c. Due to the seamless charring of the outer layer the peaks of the MLR are prevented and there is no delamination, when layer thickness is correctly designed as described.

A = width of board on edge, A>B

B = thickness of board on edge, B<A

C = thickness L of lamella

D = thickness L of lamella crossing C

E = total thickness of 3-ply lamellae

F = thickness of symmetric (5-ply) CLT element

G = total thickness of 4-ply lamellae

H = thickness of unsymmetrical (6-ply) CLT element

Claims

1. A self-extinguishing cross laminated timber (CLT) element (10) comprising a plurality of lamellae (11) arranged to form the CLT element (10), each lamella (11) comprises a plurality of boards (12) arranged side by side to form one lamella (11), wherein at least one lamella (11) of the plurality of lamellae (11) comprises a plurality of boards (12), each board having a rectangular shape cross-section with a short side (B) and a long side (A),

wherein each board (12) having its wide face(s) with the long side (A) bonded to the wide face (13) of adjacent board (12) such that during exposure to fire the plurality of boards (12) of the at least one lamella (11) char seamlessly across a thickness (L) of the at least one lamella (11), and where the thickness (L) equals a length of the long side (A).

2. The self-extinguishing CLT element (10) according to claim 1, wherein the long side (A) is at least 40 mm.

3. The self-extinguishing CLT element (10) according to claim 2, wherein the long side (A) is at least 50 mm.

4. The self-extinguishing CLT element (10) according to any one of the preceding claims, wherein the short side (B) is bonded to the long side (A) of adjacent lamella (11) such that the boards of the adjacent lamella (11) are arranged with its wide face bonded to the at least one lamella (11).

5. The self-extinguishing CLT element (10) according to any one of the preceding claims, wherein the plurality of boards (12) of the at least one lamella (11) is oriented perpendicular to adjacent lamella (11).

6. The self-extinguishing CLT element (10) according to any one of the preceding claims, wherein the at least one lamella (11) is an outer lamella of the CLT element (10).

7. The self-extinguishing CLT element (10) according to claim 7, wherein the plurality of lamellae (11) of the CLT element is arranged symmetric so that the outer lamellae (11) of the CLT element (10) have the same orientation.

8. A self-extinguishing timber structure, wherein the timber structure comprising one or more cross laminated timber (CLT) element(s) according to any one of claims 1-7.