Fire Resistance Ratings of Concrete and Masonry Structural Elements
Fire ratings of concrete and masonry structural elements considers two factors: fire endurance and fire resistance. ACI 206 provides the definition of these two terms.
Fire endurance is a measure of elapsed time during which a material or assembly shows fire resistance under determined conditions of test and performance when applied to structural elements. It should be measured by methods and to the criteria specified in standard test method for fire tests of building construction and materials (ASTM E 119).
As far as fire resistance is concerned, it is a material or assembly property to resist fire or offer protection against it when applied to the structural elements. it is characterized by the capacity to contain fire or to continue to perform a specific structural function or both.
Generally, Building Codes provide minimum needed fire resistance for members of buildings depend on the occupancy type of the structure, function of the element, the significance of the building, the contents of the building, and other fire protection considerations.
This article provides criteria of fire resistant ratings according to the standard method for determining fire resistance of concrete and masonry construction assemblies (ACI 216.1M-07/ TMS-216-07).
Fig.1: Fire Resistant Masonry Wall
Methods to Determine Fire Resistance Rating of Concrete and Masonry
As per ACI 216.1-M07, the fire resistance of concrete and masonry materials and assemblies should be estimated using one of the methods provided in the following section:
Qualification by Testing
Materials and assemblies of construction materials tested as per requirements provided in ASTM E 119, and based on results and conditions provided in such test fire ratings are determined.
Computed fire resistance
Fire resistance of elements or assemblies of concrete of masonry obtained by using procedure provided by ACI 216.1M-07 or other alternative procedures shall be deemed acceptable.
Approval through past performance
ACI 216.1M-07 does not rule out the utilization of fire ratings of elements and assemblies which have been used previously and have been proven through performance.
ACI 216.1M-07 does not prevent the application of novel and emerging techniques to anticipate the life safety and property protection implications buildings and structures.
Fire Testing of Wall Assembly
Fire testing of wall assembly based on ASTM E 119 provides four performance criteria that need to be met:
- Resistance of wall assembly against heat transmission through it.
- Resistance to the travel of flame or hot gasses through the wall adequate to start fire and burn cotton waste on the other side of the wall which is not exposed to fire.
- Decrease load carry capacity of load bearing wall
- Resistance to the impact, erosion, and cooling effects of a hose stream on the assembly after exposure to fire.
Fire Resistance of Different Concrete and Masonry Elements
The resistance of masonry and concrete members against fire is controlled by the transfer of head through the assembly which is determined by the increase of temperature on the other side of the wall that is not subjected to fire.
This mode of failure is used to formulate standardized procedure to compute fire resistance of deferent concrete and masonry elements.
Standard calculation procedure for different masonry and concrete members are provided in the following sections:
- Heat transmission in slabs
- Fire resistance rating of single Wythe masonry wall, a) Single Wythe concrete masonry wall, b) Single Wythe clay masonry wall
- Single layer concrete walls, floors, and roofs
- Multiple layer walls, floors, and roofs
Heat Transmission in Slabs
The structural simply supported concrete slab fire resistance, which influenced by constituent materials, can be determine from Figure 1 by using effective concrete cover parameter (u) as a function of design moment to nominal moment ratio (M / Mn).
Regarding continuous slabs, changes in moment distribution occurs, thus negative stresses are increased which means negative moment at supports increases as well. But in the case of fire occurrence negative reinforcement is in a greater distance to the origin of fire compare to positive reinforcement and consequently do not experience the same degree of heat as the positive steel bars and the increase in negative moment can be dealt with.
Fig.2: Fire Resistance of Concrete Slabs
The increase in negative moment might lead to yield of steel bars and the decline of positive moment creates rooms for the influence of fire. The negative steel bars should be extended adequately to accommodate moment redistribution and location shifts of inflection point.
It is recommended that, minimum 20% of the peak negative moment steel bars need to be extended along the whole span.
Fire Resistance Rating of Single Wythe Masonry Wall
The fire-resistant rating of masonry walls namely: single Wythe walls, multi Wythe walls, and wall with surface treatments depend on the influence of grouting and the effect of filling the cores of hollow units with fill materials.
Single Wythe Concrete Masonry Wall
The computation of single Wythe concrete masonry wall assemblies is formulated as per the specification of ACI 216.1M-07 and as per the Table-1.
The required equivalent thickness of concrete masonry assemblies (Tea) depend on the equivalent thickness of masonry unit (Te) and the equivalent thickness of specified finish material (Tef):
Tea = Te + Tef –> Equation-1
The unit masonry equivalent thickness is equal to the unit net volume divided by unit face area that is the multiplication of height by length of the unit.
The actual thickness of solid masonry unit is considered as equivalent thickness of the unit. The equivalent thickness of hollow masonry unit adequately filled with materials is equal to its actual thickness.
Table-1: Fire Resistance Rating of Concrete Masonry Assemblies
Single Wythe Clay Masonry Wall
Fire resistance rating of single Wythe clay masonry wall is computed as per Table-2. The equivalent thickness of clay masonry walls is computed as per the following equation:
Te = Vn / LH –> Equation-2
The actual thickness of solidly grouted clay masonry assemblies it equal to the thickness of clay unit.
Table-2: Fire Resistance of Clay Masonry Wall
Single Layer Concrete Walls, Floors and Roofs
The fire resistance rating of reinforced and plain concrete walls, floors, and roofs depend on equivalent thickness of the element and can be found out using Table-3.
The equivalent thickness of different types of concrete masonry is provided in the following:
- The equivalent thickness of solid concrete member with leveled surfaces is equal to the actual thickness of the member.
- The equivalent thickness of hollow core panels that have constant cross section along the entire length is computed by net cross sectional area divide by with of the panel.
- The equivalent thickness of solidly grouted or loosely filled with material for example, sand, expanded clay, slate, shale, or slag is equal to the thickness of the solid wall or slab.
- The equivalent thickness of tapered flanged elements is either equal to 152 mm or determined at the location of the smaller distance of the two multiply the minimum thickness.
- The equivalent thickness of ribbed surface elements is equal to either panel minimum thickness if rib spacing is not smaller than four-time minimum thickness, or net cross section area divided by width of the panel if spacing between ribs is smaller than four times minimum thickness, or apply the following equation to when spacing ribs is smaller than four times minimum thickness and greater than two times the minimum thickness.
t: Minimum thickness
Te: Equivalent thickness computed using equation 2
s: Center to center spacing between ribs
Table-3: Fire Resistance Rating of Single Layer Concrete Walls, Floors and Roofs
Multiple Layer Walls, Floors and Roofs
There are several methods such as graphical, numerical, and analytical solutions provided by ACI 216.1M-7 that used to determine the fire resistance of walls, floors, and roofs composed of two or more layers of different types of concrete, masonry, or both.
Each solution considers different possible combination of normal weight; semi light weight; and light weight concrete, sandwich panels and insulation systems, and the application of clay and concrete masonry assemblies as part of multi-wythe system.
Graphical and analytical solution: Fire resistance of solid walls, roofs, and floors constructed from different types of concrete can be determined using equation 4 or equation-5 or Figure-2.
The possible fire exposure of each element side should be taken into consideration, so two separate computations should be for each side assuming that is exposed to fire.
The smaller of the two calculations are considered to be the fire resistance except in the case where it is determined by the applicable code.
Finally, the bottom side of floors and roofs should be subjected to fire.
R: Fire resistance, hours
ttot: total thickness of slab, mm
dl: thickness of the layer exposed to fire, mm
Fig.3: Fire Resistance of Two Layer Concrete Walls, Floors and Roofs
Numerical Solutions: Floors, roofs and walls built from one layer of semi light weight of light weight concrete and one layer of normal weight concrete and each layer thickness is equal to 25 mm or larger. The fire resistance of the two layer together is determined by equation 4 if normal weigh concrete layer is exposed to fire and when the other layer is exposed to fire equation 5 should be employed.
Alternative Numerical Solution: The fire resistance of walls, floors, and roofs (not solid), which is built from two or more layer of different types of concrete or constructed from layers of concrete, concrete masonry, clay masonry, or combination, is calculated as per the following equation:
R: Fire resistance of the assembly, hours
R1, R2 and Rn: Fire resistance of each layer, hours
A1, A2 and An: is equal to 0.3
Moreover, values of (Rn) for concrete materials can be achieved from either Table-3 or from Figure-3. Table-1 used for concrete masonry, and for clay masonry Table-2 is employed. Finally, interpolation should be applied when table are used.
Fig.4: Influence of slab thickness and aggregate type of fire resistance for concrete slabs based on 139oC rise in temperature of unexposed surface
Sandwich Panels: Equation-6 is used to compute the fire resistance of precast concrete wall panels constructed from two layer of concrete between which a layer of foam plastic is sandwiched.
If the foam plastic thickness is not smaller than 25 mm, Rn0.59 in equation-6 is equal to 0.22. Moreover, the contribution of foam plastic fire resistance is zero if its thickness is less than 25 mm. the concrete layer thickness on each side of foam plastic should not be less than 25 mm.