Masonry Magazine February 1973 Page. 11
Fire Resistance Period
From the formula R = (CV)^n, the fire resistance period of a composite wall can be derived:
R = (C1V1 + C2V2 + C3V3)^n = (R1^(1/n) + R2^(1/n) + R3^(1/n))^n
Substituting the experimentally determined value of 1.7 for n and 0.59 for 1/n, the general formula becomes:
R = (R1^0.59 + R2^0.59 + R3^0.59)^1.7
It will be noted that the fire resistance period is expressed in terms of the fire resistance periods of the various wythes or laminae of the wall, which need not be of the same material and design.
If the fire resistance period of a wall is known and if it is desired to find the fire resistance period when one coat of ½ inch 1:3 sanded gypsum plaster is added, the solution is as follows:
R = (R1^0.59 + 0.3)^1.7
Where:
R1 = fire resistance period of un-plastered wall, hours
R = fire resistance period of plastered wall, hours
0.3 = an average value from tests ½ in. plaster (the value is directly proportional to the thickness of plaster, i.e., 0.6 for 1 in. plaster)
The effect of a continuous air space separating wythes by distances of ½ to 3½ inches, such as for cavity walls, is estimated by the use of the value of 0.3 for R^0.59.
Fire Insurance Cost
The fire protection benefits associated with the use of concrete masonry walls with concrete floors and roofs is reflected by the lower premiums established for insurance against fire loss, both for building and contents. The lower premiums can produce significant savings, as illustrated in Table 1 which lists average insurance rates per $100.00 for several construction types and occupancy classifications. The figures in Table 1 were obtained by averaging values from information which includes several Protection Classes in more than 50 cities. Since exact rates are not determined until a structure is built these figures are not exact and are listed merely to indicate relative differences between rates for various types of construction.
While building codes list specific minimum requirements for fire resistance of walls and partitions depending on occupancy and type of construction, it can be demonstrated where the use of materials which greatly exceed the minimum requirements may actually be less expensive. For example, an investigator in Atlanta, Georgia found that he could write off the additional cost of 12" solid concrete masonry fire walls with fire insurance savings within four years in an apartment development of 96 units which were clustered in groups of eight apartments in each of 12 buildings. In lieu of staggered-stud party walls, additional cost for construction of the concrete block fire walls was: $3120 per building x 12 = $37440.
Fire insurance premium on the 12 apartment buildings with eight units and no fire walls was $14400 per year. By putting a fire wall in each building so as to separate the units into clusters of four, the insurance
TABLE 1
Typical Fire Insurance Rates* for Various Building Types and Occupancies
| Construction | Occupancy | Rate per $100.00 |
|-----------------------------------|------------------|-------------------|
| Concrete Masonry with concrete floors & roof | Apartment | $ 0.149 |
| Concrete Masonry concrete floors wood roof | Apartment | $ 0.548 |
| Wood frame, brick veneer | Apartment | $ 1.097 |
| Pre-engineered metal building | Apartment | N/A |
| Concrete Masonry with concrete floors & roof | Office | $ 0.080 |
| Concrete Masonry concrete floors wood roof | Office | $ 0.247 |
| Wood frame, brick veneer | Office | $ 0.762 |
| Pre-engineered metal building | Office | N/A |
| Concrete Masonry with concrete floors & roof | Storage Warehouse| $ 0.116 |
| Concrete Masonry concrete floors wood roof | Storage Warehouse| $ 0.497 |
| Wood frame, brick veneer | Storage Warehouse| $ 1.399 |
| Pre-engineered metal building | Storage Warehouse| $ 0.928 |
*Annual cost per $100.00 for Fire and Extended Coverage-80% coinsurence contents not included; figures represent average from more than 50 cities covering several Protection Classes.