All codes are filled with numeric values a scientific or functional basis but the bottom line for any numeric value is that it is and the values are all arbitrary. The numbers may have a consensus number by the writers of the codes. Everything in the building codes are minimum standards, kind of a bottom line. For a designer, the numbers are something to design to and most of the time we only design to the minimum or what is required by the code.
Structural Values – Structural requirements may appear to have scientific basis, but only to achieve a certain standard. The basic standards to design to are arbitrary: why an X size earthquake or Y value wind speed or even a Z value load for any specific construction element? Some of the basis is economical as far as acceptable losses. This is the insurance companies talking. Even within the standards for earthquake design there are different values applied. Existing buildings have a lower standard than a new building. There is a simple and practical reason: making an existing building meet current standards would be impossible and economically infeasible. For that reason, we accept a lower standard.
Building Limitations – One of the biggest numeric battles have been the height and areas table in the codes. The basis has been established by the insurance industry to limit their losses due to fire. So basically, smaller buildings are limited to combustible materials and larger buildings have a higher fire resistance. That is where the logic ends. Limiting a B/office building of Type VB construction to 9,000 SF/story, from Table 503 and limiting it to a single story is just a number to design to. There is no reasoning to say there would be a bigger loss at 10,000 SF. When the International Building Code (IBC) was created, it was a blending of the original three model codes (Standard, National and Uniform Codes), which all had different values for their height and area tables. It was already strange that a building built in the northeast could be bigger than in the west for an identical design. What was decided upon was the largest or more lenient set of numbers. This was done mainly to allow existing construction to continue. Although there is no real rhyme or reason for the numbers to begin with, this was argued for years and in some circles, continues to be discussed.
Means of Egress Requirements – Again, all the numeric values associated with means of egress are arbitrary. Why two exits at 50 occupants, but 49 and one exit is okay?
Toilet Fixture Counts – The fixture count found in the code has little basis to actual function or use. The original number comes from plumbing codes that were written by plumbers and the plumber union. The reasons were more self-serving. What designers need to look at is the function and how they serve the public/users. An impact load, like an intermission at a formal theater, is different than in an office building. A formal theater where people dress up in formal attire will take longer to cycle through the toilet facilities than at a ballpark. Plumbing fixtures, in my opinion, are a convenience and not life safety, but some may disagree. Waiting longer is only an inconvenience.
So who decides on these numbers? The people who participate in code development are building and fire officials, representatives from industry and the design professional, but they are few in numbers. Most of the time, code changes do improve the code that reflects a higher level of safety. Many of the more subjective elements and conveniences, like numbers of plumbing fixtures are open to more debate. As designers, we too often take the numbers in the code as law and design to them. We should always look at the value and help the user decided if that is really best for the building. Even if it’s less, be ready to justify your design and have reasons why not to follow the code. You can do this by using the “Alternative Methods” approach provided in the code.
So if you don’t like the code, change it.