The Origin of Minimum Design Loads

by briancpotter

This is what you get if you search “1920s construction”

At the turn of the 20th century, building codes in this country were a disorganized mess. At the time, code adoption and enforcement wasn’t done at the federal or even state level – it was done city-by-city.  This resulted in a mishmash of regulations varying from nonexistant (25% of towns over 5000 people had no code or inspector) to extremely strict, depending on where in the country you were. This led to a number of problems. In areas with overly strict codes, the resulting high construction costs were causing a housing shortage. And in others areas, lack of code or lack of enforcement resulted in shoddy, dangerous buildings.

Screen Shot 2013-03-11 at 7.47.54 PM

Table of load requirements from 109 building codes across the country.

Aware of all this,  in 1921 secretary of commerce Herbert Hoover authorized the creation of a Building Code Committee, staffed with experts and tasked with the creation of standards and recommendations that might be used to streamline and cheapen the process of construction. It was, interestingly enough, organized for the purpose of “elimination of waste”, not public safety as might be expected. It’s purpose was actually to make requirements LESS strict, not more.

The committee was composed of 5 engineers and 2 architects. Instead of creating a comprehensive code, they proceeded to produce a series of reports, each containing recommendations on a different facet of construction. Their fourth report was entitled “Recommended Minimum Design Loads for Use  in Design of Buildings”

Prior to this, the loads specified in various codes were generally “not developed upon scientific data, but rather on compromises”. This report, on the other hand, was based firmly in empirical data. To begin with, the existing load requirements for over 100 existing codes were gathered and compared. Data was then gathered from a wide variety of buildings, from normal dwellings to warehouses to schools to hospitals, to attempt to find what loads they experienced during operation. The number of occupants in a room at any given time was recorded. Furniture companies were consulted in the typical furniture weights. Storable items ranging from ammunition and acid to wool and varnish were weighed. Over 500 engineers, architects, and builders were asked to contribute their professional opinions.

Screen Shot 2013-03-11 at 7.49.11 PM

Some of the many, many items that were weighed as part of producing the report.

Though safety was obviously of prime importance, the guiding intention of reducing burdensome requirements shines through in some interesting ways. Typical practice at the time was to design a building for loads it might conceivably bear in the future. Thus, a warehouse built for storing grain might conceivably be required to have the capacity to store something much heavier, like brick. The report emphases that buildings should be designed for their intended load, not their hypothetical future load, and stresses the importance of frequent inspection of existing buildings to prevent improper use. It even goes so far as to recommend the installation of signs indicating a floor’s load capacity. These recommendations don’t seem to have taken hold, as I have never in my life seen a floor load rating sign, and never once heard of an inspector examining an existing building that wasn’t already damaged.

Screen Shot 2013-03-11 at 7.50.00 PM

Good luck seeing one of these anywhere, ever.

This report long predated any type of design methodology, such as Allowable Stress Design or Load Factor and Resistance Design. There are no load combinations or load factors. Loads were only broken down into two types – dead (things that were stationary), and live (everything else, including winds). It was a far cry from more modern methods of design. However, it took the important step of rooting design loads in rigorous, empirical data.

The committee continued to produce reports of recommendations for several more years. In 1933, the work on minimum design loads was taken over by the American Standards Association (later ANSI), which continued and expanded the work. It eventually produced  standard “A58.1 – Building Code Requirements for Minimum Design Loads” in 1945. It continued this work until 1988, when it was then taken over by the American Society of Civil Engineers, which renamed it ASCE 7-88. The ASCE continues this work today, with the most recent version of the standard being ASCE 7-10. Today, nearly every building code in the country uses this document as the minimum loads buildings must be designed for.

Minimum live loads allowable for the use in design of buildings. Report of Building code committee, November 1, 1924, Bureau of standards.

Recommended minimum requirements for small dwelling construction. Report of Building Code Committee, July 20, 1922, Bureau of Standards.

Residential Building Loads: Review And Roadmap for the Future.

A Century Of Excellence In Measurements, Standards, And Technology.