Understanding Building Codes & Load Combinations in the United States

Structural design in the United States has long been governed by a complex patchwork of codes and standards, some of which are standardized nationwide and some of which are amended to varying degrees by individual states.

In ClearCalcs, we support the usage of several recent revisions of the International Building Code (IBC), International Residential Code (IRC), and ASCE 7, as well as a couple of individual states' modifications where there are particularly significant structural amendments (such as the California Building Code (CBC) in California).

In this blog post we'll examine building codes from different levels of governments and institutions in detail - including their purpose, scope & requirements - so as to help you understand why they're so important for successful structural design projects across the US.

We'll also discuss what it means to be designing under a particular code and the aspects of your design that are and are not affected in ClearCalcs.

The Legally Required Code

The building code governing your design is legally proscribed by legislation adopted independently in each individual state. In the US, all 50 states have adopted codes that are based upon the same 'model legislation', in the form of the International Building Code (IBC) and/or the International Residential Code (IRC).

Figure 1: The International Building Code (IBC) ad the International Residential Code (IRC) published by the International Code Council (ICC)

Those codes are related the entire construction of a building, including not just structural design, but also architectural design, fire design, accessibility, etc.

In most cases, when states have amended the IBC or IRC, they've amended only or primarily those latter sections and generally have not modified the structural design meaningfully.

In most states, therefore, you can accurately perform your structural design using the basic IBC or IRC. The code versions in current use by each state may be viewed for the International Building Code here and for the International Residential Code here. The actual amended text for each state is also viewable in building code management software such as UpCodes.

Note also that certain larger commercial or industrial structures sometimes make use of a clause in the IBC that allows for the ASCE 7 standard to be used in lieu of most of the IBC's load calculations, and ClearCalcs therefore additionally offers a couple revisions of ASCE 7 as a 'building code' option as well.

Material Design Standards

While the IBC and IRC are the governing legislation, those building code documents are not self-contained and do not define every step of the structural design themselves.

They generally define the loads that should be applied and the load combinations that should be used, but they do not define how to calculate the strength capacity of all the different materials - wood, steel, concrete, etc.

Instead, they refer out to standards developed by various industry bodies, such as the American Wood Council (AWC)'s National Design Specifications (NDS) for Wood Construction for wood, American Institute for Steel Construction (AISC) 360 Specification for Structural Steel Buildings for steel, or American Concrete Institute (ACI) 318 Building Code Requirements for Structural Concrete and Commentary for concrete.

In ClearCalcs, when you select a building code to use, we'll change the load combinations to reflect the building code you want to use, but we will still use the most recent revision of the relevant material standard. We do not currently match material standard revisions to building code revisions. Most states explicitly allow you to use newer revisions of the material design standards, but please check with your individual state to ensure that this is the case.

Specifically for the IRC and its local state variants, note that wood design has different methodologies allowed. You may either use the prescriptive span tables in the IRC document itself, or the IRC alternatively allows you to use the NDS - which is what ClearCalcs does.

We do not use those prescriptive span tables in the IRC document, as they are much more restrictive and generally more conservative (they have no provision for custom cross-sections, and generally do not meaningfully account for complex loading scenarios or multi-span support conditions).

Unfortunately, the IRC does not provide such an alternative design pathway for cold-formed steel or concrete; you must use the IRC's prescriptive span table methods for CFS or concrete, and ClearCalcs does not yet support this.

Therefore, if you do wish to design using the IRC, you may only do so for the wooden portions of your building.

Notable State Variations

A couple of states do make notable modifications to the structural design portions of the building codes.

One of these is California, where there are numerous additional clauses for detailing and design largely as a result of earthquake requirements. However, California also defines the need to abide by its various amendments based upon the type of building being designed and thereby the relevant government agency.

Buildings under the purview of the Office of Statewide Health Planning and Development (most medical buildings) or the Division of the State Architect - Structural Safety (mostly schools) are particularly stringent in added clause requirements, and ClearCalcs does not include support for all the numerous amendments relevant for these buildings.

ClearCalcs does, however, support the California amendments for Department of Housing and Community Development (grade 1) buildings, also known as HCD-1 buildings. However, even in the HCD-1 amendments, California also notably amends the concrete material design standard specifically. ClearCalcs now fully supports the latest California Building Code including the concrete amendments.

Figure 2: ClearCalcs structural design software fully supports the latest California Building Code (CBC) 2022 including the concrete amendments

Conclusion

Thus, we can see that building codes and load combinations are integral components when designing buildings in the United States. While a majority of states follow the same model legislation from IBC and/or IRC, there are a handful of states with notable modifications that need to be taken into consideration.

Understanding the differences between codes used at an individual state level will ensure a safe and successful outcome for any construction project.

An effective way to manage building codes is to utilize structural design tools like ClearCalcs to guarantee accuracy. All code information is integrated into the platform for easy access, so you don’t have to waste time searching for various specific codes or enforcing conflicting requirements from different states. The results obtained with this software offer quick answers to keep projects running on time and within budget restrictions.

Do yourself a favor, save time by researching once and share insights instantly with all your projects thanks to ClearCalcs. Don't miss out on this powerful tool - try ClearCalcs for free today and experience how it simplifies complex structural calculations!