Test Your Claims Knowledge: Live Loads vs. Dead Loads With Roof Claims
First published in the September 12, 2012 online edition of PLRB – Test Your Claims Knowledge
You’ve probably come across a structural claim where the terms “live load” and “dead load” get tossed around by a contractor or engineer regarding the repairs of the building. Or maybe the term “governing code” was brought up, and you weren’t 100 percent sure what they were referring to. This article will provide the basic knowledge for adjusters to become familiar with these terms and what to expect when dealing with a structural repair claim.
Question: What are building design loads and how can they differ from structure to structure?
Background: Live load and dead load are common terms used for the structural design of all buildings. In the case of dead load, we’re referring to loads that typically don’t change over time, such as the weights of materials and components of the structure itself (the framing, the flooring material, roofing material, etc.), and the weights of fixed service equipment (plumbing, HVAC, etc.). In the case of live load, we’re referring to loads that do, or can, change over time, such as people walking around a building (occupancy) or movable objects such as a flower pot on a deck. In addition to live loads, what is known as environmental loads also need to be accounted for. Environmental loads are loads that are created naturally by the environment and include wind, snow, seismic, and lateral soil pressures.
When someone refers to the ‘building code’ when discussing current structural issues, they’re typically referring to the set of International Codes developed by the International Code Council (ICC). This code set includes, but is not limited to, the International Building Code (IBC), the International Residential Code For One- and Two-Family Dwellings (IRC), the International Plumbing Code (IPC), and the International Mechanical Code (IMC).
The IBC will be used for the discussion within this article and since its creation in 2000, has been updated every 3 years. The most recent edition is for the year 2012, and the 2015 edition is currently under development. The majority of municipalities throughout the United States have adopted one edition of the IBC, so it’s always important to check with the local building department where the building in question is located in order to determine what edition is enforced. However, there are a few rural areas still around that don’t enforce any building code, or that still utilize the Uniform Building Code (UBC), but that’s a subject for another day.
In addition to determining which IBC is enforced by the local municipality, it is important to check with the building department, as they may also issue specific requirements pertaining to the environmental loads. They stipulate these requirements based on standards set forth in the IBC, but each building department can ultimately enforce the environmental loads as they see fit for the safety of their community. The building code that each building department enforces, as well as any specific requirements they also enforce (such as environmental loads) can be referred to as the governing code for that municipality. Typically these are amendments to the building code and are specified in the jurisdiction’s municipal code. We’ll get back to this a little later.
When considering structural design with regard to the IBC, an engineer has to take several important factors into account. One of these factors is called the risk category. The risk, or occupancy category, depends on the type of habitation the structure is being used for to determine the risk to human life if the structure were to fail in some way. For example, a structure that is used to store hay compared to a structure that is used as an elementary school would be in significantly different risk categories, as an elementary school must contain more elements of safety for human life. Another example: a 20-story office building and a fire station would be in different risk categories. Although the 20-story office building would contain certain substantial hazards to human life, a fire station would be considered an essential facility. The risk or occupancy category of a structure is defined in the building code and cannot be changed by a local building department.
Probably the most familiar factor for structural design would be dead load, which takes into consideration the types of materials used to build the structure and the associated material weights. While most single family homes are made out of wood framing materials, commercial structures can vary from wood, to metal, to concrete. These materials can all vary greatly in weight and need to be accounted for during the structural analysis. The fixed service equipment weights can also vary greatly, for example the existence of cranes and large HVAC systems.
Another factor an engineer must consider when designing live loads is the purpose and use of the structure, because the IBC lists minimum requirements for live load depending on these factors. For example, a single family home, a restaurant, and a stadium would all have different live load requirements. And even more specifically, the IBC lists live load requirements for differing rooms within a structure. For example, a library has a higher requirement for live load in a room where books are stacked versus a room that’s designated just for reading, because rows and rows of books weigh a lot more than a few people each reading one book while sitting on some chairs. And a school has a higher requirement for live load in a hallway than in a classroom, because 30 kids and a teacher in a classroom weighs a lot less than a whole student body standing shoulder to shoulder trying to get out of school at the end of the day.
Home after a large snow fall.
Home experiencing strong winds and snow drifts – Source.
Building damaged by an earthquake
Home damaged by an earthquake – Source.
When it comes to environmental loads, the IBC specifies different requirements depending on location. In California, seismic load requirements would govern in structural design instead of snow load because an earthquake is more feasible than a three-foot snow storm. In Florida, wind load requirements would govern because of the hurricane risk in that location. As such, in the Rocky Mountains, snow load requirements would govern for obvious reasons.
Some locations can be a little tricky and not as easy to discern. For instance, what do you think would govern in Kansas: wind or snow? This area of the US can get decent amounts of snow and huge snow drifts, but it is also prone to tornadoes and super strong winds. In this case, calculations would need to be performed to determine which loads would govern the design. Also remember, this is one instance where local municipalities can modify the loads, so the wind load in Denver may differ from the wind load in Colorado Springs, or the snow load in Vail may differ from the snow load in Aspen. Again, always check with the local building department to verify the repairs are in accordance with the local governing code.
Now that you know some of the basics regarding the IBC and structural design, you can discern how this could affect your claim. When repairing a structure, most building departments require the damaged area to be brought up to code, or at least the items that are being repaired or replaced are required to meet current code. This means that a home built in 1970 with damaged roof trusses should not be simply repaired with the same 2×6 size rafters that were installed when it was built. An engineering analysis must be performed to ensure the repaired roof meets the requirements of the code currently enforced. So, say we have two identical homes that were built in 1970 with damaged roof trusses. Here are a few differences you might encounter during repairs:
Dead load – One home has concrete tiles installed on the roof and the other has asphalt shingles. Concrete tiles weigh about 3-4 times as much as asphalt shingles, therefore the dead loads will differ during the structural analyses and could also result in different repairs for each home.
Environmental loads – One home is located in North Carolina and one is located in North Dakota. The governing load for North Carolina would most likely be wind, and the governing load for North Dakota would most likely be snow, therefore the structural analysis would again differ and could result in different repairs for each home.
In another scenario, let’s say a truck lost its brakes in a small town in Oklahoma and crashed into a restaurant and a hospital. Here are a few differences to consider when dealing with the repairs:
- Dead load – the materials used to build each of these two buildings are probably different. The restaurant can be assumed to consist of wood framing materials, where the hospital would be assumed to be made out of metal framing materials or concrete/masonry.
- Live loads – the use of a hospital differs greatly from the use of a restaurant. In the restaurant you can assume you’d have lighter loads (probably only one story, lighter furniture, lighter equipment), whereas a hospital would have heavier loads (possibly more than one story, heavier furniture, heavier equipment).
- Risk category – In the case of an emergency, people would need extra assistance when exiting a hospital, compared to people who are exiting a restaurant. And since hospitals are a treatment facility during emergencies, it’s imperative that extra precautions be taken in the design process to ensure its resistance to failure during an emergency.
Overall, determining how to repair structural damage depends on a variety of factors. An engineer should always perform a structural analysis to ensure all code requirements are met, and hopefully now you have a better idea of what that entails and what the primary factors are. For further information, copies of the IBC can be obtained at http://www.iccsafe.org/Pages/default.aspx.
Test your claim knowledge:
1. Which of the following would be categorized as a dead load?
b. Concrete tiles
2. Which of the following would be categorized as a live load?
d. Steel beam
3. Which environmental load would likely govern in southern Louisiana?
d. Lateral soil pressure
4. What factors are considered when designing structural repairs?
a. Use of the structure
c. Sun exposure
d. Electrical design
5. All local building departments follow the same edition of the IBC.
1. b,c, d
3. Trick question – either a and c could govern.
4. a, b