The Lowdown on Load Codes
By Bud Dietrich, Originally Published in Houzz
Codes for structural loads -- for floors, stairs, etc. -- establish minimum requirements for safety.
Anyone who has built, designed or remodeled a home has heard the term “built to code” and people saying, “The code requires it to be like that.” And when we hear things like this, we tend to think we’re getting a house designed and built to the highest standard.
But that’s not necessarily so. What the building codes do is establish an absolute minimum standard. This minimum may not be what you need in your home. You could, in fact, easily have needs that require the design and construction of your home to exceed code.
This is especially the case when it comes to structural items. While the code mandates that structural systems be designed to support certain minimum loads, and to do so within certain tolerances, these minimums and tolerances may not be what you actually need. Who wants to live in a house where the floors are so bouncy that you feel like you’re walking on a trampoline? And what happens when you decide to move a water bed into the bedroom next to that stack of heavy books you cherish? Will you need to have your floor joists doubled up under that big soaking tub you are planning?
It’s wise to think about particular situations like this and look at the code mandates as a starting place, not the finish.
Building codes establish many project requirements, not the least of which is the project’s structure. This holds true no matter what material the house will be built of. And a key to designing a structural system is to determine what loads, or weights, will be imposed.
So we first have to look at what the house will be made of (wood, concrete, steel, masonry etc.) to determine the dead load — the weight of materials used in the permanent construction of the house. Note that it doesn’t include items like furniture, people, toys, books, television sets etc. and will vary only a little bit over the lifetime of the house.
Next we use the governing building code to determine what the minimum live load — the impact of movable objects such as furniture and people — will be. For example, per code, the main living areas of a house have to be able to accommodate a uniform live load of 40 pounds per square foot. Bedrooms have a code requirement of 30 pounds per square foot, and roof structures have a varying live load, depending on climate (more snow equals more weight) and roof pitch (steeper roofs will shed snow faster).
But the code-mandated uniform loading may not accommodate all of your furniture and books, that large cast iron soaking tub, your water bed etc. So you’ll want to identify any items that this code-mandated requirement won’t accommodate and design the structure accordingly. Otherwise, the extra weight of these items can cause the structure to fail.
When we say a structure has failed, we don’t necessarily mean the house has collapsed. Failure can simply mean a part of the structure has failed so there’s too much deflection. This will result in uneven floors, gaps between walls and floors, and so on.
Deflection is the distance that a structural member (say, a floor joist) will bend when a load is placed on it. The greater the distance, the more the deflection and the less level the floor.
In addition to holding up a certain load, a structure has to stay rigid and keep its shape. But that’s nearly impossible, as any structure will start to deflect the moment any load is placed on it.
The code requires that this deflection be limited to L/360, where L is the length of the unsupported span. This means that for a floor structure that spans 18 feet (not uncommon in newer homes with open floor plans), the maximum allowable deflection is 0.6 inches.
In other words, a floor can sag more than a half inch and still be deemed OK. So some architects and engineers will use L/480 to calculate the allowable deflection. For an 18-foot span, using L/480 would limit the amount of deflection to 0.45 inches.
While the difference between 0.6 and 0.45 inches may seem insignificant, it really isn’t when it comes to a floor structure that gets walked on all the time.
If you’re designing or remodeling a house, have a conversation with your architect and builder about what you plan to put into your home and how the structure will accommodate it.
January 14, 2014