One of the biggest advantages of floor trusses is that they can span further than conventional framing (i.e. 2X10’s or 2X12’s) or I-joists. Below is a chart that we use for quick reference to match floor truss height and spacing with any given span. Bear in mind that this span chart shows the absolute maximum span for a truss with no HVAC chase.
This chart is for reference only; any truss design must be approved by an engineer before it should be built.
If you would like to print this chart, you can download the pdf below.
Floor Truss Loading Information
While this gets into the nuts and bolts of floor truss design, it is vital to truss design. It’s also important for our clients to have a working knowledge of this information. Our typical floor truss loading is 40-10-0-10. Translated, that is:
- Top Chord Live Load: 40 pounds per square foot
- Top Chord Dead Load: 10 pounds per square foot
- Bottom Chord Live Load: 0 pounds per square foot
- Bottom Chord Dead Load: 10 pounds per square foot
The best way to understand the difference between live and dead load is that a live load is a “moving” load. Dead loads are static (i.e. they are always there). So live load accounts for furniture, people, pets, appliances, elephants, etc. Dead loads account for things like sheathing, flooring, drywall, fasteners, duct work, etc.
Deflection is another important loading term. We design our floor trusses to L/360 Live Load Deflection as standard, and will go higher if requested. While it’s easier to ignore numbers that look like formulas, this is an important number to understand.
Let’s look at what L/360 means. L/360 means that the deflection (i.e. bending or sagging) of the truss from the live load is limited to 1/360th of the overall span. To calc that out, a truss that is 240″ long (or 20′ 0″), under total live load, may deflect only 1/360th of the span. In this case, that is is equal to .67″ (240″ / 360 = .66667″).
Just how much weight is the truss carrying? We’ll assume spacing of 24″ O.C. for easy figuring. The truss carries half the distance to the next truss on either side, which equals 24″. 2′ of spacing X 20′ of span = a total carried area of 40 square feet. We can multiply that times the Live Load of 40 psf. 40 X 40 = 1600. Therefore, according out our design standards, our 20′ truss can carry 1600lbs of live load without that load causing it to deflect, or bend, more than .67″. You can do these same calculations to any truss span, and design to any deflection ratio you would like.
Here’s the thing to remember: the higher the deflection ratio, the more load the floor can carry before maxing out its deflection. Higher deflection ratio = stiffer floor.
Modifying Floor Trusses
Sometimes things just don’t fit the way they are supposed to. Perhaps a foundation wall was not in the right spot, or changes were made to the plan after the trusses were manufactured. Occasionally, even truss manufacturers make mistakes. No matter what the reason, there is one thing you need to remember if changes have to be made to existing trusses: never modify trusses without prior engineering approval.
Truss repairs can be costly and time consuming. Modifying trusses before you have a clear engineered solution to the issue will likely lead to more work later. In the event of a problem, we will help you look at all your options to determine the best solution. Any fix must be approved and sealed by the engineer in order to pass inspection approval. It usually takes about 24 hours for the engineers to get a repair designed.
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Strong-back bracing in Floor Trusses
We recommend Strong-back braces at no less than 10′ on center. The first brace should be no further than 10′ from the bearing point, and braces should then be placed at regular intervals of no more than 10′ until the next bearing point is reached.
Strong-back braces are simply a 2X4 or 2X6 brace, placed on a vertical web, and is oriented perpendicular to the trusses. The purpose of Strong-back bracing is to create more stability in the floor. The way Strong-back bracing accomplishes that is by tying the trusses together so that they accept loads as a unit, instead of each truss deflecting individually as a load moves across the floor. While Strong-back braces are not required by the engineers, they greatly enhance the “feel” of the floor. If a floor feels as though it has too much “give”, add more braces to improve the stability.
When installing top chord bearing floor trusses, it is wise to plan ahead and insert the bracing material into the trusses as they are installed. You may find it difficult or even impossible to insert the braces after all the trusses are installed.
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HVAC Chases in Floor Trusses
HVAC Chases, or duct runs, may be designed into floor trusses to allow for plumbing and/or heating and air conditioning ducts to be installed in the floor.
HVAC Chases are typically 24″ wide, and as tall as the height between the top and bottom chords will allow. You can usually assume that the height of a HVAC Chase is 3″ less than the overall height of the floor truss, however, when the truss is near its maximum span distance it may require double top or bottom chords, which further reduce the HVAC Chase height. It’s always best to verify the HVAC Chase’s height, width, and location with the one who will install the duct work and/or plumbing.