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Old 01-04-2009, 08:48 PM   #1
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Stiffening Floor Trusses


My first post to this site; thanks in advance for any advice given.

In my 30-year old home, I am planning to add a 17' stiffening beam in my finished basement to support several 2x4 16" trusses. The beam will be placed perpendicular to the trusses at the midpoint of the 24' truss span and located under a vertical member of the truss. Purposes of the beam are (1) to stiffen and de-bounce the floor and (2) to correct a 3/4" deflection that has occurred in the last several years due to increased load in the kitchen above (new pantry, cabinet and counter tops). To maximize headroom, I am planning to use a 4x6 steel I-beam that will be supported in 3 locations with the maximum unsupported span being 13' 6". I plan to use 4" dia. jack posts as supports.

My questions: 1. Are jackposts reasonable to use? Do they meet residential code? 2. Can I place the jackposts directly on the basement slab or is a new footing needed? Could I use a piece of steel (say 1' square x 1") to better distribute the jackpost load? 3. Does this sound like an overall acceptable solution to my two problems?

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Old 01-05-2009, 06:07 AM   #2
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Stiffening Floor Trusses


Engineered I beams are supposed to be stronger that stick built, but I have heard they tend to flex more. A new beam sounds reasonable. But I don't think jack posts are allowed, at least not here, check local code. And yes they would need a seperate pad to virgin earth. That beam could be wood. might be easier to manage in a already built structure.

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Old 01-05-2009, 08:05 AM   #3
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Stiffening Floor Trusses


Whenever a beam is added to remove or control deflection, loads are incurred at the beam's bearing points. Steel lally columns are appropriate, but must be supported by pad footings, not the basement slab alone. The size and depth of the footings is determined by the imposed loads and your soil conditions, and an engineer is needed to size them for you. The engineer will also size and specify the size of the columns based on the loads.

Hopefully you have an engineer working with you on the size and placement of the beam as well as the potential structural effects on the trusses.

As for distributing the load of the posts with an additional plate...
No, not on the floor slab. That would be unconventional and ideally you want structural independence between the post and the slab, with the slab sitting on a pad footing beneath or in the floor. That way if the floor heaves it doesn't push the structure upwards with it.
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Old 01-05-2009, 10:43 AM   #4
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Stiffening Floor Trusses


Thanks to both of you for prompt replies. I appreciate your time and advice. regarding the footings needed for lalley posts, I have gotten differing advice, including the need to go down 4' to avoid frost heave here in Minnesota. My thought however, is that the footings will be within the interior of the home where the slab and underlying earth will not be subject to freezing. If that's true, do I need a "frost footing" or rather one that will properly bear the load?
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Old 01-05-2009, 10:49 AM   #5
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Stiffening Floor Trusses


Since the pad footings are on the interior of a home they don't need frost protection or the frost depth. Crawlspaces would be an exception to this if unheated.

Soil types and imposed loads vary, but around here the smallest pad footing will be 8" thick and 24" square. The code doesn't allow anything less than an 8" thickness. Most pads are 36" square and 12" thick, some go larger occasionally. They're also reinforced with a mat of rebar.
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Old 01-05-2009, 11:49 AM   #6
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Stiffening Floor Trusses


Thanks for the quick reply. I appreciate your help.
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Old 01-05-2009, 08:35 PM   #7
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Stiffening Floor Trusses


I understand you are placing this new beam under a vertical member in the floor truss, however trusses are engineered for a specific condition. That vertical member was not originally sized to be a bearing point and putting a beam there will change the way the loads from above travel through the truss members. Those metal plates holding the truss together were each specifically sized for the loads at that particular location. I would highly recommend you contact an engineer about beam, column, and footer sizes as well as the impact on the trusses. The name of the truss manufacturer should be stamped right on the trusses themselves, you can start by contacting them.
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Old 01-05-2009, 09:46 PM   #8
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Stiffening Floor Trusses


Thanks for your additional input. I'll contact the truss manufacturer to see what can/should be done.

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