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Old 04-24-2010, 06:11 AM   #1
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Floor Truss Project


I've just boxed in floor trusses with 5/8 plywood to stiffen them up. They are 14" deep and span 20.5 feet covering a total of 294 sq ft under an office above a garage. The ply is screwed and glued to each side with cutouts for a 7" hvac duct and wires. I also added two rows of 2x4's at 1/3 and 2/3 across the span short side up attached to the bottom of the top chord and top of the bottom chord of each truss. Then I coverd both sides of the 2x4's with 5/8 ply as well. The floor in the office which felt like a trampoline now feels like concrete. All the plywood has added 675 lbs to the structure with the 5/8 sheetrock and insulation the total weight added should be about 1452 lbs. Before this there was only 1/2 inch sheetrock which weighed about 500lbs attached to the trusses. There are 8 trusses in the floor and are rated 15lbs/sq ft dead load and 40lbs/sq ft live load. Have I overloaded the structure?

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Old 04-24-2010, 11:11 AM   #2
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Floor Truss Project


Chris,
I've no idea to answer your question, but out of curiosity, why are you asking about it now, after the fact?

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Old 04-24-2010, 11:35 AM   #3
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Floor Truss Project


Gotta' have diagrams or photos.

Stress causes strain; how much sag do you now have for your existing dead & live load?
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Old 04-24-2010, 11:58 AM   #4
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Floor Truss Project


When you added the plywood, you converted the truss system into a box girder. A truss is a statically determinate structure, and each individual member carries either tension or compression, but no bending moment. By adding the plywood, you convert the structure into a statically indeterminate system, with the plywood carrying moment, and the truss elements now carrying moment.

Analysis of box girders is notoriously difficult, engineers spend entire careers learning how to carefully and correctly analyze loads, deflections, and reactions on box girders, since they are used in a variety of mission critical applications (bridges, high rises etc.). So you are now the proud owner of a do it yourself, undesigned box girder. Since it did not fall down before, it is unlikely to fall down now, since you have added strength along with weight, however I have no idea if you have overloaded a specific member. I can tell you that since the structure is now statically indeterminate, it is capable of building up significant stress due to changes in moisture content of the wood, temperature changes, and movement of the supports. Analysis of these issues would require a sophisticated engineering analysis, the cost of which is almost certainly more than you would care to pay.
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Old 04-24-2010, 02:49 PM   #5
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Floor Truss Project


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Originally Posted by bob22 View Post
Chris,
I've no idea to answer your question, but out of curiosity, why are you asking about it now, after the fact?

I was thinking it might have turned into an unsafe structure requiring serious attention. However I did consult a structural engineer who said putting the ply on would be fine in fact he advised doing both sides.
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Old 04-24-2010, 02:57 PM   #6
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Originally Posted by Daniel Holzman View Post
When you added the plywood, you converted the truss system into a box girder. A truss is a statically determinate structure, and each individual member carries either tension or compression, but no bending moment. By adding the plywood, you convert the structure into a statically indeterminate system, with the plywood carrying moment, and the truss elements now carrying moment.

Analysis of box girders is notoriously difficult, engineers spend entire careers learning how to carefully and correctly analyze loads, deflections, and reactions on box girders, since they are used in a variety of mission critical applications (bridges, high rises etc.). So you are now the proud owner of a do it yourself, undesigned box girder. Since it did not fall down before, it is unlikely to fall down now, since you have added strength along with weight, however I have no idea if you have overloaded a specific member. I can tell you that since the structure is now statically indeterminate, it is capable of building up significant stress due to changes in moisture content of the wood, temperature changes, and movement of the supports. Analysis of these issues would require a sophisticated engineering analysis, the cost of which is almost certainly more than you would care to pay.

I actually consulted with a structural engineer and paid him 350 for his opinion. When I mentioned putting ply on to stiffen the trusses he said to put it on both sides. Now I'm really concerned by your response that I've done something which is totally unsafe. Should I now tear everything out?
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Old 04-24-2010, 04:59 PM   #7
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Floor Truss Project


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Originally Posted by Yoyizit View Post
Gotta' have diagrams or photos.

Stress causes strain; how much sag do you now have for your existing dead & live load?

There is no sag now nor before stiffening the trusses. The floor was really deflecting that's the reason for this fix that may now be a nightmare according to the Civil Engineer. I'll post pictures and diagrams tonight.
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Old 04-24-2010, 07:29 PM   #8
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Floor Truss Project


I don't guess there's any easy way to test your new structure, and weed out overloaded members.

Last edited by Yoyizit; 04-24-2010 at 07:39 PM.
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Old 04-27-2010, 03:19 PM   #9
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Here are the photos. After calculating I'm adding a total of 1200 lbs to the structure. All suggestions would be appreciated. I'd like the Civil Engineer to look at these as well. My engineer is not returning calls.
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Floor Truss Project-garageceilingviews2a.jpg   Floor Truss Project-garageceilingviews3a.jpg   Floor Truss Project-garageceilingviews4a.jpg  

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