[lep]

Our 2-story garage was built in 2006, and we are now finishing out the second floor. The permit was pulled and everything is on schedule. Because the garage measures 20' x 30' the ceiling joists for the second floor ar 20' long, and apparently only 18' ceiling joists are now available(?)

At any rate, the contractor and I noticed the following:

1). When you close the ladder (with springs) going to the attic the ceiling joists tremor somewhat (or vibrate) -- probably because they are 20' long.

2). When standing in the attic, if you jump a little, the ceiling joists flex slightly, which means that sheet rock joints (on ceiling of 2nd floor, screwed into ceiling joists) could fail over time.

We thought that we could drop vertical "purlin" 2x4's from the rafters down to the ceiling joists, and cut holes in the attic floor plywood so the 2x4's could be nailed in to the ceiling joists. We also thought of altering every other ceiling joist -- too many and you couldn't store anything in the attic. As long as there are 2 purlin joists per ceiling joist.

The contractor suggested jacking up the rafters because they are bowing, but my thoughts are just to tie in the vertical purlin 2x4's dropped from the rafters down to the ceiling joists -- which will help minimize flexing of the long 20' ceiling joists.

Couple questions:

-Do the rafters need to be jacked up, or should the purlin 2x4's simply be attached between the rafters and ceiling joists without jacking up?

-Aren't purlin joists angled like 45 degrees?

-For 20' ceiling joists, how far from the ends of the ceiling joists should the purlins connect, and what about the rafters (how far from the bottom ends of the rafters should the purlins connect)?

 

[lep]

Also, the ceiling joists are 16" on center, so I am thinking that we could just drop vertical 2x4's from the rafters every three joists (3 x 16" = 48" --> max spacing) and do this at 5' from the ends of the ceiling joists. This might not be true purlin hangers, but it would stabilize the ceiling joists from vibrating when you jump in the attic or let the ladder bang to hard when closing. The goal is to prevent the sheet rock joints on the 2nd floor ceiling from popping out because the joists are so long (20').

 

[Bud9051]

All of your questions will be answered when you get a properly engineered design for your project. I'm surprised they gave you a permit without the engineering.

Seriously, you don't want that coming down with people in it and better to have the plans on how to do it right before the inspector discovers it was wrong.

Bud

 

[lep]

"All of your questions will be answered when you get a properly engineered design for your project. I'm surprised they gave you a permit without the engineering."

Didn't have enough time to respond appropriately. The current structure met code in 2006. Nuff said? Regarding finishing out the 2nd floor, we have a *new* (2015) engineer-stamped letter to the city verifying structural integrity, as well as architect plans (2015) that were submitted to city. *All approved*

"Seriously, you don't want that coming down with people in it and better to have the plans on how to do it right before the inspector discovers it was wrong."

The structure is not "coming down with people in it." It was approved by the city.

The intent was not to request regulatory oversight, but rather how to recover from having a garage with 20' ceiling joists on the 2nd floor which met code in 2006.

Therefore, again, how can vibrations among 20' ceiling joists be tempered either by 45-degree-based Purlin hangers or vertically hanging 2x4's between the rafters and 20' ceiling joists? A concern I had was that by placing Purlin hangers every 48" there is potential that the ceiling joists between the 48" max spacing may nevertheless bend if they are not supported.

Looking for responses from members who can wade through regulatory issues and theoretical statements surrounding structural failure.

Therefore the question is: you are stuck with 20' ceiling joists in the attic of a 2-story garage that met code in 2006; you notice that jumping in the attic results in large vibrations among the ceiling joists; so how can the vibrations be tempered?

 

[lep]

Also, structural inspector visited last week - and no recommendations for dampening vibrations among the 20' ceiling joists on the second floor. Thus, this is above and beyond inspection. We are trying to calm down vibrations among 20' ceiling joists on the 2nd floor of a garage.

 

[ront02769]

Two things

First off, to the moderators, is the quick reply thing dead???? Or just on my iPad with the new IOS?

Second, I missed the part where you told us the depth of the ceiling joists? Two bye something?? And I can tell you for a fact that any real lumberyard will produce two bye four, six, eight, etc. in 20' length....at least in New England. Ron

 

[Gary in WA]

IMO, ask an engineer for the liability involved (w. added roof loads) as the city usually has a disclaimer somewhere in the permit... add some 2x to the bottom edge of the joists to strengthen them;eg; http://www.johnbridge.com/vbulletin/showthread.php?t=54371&page=2

Other ways; http://www.finehomebuilding.com/pdf/021184090.pdf

Gary
PS add two Paralams there.

 

[lep]

Thanks for the replies. The ceiling joists on the second floor are 2"x8" and 20' long. If that helps in your computations, then that's great. Can't argue whether 20'-long 2x8's are available in your region. Am asking about calming down vibrations among 20' ceiling joists.

 

[Daniel Holzman]

I am a bit confused by the thread. The OPS indicates that he has a garage with 20 foot long joists, 2x8 inch wood. The OPS apparently wants to finish the floor, and is concerned that the joists are vibrating.

I am not clear if the garage was designed to support load above the joists. I ran a quick check assuming standard strength joists, and 20 foot 2x8 joists spaced 16 inches on center are not adequate to support 10 psf dead load and 30 psf live load, which would be typical loads for occupied space. So without accurate, complete information about the proposed loading, I don't see how anyone on this forum can comment on methods for removing the bounce from the joists. Before evaluating bounce, it would seem to be more important to make sure the joists can hold up the load, whatever it may be.

 

[lep]

Hi Daniel,

It's the attic floor of a 2-story garage, not the 2nd floor (occupancy floor). The 2nd floor (occupancy floor) has 2x12's below it which are 16" on center, and the ceiling joists on the second floor (i.e. attic floor) are 2x8's that measure 20' long. I don't think attic requirements are the same as occupancy requirements(?) Thanks for your help!!!!

L

 

[Bud9051]

Well, I'm with Danial, I don't understand what you are doing. @lep "and we are now finishing out the second floor." If you are going to add flooring for some storage above the second floor that's one thing.

In any case, having approved drawings for the work you are describing means that is the way you must do it. What does your permit describe for the work you want to do? Was that part of the engineering stamp? Sure sounds like you are re-engineering the structure.

Bud

 

[lep]

Hi Bud,

That's right, this is above what the approved engineering and plans call for. We'll ask the city structural inspector about this when he closes out the job on his next visit.

 

[Mingledtrash]

Hi Daniel,

It's the attic floor of a 2-story garage, not the 2nd floor (occupancy floor). The 2nd floor (occupancy floor) has 2x12's below it which are 16" on center, and the ceiling joists on the second floor (i.e. attic floor) are 2x8's that measure 20' long. I don't think attic requirements are the same as occupancy requirements(?) Thanks for your help!!!!

L

Attics by themselves are not the same as occupancy, but the moment you sheathe it, its requirements change. Table 1607.1 of the international building code 2015 states that attic with storage needs a 20 psf live load rating. Now if we jump over to our span charts in Table 2308.7.1(2) ceiling joist spans uninhabitable attic with limited storage you would need minimum #2 doug fir 2x10 on 12 in centers to span this.

right now you have 16 in. centers you are close to the max span of 23 feet for a 2x8 for attic without storage requires a 10 psf live load.that is why you have bounce and vibration issues.

 

[lep]

@Mingledtrash -- thanks for looking up those values in the tables. Also sounds like 2x10's should have been used at 12" centers -- which would give a 20psf. Thus far, none of these structural issues were on the radar of the city's structural inspector, and Something that came to mind regarding purlin hangers is that they probably can't be used because:

1- the 2nd floor is a totally open space without internal walls
2- without internal walls, there are no horizontal studs immediately below the ceiling joists (the attic floor).

Therefore, the typical setup for purlin hangers is maybe in a house where all of the ceiling joists are supported by vertical and horizontal wall studs. In such a scenario (structure) you can knock yourself out with all you want to jack up the rafters with bracing -- since the wall studs everywhere will support the ceiling joists you are working from. Not so in this garage; those 20' ceiling joists span across the entire area without internal wall support. So the mere idea of using purlin bracing to support the rafters probably won't work and probably shouldn't be used.

In the end, I think the only relatively safe move is to:

1) Drop vertical 2x4's from the rafters (spaced every three ceiling joists = 48" max)
2) Ensure two 2x4's are used per selected joist and place them 5' from the ends of the ceiling joists (rafter nail-in will occur vertically above the tie-in to the ceiling joist)
3) nail the dropped 2x4into the ceiling joists -- only to dampen vibrations.

The structure was built in 2006, so that has been plenty of time for the wood to settle. Using the above approach, I would be locking in the long ceiling joists from vibrating so much and wouldn't (I think) be placing any additional stress on the rafters or the ceiling joists. Essentially, this would be like locking the current vertical locations of the ceiling joists via the rafters -- only to minimize the flexing when in the attic. So in a way, it's almost cosmetic -- without stress, it wouldn't be purlin bracing, but locks of the ceiling joists from flexing.

Now for the interesting question, what happens if you place additional weight on the rafters?

 

[ron45]

Just my opinion..
But I think you need to reconsider.

 

[lep]

okay, what's the alternative solution? [there is enough information"- a) it's a 20'x30' 2-story garage with 20' 2x12 ceiling joists placed 16" on center that span 20' under the second floor, and 20' 2x8 ceiling joists placed 16" on center under the attic plywood floor; b) there are no interior walls upstairs, so essentially like a barn --> purlin hangers are innapropriate as there is no internal wall support for the 2x8 ceiling joists that purlin hangers would rest on; c) the long 20' ceiling joists under the attic floor need to be vertically stabilized (they are covered with nailed-on plywood so they are stable horizontally although may twist)].

 

[Daniel Holzman]

What load are you designing for in the attic?

 

[lep]

Probably not much more than 10psf. If the attic measures 20x30' then how does the 600 sqft with 10psf (on average) work out for the total allowable weight supported?

 

[Daniel Holzman]

The dead load alone is generally taken to be about 10 psf, so you need to add the design live load to the dead load to come up with the total load. Your building inspector will be able to tell you what code minimum loading is, and that is what you design for. Generally loads are code driven, and are not based on the actual things you are going to put up there, so bear that in mind.

 

[Mingledtrash]

I do not know what your budget is, but the best solution, I can think of, is to get a company that makes structurally engineered trusses to design and turn your existing rafters and ceiling rafters into a truss. The area i am in has several companies that have portable gusset plate presses that they bring on site for truss repairs when they are damaged in shipping and installation. They could design and do a fix but it could cost a couple grand. Here is a picture of what i am thinking you could turn your existing rafters into. At the same time they could jack up the rafters so as to add camber to the system. Its possible you could do this every other joist or every 4 feet just depends on what there engineering software says.