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Re: Converting ceiling joist to floor joist.

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Bill, thanks for your comments and confirmation. 

I especially appreciate your advice that:  "I always design the floor to be stiffer than code levels."  I'm similarly conservative and use L/480 for live load deflection, L/360 for total load deflection, and 0.50" maximum total deflection, period, except in *very* limited circumstances.  The only time I cut things close I had a very unhappy client---ME!  I went cheap in my own attic conversion.  Live and learn! 

Trus Joist MacMillan has a very good little essay in their floor joist catalogue:  "A Word about Floor Performance," which cautions that people's perception of floor performance is "very subjective."

Ralph

In a message dated 9/17/05 10:18:08 AM, bcainse(--nospam--at)aol.com writes:
Djohan-
Ralph gives you some very good advice on separating the new floor from the existing ceiling.
 
Another way to do this is to install a plate on top of existing ceiling joists at the bearing walls and build your new floor on top of that plate.  This does not require the new floor joists to run the same direction as the ceiling joists and provides a space to run electrial, although often you want to run them in the same direction as ceiling joists as they are usually spanning the shorter direction. Where this solution becomes more difficult than what Ralph suggested is in the development of your lateral system shear transfer (although not much more difficult, it is just a bit fussier). You also, as with Ralph's suggestions, achieve a separation which is good for noise isolation. 
 
Ralph identified the immediate problem of composite action (damage to an existing plaster ceiling), but you can also have longer term issues during occupancy in that the structural action of composite existing ceiling joists/new floor joists, in addition to problems he noted during during construction, can also produce problems during occupancy for an old plaster ceiling which remains in place. Typically, ceiling joists never receive their design loadings unless the space is used for storage.  Floor joists, on the other hand, can achieve nearly design level loadings from live load in local areas if one has a well attended party on the floor, or has a heavy bookcase, etc.  The additional loadings require very careful attention to deflections (Thus you probably won't attain the desired performance with a minimal composite situation). Remember, the code loadings are MINIMUM values.  I always design the floor to be stiffer than code levels. The cost differential is very small for this improved perfoprmance.
 
To summarize: isolate, isolate, isolate the new floor from the old ceiling.  And by all means provide a structural floor system that will not only meet the letter of the code but will provide good serviceability as well.  The cost difference to do it right is usually small and often results in overall savings as Ralph noted.
 
Regards, 
Bill Cain, S.E.
Berkeley CA
 
 
-----Original Message-----
From: Rhkratzse(--nospam--at)aol.com
To: hadiprawira(--nospam--at)yahoo.com; seaint(--nospam--at)seaint.org
Sent: Sat, 17 Sep 2005 12:42:57 EDT
Subject: Re: Converting ceiling joist to floor joist.

I believe the smart way is to build a new floor above the existing ceiling, which is left in place.  Space the new floor joists 1/2" or 3/4" away from the ceiling joists both vertically and horizontally with plywood spacers, so that when you nail the new floor sheathing to the new joists you don't damage the existing ceiling.  This is especially important with old plaster ceilings that can be knocked loose if you nail onto the joists.

This method provides a floor that is separate from the ceiling, which is good for sound separation.  Also it provides space for wiring to be threaded through.  Obviously it requires that the new joists run the same direction as the old joists if they are set down between each other, which may not always work.

Unfortunately, sometimes the "smart" thing to do is to just demolish the existing ceiling and build a new floor in its place.  (I say "unfortunately" because this uses more material and is thus wasteful, but it may be the fastest and maybe even the cheapest way to accomplish the job, especially if labor is expensive.)  On many of my jobs like this the people want to live in their home during the work and leaving the ceiling in place is a real plus.

Now, I believe what you're really asking is how to somehow strengthen existing 2x4 or 2x6 ceiling joists with something smaller than a new 2x12--possibly by adding a new 2x6 on top of an existing 2x6--to provide a floor joist.  I don't think that's either feasible or "smart" unless the labor is free and you don't care at all about squeaking floors.

Ralph Hueston Kratz, S.E.
Richmond CA USA

In a message dated 9/17/05 8:01:32 AM, hadiprawira(--nospam--at)yahoo.com writes:


Fellow Engineers,
 
A fellow architect was asking me, what is the smart way to add a new story on top of an existing structure without replacing/sistering the whole ceiling joist (2x8 @16"oc) to accomodate new loadings (need 2x12@ 16"oc) assuming the wall and the foundation below are adequate.
 
The question sounds very simple, but is it practical to build up a composite member from ex.  ceiling joist? what is the practical way to go? Anybody?
 
As always, thank you in advance!
 
Djohan.