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Old 07-25-2012, 02:55 PM   #1
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Strengthen Ceiling Joist


I have read several threads about different ways to strengthen the ceiling joist, but they do not address my situation, so I'll post again.

Our house is 80 years old with the ceiling joist 2x4, 16"OC span 26' cross with a bearing wall in the middle. Thus, the span is about 12.5'.

We want to strengthen the joist and put 1/2"-3/4" plywood to make floor. Here are our possible options:

Option 1. Sistering with 2x8
Option 2 add 2x6 on top of the existing 2x4 with glue and Simpson LPT4 strong tie (2' spacing)
Option 3- Option 2 instead of Simpson LPT4 strong tie, use OSB or Plywood to joint the 2 pieces

The disadvantage of option 1 - sistering is not possible in some of the joists. Option 2&3 is structural doubtful but would be the easiest and cheapest approach.

The ceiling is about 7' high, so ideally, I would like to have a library up there (very heavy load).


What would be the best approach to strengthen the joist with a minimal cost?

Are there better options?
Thanks,

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Old 07-25-2012, 03:24 PM   #2
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Strengthen Ceiling Joist


Structurally, new solid joists (as in option 1) would be better than trying to deepen the existing joists by adding pieces on top.
Aside from that, 2x8 on a 12'6" span may be pushing it. Your local code will stipuate loadings to be allowed for. As you are putting a library up there, your loads may be considerably higher than normal domestic loading (you are doubtless aware of how heavy stacks of books are). Personally I would think at least 2x10.
Will the top plates of the existing stud walls (external and internal) take the increased loads?

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Old 07-25-2012, 03:27 PM   #3
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Strengthen Ceiling Joist


Library can be a heavy load.
Do you need permits?
Talk to the building inspector.
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Old 07-25-2012, 03:31 PM   #4
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Strengthen Ceiling Joist


I worked on a job removing a roof and adding a second floor. At the time an engineer said something like option 3 was good enough. Another 2x glued to the top of the first one, and 3/8 " ab plywood strips, slightly smaller than the total height, glued and screwed to each side, to both joists, staggering joints 4' on each side. Codes have changed since then so you should check if it is OK now.
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Old 07-25-2012, 03:56 PM   #5
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Strengthen Ceiling Joist


We understand that 2x6 on top of 2x4 is not equal to 2x10 but how is the strength of the two combined in comparing to a single 2x8?

If we were

Tony mentioned that 2x8 is already reached the limit for a 12.5' span.

It is not likely that I'll have a complete library up there but probably boxes of old college text books and other old books.

If the two combined is as strong as 2x8, I would prefer that option for I would add insulation to it, so 9.5" of the combined joists is better than 7.5" of 2x8.

additional comments would be appreciated
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Old 07-25-2012, 05:09 PM   #6
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Strengthen Ceiling Joist


Quote:
Originally Posted by cocxanh View Post
We understand that 2x6 on top of 2x4 is not equal to 2x10 but how is the strength of the two combined in comparing to a single 2x8?

One 2x8 on its own would be about 40% stronger than the 6" on top of the 4", assuming that one just sat on top of the other, with no connection at all.
However, if they are joined together with either metal or plywood straps, their combined strength will be considerably enhanced. It could then approach that of the 8", though much would depend on how effectively they are joined together.
If you chose this method, your Inspector could ask for details/calcs from an SE, which will cost. It might be cheaper in the long run to use new 2x10.
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Old 07-25-2012, 05:21 PM   #7
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Strengthen Ceiling Joist


Unfortunately it is complicated to compute the strength of a composite beam (that is what you call it when you combine two separate elements together). The reason it is complex is that the strength of the beam depends to a large extent on the strength of the joint between the two elements. In your case, if you glue a 2x4 on top of another 2x4, you will have a beam that is 1.5 inches wide and about 7-1/4 inches tall, depending on the exact dimensions of the 2x4. This is roughly the size of a 2x8, so the question is, why would the composite beam not be as strong as the solid 2x8?

The reason is that the two 2x4's tend to slide apart at the point of connection, in this case at the midpoint of the composite beam. The force tending to pull them apart is called horizontal shear. If the glue is strong enough to resist the horizontal shear, then the two beams act the same as a 2x8. If the glue is not strong enough, the glue will crack, and the combined strength is only slightly greater than the strength of a single 2x4. So in building composite beams as you propose, the critical design feature is to make sure that the horizontal shear that will develop when you load the beam with your books can be handled by the glue, or the OSB, or plywood, or whatever method you use to fasten the 2x4's together.

It is notoriously difficult to predict the properties of glue over the long term, witness the catastrophic failure of glued in place bolts holding up ceiling panels in the Central Artery tunnel in Boston, which lead to a fatal accident involving a massive concrete panel falling onto a motorist. Obviously your situation is different, but I never recommend glue alone for a critical application such as yours, and in fact standard engineering practice is to assume that the glue is at BEST useful in increasing the horizontal shear safety factor, but should be IGNORED for purposes of design.

You need to devise a connection strategy that provides full horizontal shear capacity for the connection. This would typically require nailing or screwing in position plywood, using sufficient fasteners closely enough spaced to meet the required shear criteria set forth in your local building code.

This is more work than most people are willing to undertake, hence the standard of practice is to replace the 2x4 with a properly sized sawn joist, or possibly use an LVL or an I joist if you need the extra strength. You can sister the new joist in place, but you will gain virtually no strength from the 2x4, so it may not be worth it, as you still need a proper connection to the walls on either end.
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Old 07-25-2012, 06:36 PM   #8
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Quote:
Originally Posted by cocxanh View Post
If the two combined is as strong as 2x8, I would prefer that option for I would add insulation to it, so 9.5" of the combined joists is better than 7.5" of 2x8.

additional comments would be appreciated
if you have room (height) for 9.5" go with a sistered 2x10 and call it a day. will give you the strength you need for live loads and be quicker that piecing together various parts to try and create a new floor assembly.

just my humble opinion ....

Good luck!
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Old 07-25-2012, 09:20 PM   #9
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Strengthen Ceiling Joist


A few things - adding onto the top of the 2x4 and getting a splice connection on both sides will depend on compatible lumber thicknesses. framing lumber over time has shrunk in size and sawmills 80 years ago produced thick 2x stock. You might have to plan on splicing just one side of the built up joist if that would even be strong enough. also the 12'.5" ceiling span there now probably has a definite deflection and putting straight lumber on top of a deflected 2x4 joist causes more issues.

2x10 sistered in next to the 2x4 joists would be ideal but if finished upstairs ceiling height is an issue consider double joists for the floor.
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Old 07-25-2012, 09:31 PM   #10
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Quote:
Originally Posted by Daniel Holzman View Post
....The standard of practice is to replace the 2x4 with a properly sized sawn joist, or possibly use an LVL or an I joist if you need the extra strength. You can sister the new joist in place, but you will gain virtually no strength from the 2x4, so it may not be worth it, as you still need a proper connection to the walls on either end.
We would not want to touch the existing joists for they would create a disaster on the existing ceiling (this is 80 years old with stucco style - concrete and wood). Our floor joists are only 2x10, so remaking the the attic with 2x10 for storage purpose would be over kill.

We probably go with the stacking of 2x6 on top of the 2x4 with proper fasteners and glue, and be attentive to the storage load. One of main purposes in doing was to be able to proper insulate the attic and make use of the space.

Since the attic is less than 84" hight, it cannot be converted to a living space (according to code), so we don't think the inspector is going to be too picky about using it as storage.

We have learned much from all the comments. Thank you all!
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Old 07-25-2012, 09:42 PM   #11
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Strengthen Ceiling Joist


so sister on a 2x8 .... would be a lot faster and lot stronger. according to the International Residential Code if attic storage is accessed by means of fixed stairs the attic is to be designed for 30 pounds per square foot for live load plus the weight of the materials for dead load.

See the link below for the span tables from the 2009 International Residential Code and the 30 pounds per square foot live load requirement for attics accessed by fixed stairs. Your code may differ.

http://publicecodes.citation.com/ico...002_par017.htm

Good luck!
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Old 07-26-2012, 11:38 AM   #12
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Cocxanh; The best way to do your floor would be to use new joists as others have advised. However, if you are dead against this and just want to add joists on top of the existing ceiling joists, read on.
You could connect them with plywood but, as one poster said, new and old joists will be different thicknesses.so plywood can only go on one side. This might make the joists squeak. You also have to get the nailing/screw pattern right.
I deepened the ceiling joists in my own home some years ago, as on the attached sketch.
The sizes are different to yours, as the spans are slightly less, but the principle could be modified. Basically, I glued and screwed an additional joist on top using 4" x 12s guage screws at 12" centres. I had to bore 1" diameter holes to get to the right depth for the screws. The floor is now good and stiff and well-suitable for storage, and there has been no deflection of the ceiling downstairs.
(Needless to say, I didn't advise our local authority, but it doesn't matter now as its well out of time!)..
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Strengthen Ceiling Joist-mini-scan0172.jpg  
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Old 07-27-2012, 07:18 PM   #13
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Quote:
Originally Posted by cocxanh View Post
We understand that 2x6 on top of 2x4 is not equal to 2x10 but how is the strength of the two combined in comparing to a single 2x8?

If we were

Tony mentioned that 2x8 is already reached the limit for a 12.5' span.

It is not likely that I'll have a complete library up there but probably boxes of old college text books and other old books.

If the two combined is as strong as 2x8, I would prefer that option for I would add insulation to it, so 9.5" of the combined joists is better than 7.5" of 2x8.

additional comments would be appreciated
It may be close to a 2 x 10 because of the layer of plywood gusseting each side. The engineer on our job wanted every component glued together. You could probably add solid bridging, (blocking), mid span transferring the load to several joists at a time. Because we had wires over the top of the first set we added 3/4" x 1-1/2" with a space for each wire to avoid any notching.
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Old 07-27-2012, 08:28 PM   #14
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Strengthen Ceiling Joist


cocxanh,

check out this section from the 2009 International Residential Code (basis for the Cali Building Code). If your attic is accessed by fixed stairs it must be designed to carry 30 pounds per square foot live load. You can check with your building department for your specific requirements.

http://publicecodes.citation.com/ico...002_par017.htm

This link will give you the span table for joists.
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Old 11-07-2012, 01:27 PM   #15
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I take "challenged" attics, and make them wonderful.

Here are two examples I will address where I found cracked 2x6 floor beams, and repaired them as composite 2x10:
Laurelhurst Attic
NW Portland Attic

Here is one of many examples where I strengthened 2x4 bottom elements of a truss attic, to carry loads as 2x10.
https://docs.google.com/open?id=0Byj...GUxcy00SVU4bU0
A 2x4 bottom element can NOT carry load, and if better beams aren't equal to 2x10, they are yet good enough.

In doing what I must, to the best of my ability, I have not yet felt compelled to have my methods passed by a structural engineer. As a career mechanical engineer, my PE license, never used, expired. Yet, I feel I have basis for confidence in what I do. In addressing this forum, I hope to get broad approval useful to others, as from commenting current PE's.

Here are photo examples, with captions:
https://picasaweb.google.com/1085337...teBeamExamples

Will readers please offer constructive comment upon my methods? May we share in applying my examples.

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