Smart questions
Smart answers
Smart people
Join Eng-Tips Forums
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

jmme (Civil/Environmental) (OP)
26 May 11 14:39
Currently there are 5" dia. logs (18.5 feet long) spaced 36" o.c. being used as rafters for a building with a 12:12 pitch and a metal roof.  The owner wants to add a sheetrock ceiling so he wants to sister a 2x to the log so he can run his furring strips and install sheetrock.  Below are my calculations based on removing the logs and installing regular 2x rafters.

Ground snow load = 55 psf
Sloped Roof snow load = 17.3 psf

W = 17.3 (LL) + 10 (DL) x 3' spacing = 81.9 #/ft.
M = (81.9 x 18.5^2) / 8 = 3504 #-ft = 42,045 #-in.

This would require two 2x10's.  However, the 5" logs will carry some of the load.  What load can a 5" log handle?  

Am I going about this the right way?  Thanks.  
boo1 (Mechanical)
26 May 11 14:55
is the span 37 feet?
a2mfk (Structural)
26 May 11 14:56
I don't understand why you are basing the calcs on removing the logs?

I believe you could just leave the logs alone and then design 2x rafters that are only for supporting the gypsum board, therefore you are not modifying the roof other than the ridge beam (assuming there is one).

Sistering 2xs to round logs has its complications I would think.
BAretired (Structural)
26 May 11 15:08
How did you arrive at 17.3 psf snow load?  
How are the rafters supported at the top?
Are the existing rafters tied?
How many years have the 5" logs been supporting the roof?
Could the metal roof be considered an unobstructed, slippery slope?

BA

jmme (Civil/Environmental) (OP)
26 May 11 15:21
The span is 18.5' and the metal roof can be considered an unobstructed slippery slope.

I arrived at the 17.3 psf snow load by using the following
Pf=0.7 x Ce x Ct x I x Pg
Pf=0.7 x 0.9 x 1 x 1 x 55
Pf=34.65 psf

Ps=Cs x Pf
Ps=.5 x 35.65
Ps=17.3 psf

There is a collar tie at the top of the rafters.

The logs have been supporting the roof for 20+ years.
a2mfk (Structural)
26 May 11 16:01
So collar ties, without a ridge beam?

I think I'd design a ceiling support system independent of the roof. That way you are not touching the logs, which like you said have been there for 20+ years...

However, if your calcs show the logs are way undersized, you now have some responsibility to correct this, depending on your local code requirements and the amount they are overstressed. At the bare minimum, you should tell the owner in writing your results.
MiketheEngineer (Structural)
26 May 11 16:06
All good ideas.  But ONLY God knows what the logs can carry!!  You could compute the Section Mod and assume say 750 psi for fiber bending stress(pretty conservative) and go from there.
BAretired (Structural)
26 May 11 17:00
I of 1 - 2 x 10 = 1.5*9.25^3/12 = 98.9 in^4

I of 2 - 2 x 10 = 198 in^4

I of 5" log = pi*5^4/64 = 30.7 in^4

If the dimension lumber is sistered to the existing logs, the logs will carry very little because they are not anywhere near as stiff.

It is surprising that 5" diam. logs at 3'-0" centers have survived  for the last 20 years, but I don't think the logs should be deemed to contribute to the strength of the roof.  You could place a 2x10 each side of each log and nail them together but you will not be easily able to provide for air circulation within the ceiling space.

Perhaps the best option is to replace the existing roof with new construction.

BA

dhengr (Structural)
26 May 11 18:53
Jmme:

There's lots of missing information in your post so far.  I'd like to see some sketches: particularly the conditions and details of the log rafters at the ridge, collar ties and their distance below the ridge, and the rafter bearing detail at the ext. walls, all with some dimensions and member sizes.  What's acting as a rafter tie at the top of the walls, the rafter bearing elev.?  Are the rafters sway-backed after 20+ years?  Are there 2x4 purlins at 2' o/c, or some such, on top of the rafters for the metal deck attachment?  If the roof is a 12/12 pitch and the rafters are 18.5' long, then the bldg. is about 26' wide, and the rafter run is 13'.  The rafter span length is the same as the run for this bending calc., and the loads should be in lbs./sq.ft. on a horiz. projection.  The snow load already is assumed to be on a horiz. projection, but the DL's have to be converted from loads in the roof plane to loads on a horiz. projection.  So, the DL's will be (1.414)( 10 lbs./sq.ft.  or some such).

Are you going to vent this roof, or insulate it?  You may want the new rafters dropped to allow for venting, insulation and a vapor barrier.   I would want to study how I was going to detail the new dimensional rafters at their bearing on the walls, the ridge, around the collar ties, etc.  Rafter ties, again?  Do I want the log rafters and the new 2x rafters to act together, or as separate systems?  You can get lots of 2x rafters at 16" o/c btwn. the 3' o/c log rafters if you lay them out right.
 
HeavyCivil (Structural)
27 May 11 8:41
I know that proper design dictates either collar ties or a ridge beam, but there are rafter systems that can work with neither.

Balloon framed, timber framed, or platform framed where rafters do not bear on a knee wall all properly tie the first story frame/framing with floor joists. If rafters can be fastened at their bearing point such that outward loads are resisted, they're okay. Obviously this only works at more modest spans.

My house has stood up to 70 psf pg for 75 years with 5" half-sawn log rafters , no ridge pole, no collar ties. It's hard to justify messing with something that has worked so long, especially when my homeowners (not professional liability) insurance is the only thing on the line.   
a2mfk (Structural)
27 May 11 9:29
VTEIT "It's hard to justify messing with something that has worked so long, especially when my homeowners (not professional liability) insurance is the only thing on the line."

Well, that and your neck...

But I see your point, and I too have seen many older residential roofs without much in terms of collar ties or ridge beams, only the ceiling acting as a bottom chord. One of the reasons I suggested leaving the roof alone and framing out a separate ceiling as an option. But others have brought up points about insulation, ventilation, etc. that the homeowner may not have fully considered.
jmme (Civil/Environmental) (OP)
27 May 11 13:05
Thanks for all the replies.  There is going to be closed cell insulation installed so ventilation will not a problem.   
jmme (Civil/Environmental) (OP)
27 May 11 15:43
Here is a picture looking up at the existing structure.  I will add a couple more photos from different views.  Thanks.
jmme (Civil/Environmental) (OP)
27 May 11 15:45
Here is another picture showing the bottom of the log rafters.   
BAretired (Structural)
27 May 11 19:16
jmme,
You asked if you were going about this the right way and I think the answer is no.  The existing rafters are tied at mid length by a piece of dimension lumber.  The bottom of the rafters bear on a timber beam which seems to be tied to the corner of the building and may be tied to another wall about ten feet over from the outside wall.

If the top of the wall is laterally braced, the rafters, under snow load will carry a combination of compression and bending.  If the top of wall is not braced, the rafter moment is much larger.

I suggest you draw a cross section of the building complete with dimensions, then decide whether or not the wall is braced laterally at the bearing point of the rafters.  Only then can you perform any meaningful calculations.  

BA

jmme (Civil/Environmental) (OP)
28 May 11 10:10
BAretired, thanks for the input.

Is the dimensioned lumber doing much from a structural aspect?  The owner would like to remove the dimensioned lumber so he can have a catherdral ceiling.  Would it be better to just move the dimensioned lumber up higher and have his ceiling be a trapezoid shape?

The pitch is 12:12 and it is a metal roof so the snow is not going to stay on the roof.   
BAretired (Structural)
28 May 11 12:25
The collar ties prevent the rafters from spreading, so they cannot be moved up or removed without endangering the structure.

BA

jmme (Civil/Environmental) (OP)
28 May 11 13:04
What if larger (2x6 or something) were used for the collar ties?  Could they be moved up then?  Is there somewhere I can get more information regarding collar ties and where they should be located?  Thanks.
jmme (Civil/Environmental) (OP)
28 May 11 13:10
As the homeowner doesn't want to spend a lot of money I am thinking instead of making it a cathedral ceiling he would be better of just installing some framing material to attach the sheetrock to.  Would it be ok for him to install 2x6 sistered to the logs and have the ceiling be at the height of the existing collar ties?  Basically the 2x6 would be in a trapezoid shape and then the furring strips/sheetrock would be attached to them.  This wouldn't have any affect on the logs supporting the roof would it?  
BAretired (Structural)
28 May 11 15:23
The ceiling adds weight to the existing roof which increases the compression and bending in the existing rafters and increases the tension in the collar tie.  

You are playing with dynamite if you add any weight to a structure like that.  The best advice you can give the homeowner is to have it analyzed by a structural engineer.

BA

jmme (Civil/Environmental) (OP)
28 May 11 23:08
Thank you.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close