Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Need some advice for a rafter tie splice

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Please respond to me privately at dennis.wish(--nospam--at)verizon.net unless you feel
that this e-mail has merit to anyone else who may benefit from it.

Problem:
A client has reframed the roof of a garage without a permit and must bring
the structure up to code through engineering. The method of construction
contains a few non-conventional connections that are creating a problem in
my retrofit design and detail.

1. The dimensions of the garage are 24-feet wide and 21-feet deep. The
structure is existing and while it probably does not comply with current
code for lateral design, the building department has only asked the owner to
have an engineer justify or retrofit the portion of the garage that was
modified - the roof framing. The weight of the new dead load materials is
actually less than the original design and the live load will not have
changed. Because the total load is less than or equal to what the original
roof was designed for, the building official is not asking that lateral
design be considered as the performance will not have changed other than in
the gravity load design.

2. The original roof was a flat roof and this was removed and replaced with
2x6 rafters @ 16" o.c. sloped to 1.5" per foot. The contractor installed a
2x12 DF#2 ridge board to span the 21-feet and believed this to be adequate
as a ridge beam. He did not install any ceiling joists/ties. Therefore there
is an outward thrust on the exterior walls and the potential for the 2x12
ridge to deflect as much as 5-inches under the weight of the new foam roof.

3. The 2x6 DF#2 rafters are extended over the bearing walls to create a
21-inch eave. The 2x6's were end nailed through the 2x12 on one side. The
2x6 on the other side of the 2x12 was aligned end to end with the other 2x6
and toe-nailed through the 2x12 into the sides of the 2x6.

4. The 2x6 was not seat cut on the double 2x4 bearing wall and sits at a
7+degree angle (1.5:12). Solid blocking was installed by at the same angle
of the 2x6 rafters and set with the inside edge of the 2x6 block at the
outside edge of the 2x4 stud wall. The 2x6 rafters were connected to the 2x4
double top plate using a Simpson A34 or A35 and the blocking was toe-nailed
to the rafters.

5. The roof was sheathed with 19/32" APA rated exterior glue plywood and
nailed to the rafters at 6:6 nailing using 8d sinkers as I suspect rather
than common nails.

6. The panels were not blocked but the pattern was laid properly with the
ends of panels on the rafters and each row of panels staggered. Field
Nailing was at 6" o.c. rather than 12" required.

Intended Corrections:

A. The weight of the foam applied over the 19/32" plwd is only 0.25-psf for
1-inch of foam and only a maximum of 1-inch thick foam can be applied before
it must be stripped off and replaced. The panel nailing is considered
adequate.

B. The slope of the roof with the 2x12 DF#2 Ridge board still allows me
clearance to install 2x6 DF#2 rafter ties. The problem is the tension in the
ties will require a rafter tie at every rafter. Based on a live load of 20
psf and a minimum dead load of 10-psf (much higher than actual) the reaction
at the bearing walls is 556-lbs and because of the low slope of the roof,
the horizontal load that causes thrust in the exterior walls is 4436.88
pounds.

C. This requires the splice of (2) 2x6's to make up the 24-feet across the
garage. I can face nail the tie to the rafters with 16d x 3" long nails and
following the AITC requirement for single shear with Z=133 pounds per nail
and a permanent wind or seismic load factor Cd of 1.6 and because we are
here in the desert where over half of the year the temperature can be easily
over 110 degrees in an unconditioned space, the Ct factor is 0.8. Therefore
I would need 26 nails (Hankinson's formula was not considered for nailed
connections according to my interpretation of the AITC Table 7.52 for Common
Wire Nails).  Simpson makes a 16d nail that is 3-inches long so embedment is
not an issue as much as shank size since 10d nails would require almost
twice the number at each connection.

D. HERE IS THE KICKER OF THE PROBLEM - THE RAFTERS ARE ALL ALIGNED ON EACH
SIDE OF THE 2X12. THE FRAMER IS NOT ADVERSE TO ADDING A SIMPSON LSU26
(SLOPED BOTTOM FLUSH JOIST HANGER) ON THE SIDE WHERE THE NAILS ARE
TOE-NAILED TO THE 2X12, BUT I MUST SPLICE THE 2X6 RAFTER TIES END TO END AND
THE LOAD IN TENSION/COMPRESSION IS 4,436.88 POUNDS. I DON'T WANT TO SISTER
ON A PARTIAL 2X6 TO ONE SIDE SINCE IT IS LATERALLY UNSUPPORTED AND I WORRY
ABOUT TWIST IN THE TIE AT THE CONNECTION WHETHER I USE NAILS OR SCREWS - SO
I WANT TO USE A BOLTED CONNECTION BUT EITHER NEED TO FABRICATE A PLATE FOR
EVERY SPLICE OR USE A SIMPSON STRAP CONNECTION (MY THOUGHT IS A STRAP ON
EACH SIDE AT TOP AND BOTTOM OR FOUR STRAPS TOTAL PER SPLICE) TO PICK UP THE
4436 POUND LOAD.

Now I need some advice. I will come back in and connect the blocking to the
top plate of the wall using a Simpson RBC clip bent to the angle of the
block to top plate to restore any shear transfer (the plywood is boundary
nailed to the blocking).

Can anyone help me with a reasonable solution to the rafter tie splice
problem? I spoke to the framer and he has no trouble with the nailing of the
ties to the rafters using a palm nailer. He can do it quickly and easily. He
also has no problem with a splice or adding the hanger on one side since
this would repair the problem he caused much less expensively than tearing
it all out. 

NOTE: I SUGGESTED AT CONTINUOUS TIMBERSTRAND 1-1/2" MEMBER BUT HE FEELS THAT
THE COST OF THE LUMBER AND SPECIAL ORDER WOULD BE MUCH MORE EXPENSIVE FOR
HIM THAN TO MAKE THE SPLICES IN THE FILED. 

I am late on getting this out because of the small issues in his method of
construction that created a much more difficult means of installing the ties
as well as the high tension load to the tie due to the very low slope of the
roof. I can use some advice but I need it quickly and will check incoming
e-mail much faster than I do the list digest mode. SO PLEASE E-MAIL ME
DIRECTLY AT DENNIS.WISH(--nospam--at)VERIZON.NET.

Thank you,
Dennis

P.S. THE ENDS OF THE GARAGE ARE GABLED ENDS AND ARE NOT A PROBLEM. THE
FRAMER THOUGHT THE 2X12 SPANNING 21-FEET WOULD BE SUFFICIENT AS A BEARING
RIDGE UNTIL I WAS CALLED IN TO EXPLAIN THE DIFFERENCE BETWEEN A RIDGE BEAM
AND A RIDGE BOARD. ALSO I TRIED TO RUN THIS WITH THE TIES AT 4'-0" BUT AS
YOU CAN EXPECT, THE TENSION/COMPRESSION IN THE SPLICE AN AT THE RAFTERS WAS
OVER 12,000 POUNDS.


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********