Timber roof on a RC concrete slab.

[kellez]

Hi,

I am designing a timber roof to be supported by a RC concrete slab. As you can see from the picture the timber frame is simply composed
of inclined beams supported by timber columns that connect to the RC slab.

Now my question is, what is the best way to connect the timber columns to the Rc slab? I am thinking of steel brackets but not really sure.

 

[kellez]

Given the owner's desires regarding space usage, I'd get rid of the columns beneath the ridge and let the rafters cantilever up to a ridge board that is supported by the cantilevers without the posts.

Ok, therefore if i remove the timber columns beneath the ridge board, the rafters will actually carry the load of the ridge board, thanks for the advice

 

[dhengr]

Kellez:
My ‘bearing line’ is what you call a ‘wall plate,’ same thing. The wall pl./sill pl./sole pl. is bolted to the conc. slab, and then the rafters bear on the sill pl. with a horizontal seat cut, or birds mouth cut, and then they should also be tied down to the sill pl. Otherwise, you seem to understand basically what I’m suggesting. Assuming you have access to dimensional lumber and manufactured lumber like I-Joists and that your carpenters understand this kind of framing, this should be a more practical way to go. I would be tempted to frame the gable ends above the conc. slab with stick framed stud walls also. It’s easier to drop the top plates on these walls so the rake soffit ladder framing can cantilever out over that wall. Furthermore, doing that part of the gable ends, above the conc. slab with conc. just really keeps the conc. crew around for another cycle of forming and pouring, above the slab. Get them done and out of there after the slab and let the carpenters do their job above the slab. That’s less coordinating and crew congestion above the slab. Given your background, you would do well to get yourself a good timber/wood design textbook, and study it and some wood design codes.

 

[dik]

Now to remove the concrete floor and walls and use SIPs... so you have timberframe visible from the lower level.

Dik

 

[kellez]

Now to remove the concrete floor and walls and use SIPs... so you have timberframe visible from the lower level.

hahahah, well we are not used to timber frame structures in my country, we do love our concrete houses

 

[kellez]

Furthermore, doing that part of the gable ends, above the conc. slab with conc. just really keeps the conc. crew around for another cycle of forming and pouring, above the slab. Get them done and out of there after the slab and let the carpenters do their job above the slab.

I am actually planning to use masonry bricks for the gable walls and not RC concrete. i am also planning to tie the roof to the gable walls using timber as bracing embedded within the masonry of the gable walls.

 

[kellez]

Here is my latest design of the roof, I have replaced the 2 purlins with 1 stud wall and also removed the timber columns that supported the ridge board.
I have also increased the number of rafters with spacing at 50cm.
Now the rafters are supported by the wall plate at the roof edges and by the stud wall at midspan (almost midspan), the top of the rafters cantilevers on the ridge board

what do you guys think?

Keep in mind that the design of the timber members is not finished and the section dimensions will change during the design.
also the design of the stud wall is not finished and i will probably add some horizontal bracing at mid-height of the stud wall.
I will also create some openings with a header on the stud wall for access to the outer storage space under the exterior portion of the roof.
For the gable walls I will use masonry bricks and i am also planning to tie the roof to the gable walls by either using timber bracing embedded or use some steel brackets embedded.

 

[hokie66]

Makes sense to me. The gable should also be framed in timber, with the brick as cladding if desired. Typical brick veneer construction. You don't want single skin, unreinforced brick there.

 

[KootK]

I think you're there. What will the roof sheathing be in your market? If it's plywood, it would make sense to space your framing at 610 mm.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

KootK,

He can answer, but I would guess tiles on battens, thus no sheathing.

 

[kellez]

The gable should also be framed in timber, with the brick as cladding if desired. Typical brick veneer construction. You don't want single skin, unreinforced brick there.

I forgot to mention the type of bricks i am using for the gable walls, please see the picture below, all dimensions are in mm,
do you still think we cannot use single skin unreinforced bricks for the gable walls? The gable walls will also be tied to the roof frame

http://kebe-sa.gr/e/portfolios/no250/

 

[kellez]

What will the roof sheathing be in your market? If it's plywood, it would make sense to space your framing at 610 mm.

For roof sheathing I am using OSB3

Properties
8ft x 4ft
Length : 2.44m.
Width : 1.22m.
Thickness : 18mm.
Material : OSB.

Here are the details

I think i will need another layer of vertical battens between numbers 4 and 5 to hold the waterproof membrane down and also provide space for airflow beneath the tiles.


Why does it have to be at 610mm? Isnt the roof sheathing installed perpendicular to the rafters? as shown in the picture below?

 

[KootK]

In my area, we like the sheathing joints to land on top of the framing members. Hence the 2' spacing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

I can't really comment on your roofing system, because it is new to me. Is this customary in your country? I have never seen roof tile battens installed above rigid insulation and a membrane. How long are the batten screws to the rafters? I would be a bit concerned about the screws bending.

 

[kellez]

In my area, we like the sheathing joints to land on top of the framing members. Hence the 2' spacing.

Ok i understand however Some manufacturers suggest installing the sheathing perpendicular to the rafters.

I can't really comment on your roofing system because it is new to me. Is this customary in your country? I have never seen roof tile battens installed above rigid insulation and a membrane. How long are the batten screws to the rafters? I would be a bit concerned about the screws bending.

You have a valid point there, I am not really sure about the system either and i have not made up my mind yet, i am still working on it, i think this is mostly used on small roofs with no access, therefore, makes it easier installing the insulation on top of the sheathing.
I was thinking that if i could use this system it would be possible to save some money by not having to plasterboard the whole roof ceiling in order to hold the insulation in place.

installing the insulation above the roof sheathing is more straightforward and as you can see all thermal bridges due to the rafters are blocked.

 

[kellez]

Hello guys, any ideas on how thick the wall plate should be and what spacing for the anchor bolts shall i use??
I know i should use the reaction forces from the analysis to design these but i am just curious to see what is common in practice.

 

[Lomarandil]

Kellez, with 8'x4' sheets, you would still run them perpendicular to the rafters, but if your rafters are at 2' spacing, that allows the joints (the ones running up and down slope) to also be aligned with the framing.

----
The name is a long story -- just call me Lo.

 

[kellez]

Kellez, with 8'x4' sheets, you would still run them perpendicular to the rafters, but if your rafters are at 2' spacing, that allows the joints (the ones running up and down slope) to also be aligned with the framing.

Yes that is exactly what i am going with but i forgot to post an update about the spacing of the rafters.

I am using 2440mmx1220m (8'x4') boards, therefore, my spacing needs to be at 610mm (2').
When installed perpendicular to the rafters one board will span 4 openings, therefore, 4x610mm = 2440mm or if you will 4 x 2' = 8'


Wall plate and Anchor bolts

Any ideas on how thick the wall plate should be and what spacing for the anchor bolts shall i use??
I did find some info for anchoring stud walls on a concrete foundation but not for a timber roof.

it is suggested to Anchor sill/wall plates with 0.5in.(13mm) anchor bolts equipped with 0.229-in (6mm) thick, 3-in. by 3-in.(76x76mm) square plate washers.
Space the bolts from 32 in. (80cm) to 48 in. (120cm) on center. The IRC requires a minimum spacing of 6 ft. for houses subjected to wind speeds up to
110 mph, but tighter spacing greatly improves wall performance

 

[KootK]

For shear transfer purposes, wall plates 1.5" think are usually sufficient. Your detail out at the eves may require something thicker for geometric purposes though, I don't know. Bolt spacing is something that can be designed based on the load requirements.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kellez]

For shear transfer purposes, wall plates 1.5" think are usually sufficient.

I will probably use a wall plate of 2" (50mm) thick.


Rafters - Wall plate connection

For connecting the rafters to the wall plate, I will use a birds mouth cut on the rafter to sit on the wall plate,
then what are my options for the connecting the rafter to the wall plate?

shall i use L-shaped brackets on both sides of the rafter in addition to vertical nails (at an angle) passing through the rafter and wall plate?

is it ok if the heel goes down into the concrete slab?

 

[kellez]

After some research These are some of the options i found out regarding RAFTER - WALL PLATE connections, what do you guys think?
I personally prefer a combination of numbers 3 and 4 ---> 3) birdsmouth cut with L-shaped brackets in addition to 4) screws or nails


1) SIMPSON STRONG TIE Rafter-to-Wall Connectors for Solid Sawn Lumber NO NEED for birds mouth cut

https://www.strongtie.com/raftertowallconnectors_solidsawnlumberconnectors/vpa_connect/p/vpa

2) BEVELED WALL PLATE with no birdsmouth cut on the rafter.

3) Birds mouth cut with L-shaped brackets on both sides of the rafter

https://www.strongtie.com/deckconnectors_decks/ml_angle/p/ml

4) Birdsmouth cut with nails or screws.

5) Birdsmouth cut with h3 brackets on both sides

https://www.strongtie.com/hurricanetiesforplatedtruss_trusstiedowns/h-tsp_productgroup_wcc/p/h.tsp