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Timber roof on a RC concrete slab.

Timber roof on a RC concrete slab.

Timber roof on a RC concrete slab.

(OP)
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.





RE: Timber roof on a RC concrete slab.

Hi, You can anchor your timber posts to concrete floor as below.

Also you can make a slot at the bottom of your post and connect using steel connectors as below.

RE: Timber roof on a RC concrete slab.

Depending on your uplift, you could use a light gage clip on each side of the post similar to this...
https://www.steelnetwork.com/Product/StiffClipCL

Or you could just buy a bolted base like a Simpson ABW.

RE: Timber roof on a RC concrete slab.

I'd go with a Simpson post base, like the ABA...link These are available in N America and elsewhere.

A 'U' shaped steel bracket would also work, or simply steel angles anchored to the floor and post base, as long as you can avoid direct contact between raw wood and concrete.

RE: Timber roof on a RC concrete slab.

I've used the lower one for concealed fasteners with 2 - anchor rods into the concrete. all hidden... worked well. Any chance you can make those timber roof members into trusses and cut back on some of the vertical elements?

Dik

RE: Timber roof on a RC concrete slab.

(OP)
Thank you everyone, i think i will go with something similar to Simpson AB base.

Quote (kipfoot)

A 'U' shaped steel bracket would also work, or simply steel angles anchored to the floor and post base, as long as you can avoid direct contact between raw wood and concrete.

One more question is it really necessary to keep the timber out of contact with the concrete in a location such as the roof slab?

Quote (dik)

Any chance you can make those timber roof members into trusses and cut back on some of the vertical elements?

Initially my design was composed of roof trusses but the client changed his mind and asked for usable roof space, therefore i left an empty space in the middle of 2.50m.
I decided to also remove the bottom chord of the truss so that the concrete slab can be used as the floor without worrying about the bottom chord.

Having removed the bottom chord and all diagonal bracing form the truss, I now have only vertical elements, which i think is not sufficient to provide the required lateral stability of the frame.
I now need to think of a way to provide additional lateral support to the frame, i think my options are

1) Decide on the direction of the connection of the timber post with the concrete slab.
2) Decide on the connection between the timber post and timber chord (beam)
3) Decide on the connection where the two timber beams meet at the peak
4) Add diagonal bracing between the two timber posts.





RE: Timber roof on a RC concrete slab.

As long as the rafters are attached to the slab at the roof edge and you have a roof diaphragm, you are stable.

added edit - and it would be helpful to have collar ties above the center two posts

RE: Timber roof on a RC concrete slab.

Can you put a couple of supports at the roof with a beam at the ceiling level and maybe a couple of columns supporting the ceiling beam. A large number of posts on the main floor will quickly become a pain...

Dik

RE: Timber roof on a RC concrete slab.

(OP)

Quote (dik)

Can you put a couple of supports at the roof with a beam at the ceiling level and maybe a couple of columns supporting the ceiling beam. A large number of posts on the main floor will quickly become a pain...

Dik

Hmmm, i see what you mean, too much drilling on the main floor...currently i have 48 timber posts that need to be fixed on the concrete floor.Too much drilling and too many Simpson bases
I think i need to halve that.

Ok so you are suggesting to use a main beam running from one gable wall to the other gable wall with a couple of timber posts for support.
what do you suggest i do with the current 48 timber posts? shall i keep only one in the middle of the main rafter?


RE: Timber roof on a RC concrete slab.

You probably need a ridge board as well.

I would replace the lines of columns with lines of beams, then support said beams at a minimum 8ft on center. The exterior rafter eave condition will probably need a (2)2x6 PT nailer. Then use hurricane ties here.

RE: Timber roof on a RC concrete slab.

(OP)

Quote (EngineeringEric)

You probably need a ridge board as well.

due to the fact that the client asked for a usable space, instead of using a ridge board (that needs timber posts in the middle of the roof)
i decided to use bracing connecting all rafters to one another (see picture, its an update of the original design). Additional bracing will also be provided by OSB sheathing.

However I will have to reconsider the design due to big number of timber posts used.

Quote (EngineeringEric)

I would replace the lines of columns with lines of beams, then support said beams at a minimum 8ft on center. The exterior rafter eave condition will probably need a (2)2x6 PT nailer. Then use hurricane ties here.

I will consider this with my new design.



RE: Timber roof on a RC concrete slab.

Too many posts cluttering up the floor space...

Dik

RE: Timber roof on a RC concrete slab.

(OP)

Quote (dik)

Too many posts cluttering up the floor space...

Yes this is the main issue with my design above, I will try to reduce the number of timber posts.

Now i see were I went wrong with the design...I actually tried to reproduce the design of a trussed rafter but removing certain components that actually were not required due to the concrete slab below and i ended up with too many posts that need to be fixed on the concrete floor which is too much work.

RE: Timber roof on a RC concrete slab.

Kellez:
Cast a 6" wide by 3.5" high curb with and around the RC slab. That uses a 2x4 for the inside forming. Then use 2x8's (dimensional lumber) for roof rafters at 24" o/c, with a ridge board. The rafters have a vert. plumb cut and a horiz. bearing seat cut to fit the curb and slab, and distribute their vert. and thrust reactions. Scab on 2x4 or 2x6's as tails for the eave overhang. At the gable ends, set the first interior rafter and the gable end framed walls 3.5" lower than the roof plane of the other rafter tops. Then build a 2x4 ladder which cantilevers out for the rake soffit and trim. Since you didn’t bother to tell us anything about loads, building dimensions, all the important design info. the above is for concept only. Why not give that important design info. if you want meaningful answers, remember we can’t see exactly what you’re thinking. What training, and experience do you have, and where did you get it, regarding structural design in wood and concrete and their construction methods?

RE: Timber roof on a RC concrete slab.

Why not just use a typical stud bearing wall on each side of the 2.5 metre wide usable space? Just bolt down the bottom plate, and tie the studs and rafters down as you would for any typical wall.

RE: Timber roof on a RC concrete slab.

(OP)

Quote (dhengr)

Why not give that important design info. if you want meaningful answers, remember we can’t see exactly what you’re thinking. What training, and experience do you have, and where did you get it, regarding structural design in wood and concrete and their construction methods?

Hi dhengr, I have an MSc in Civil Engineering and the only experience I have is with RC concrete structures and seismic design of RC structures. I have not designed a timber roof before and i have not seen anything up close similar to the one i am designing. However, I can do the modeling analysis and design once i decide on the layout of the roof.

New Design

After all the comments I have changed my design

As you can see I have used purlins to support the weight of the rafters and reduced the number of timber posts which support the timber purlins.
Now I have 20 timber posts instead of 48 as with the old design
I have also included a ridge beam with ridge colar which is supported by timber columns, Please have a look at the pictures below.

What you guys think about the new design, can you see any major issues? Please, guys, try to help me out a bit more with your advice, i really need your help with this one.


Roof Dimensions

Roof Length = 12.15m
Roof Width = 10.00m
Roof Height = 2.60m

Loadings on the roof are:

Permanent Loads:
1. Selfweight of timber frame
2. Roof coverings = 0.93kN/m2

Wind Loads
3. Wind Pressure = 0.80kN/m2
4. Internal wind pressure = 0.15kN/m2

Imposed Loads
5. Roof (Category H) = 0.4kN/m2











RE: Timber roof on a RC concrete slab.

Kellez:
Do you have roof sheathing material which will span about 4' (12.15m/10)? Why are you insisting on some form of timber framing, wouldn’t stick framing with dimensional lumber be less expensive and less complicated? Why not put a framed bearing wall at about the quarter point of the roof span, about 8' from the exterior rafter bearing (10m/4)? At this location, you are pretty much out of useful headroom up in that space, with only about 4' clearance. Then frame the roof with 20' - 2x8's, or some such, at 2' o/c, and continuous over the framed bearing wall; or two 2x8's, lap spliced over the bearing wall. Full length, 20' +/-, I-Joists would work for the rafters too. Now, the center part of the space can be column free, a much more useable space, and you really only need a ridge board up at the ridge. The framed bearing wall has studs at 2' o/c, and can have a couple 4' openings with a header for access into the outer storage space under the exterior portion of the roof. You must pick up some roof framing thrust and other lateral loads out at the exterior bearing line.

RE: Timber roof on a RC concrete slab.

(OP)
First of all thanks a lot for your time, I really appreciate this, i have learned a lot from these Forums and its people like you that make this such a great place. Thank you


Quote (dhengr)

Why not put a framed bearing wall at about the quarter point of the roof span, about 8' from the exterior rafter bearing (10m/4)? At this location, you are pretty much out of useful headroom up in that space, with only about 4' clearance.

Ok i understand that you want me to remove the 2 purlins that run across the two gable walls and replace them with a load bearing stud wall at a quarter of the roof span 10/4m = 2.5m


Quote (dhengr)

Do you have roof sheathing material which will span about 4' (12.15m/10)?

Quote (dhnegr)

Then frame the roof with 20' - 2x8's, or some such, at 2' o/c, and continuous over the framed bearing wall; or two 2x8's, lap spliced over the bearing wall. Full length, 20' +/-, I-Joists would work for the rafters too.

You also want me to increase the number of rafters (and decrease their size) so that the span for the OSB board is also reduced.

Quote (dhengr)

You must pick up some roof framing thrust and other lateral loads out at the exterior bearing line.
I did not quite get that, by exterior bearing line you mean where the rafters sit in the RC slab? (see picture below)








RE: Timber roof on a RC concrete slab.

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.

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.

RE: Timber roof on a RC concrete slab.

(OP)

Quote (KootK)

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

RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

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

Dik

RE: Timber roof on a RC concrete slab.

(OP)

Quote (dik)

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

RE: Timber roof on a RC concrete slab.

(OP)

Quote (dhengr)

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.

RE: Timber roof on a RC concrete slab.

(OP)
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.









RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

KootK,

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

RE: Timber roof on a RC concrete slab.

(OP)

Quote (hokie66)

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/



RE: Timber roof on a RC concrete slab.

(OP)

Quote (KootK)

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?




RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

(OP)

Quote (KootK)

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.

Quote (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.

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.

RE: Timber roof on a RC concrete slab.

(OP)
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.


RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

(OP)

Quote (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.

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




RE: Timber roof on a RC concrete slab.

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.

RE: Timber roof on a RC concrete slab.

(OP)

Quote (KootK )

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?

RE: Timber roof on a RC concrete slab.

(OP)
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_s...




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_...




4) Birdsmouth cut with nails or screws.




5) Birdsmouth cut with h3 brackets on both sides
https://www.strongtie.com/hurricanetiesforplatedtr...




RE: Timber roof on a RC concrete slab.

Has this thread ever evolved... I'd move the membrane between layers 2 and 3. I'd also avoid contact between concrete and lumber/timber and use PWF material to minimise the chances for dry-rot.

Dik

RE: Timber roof on a RC concrete slab.

For the rafter connection, I'm partial to option 5 to be honest. And please review your birdsmouth cut in your rendering, there doesn't appear to be much meat leftover. Your overhang could cause failure of that narrow section.

RE: Timber roof on a RC concrete slab.

birdmouths are normally restricted to 1/3 the member depth...

RE: Timber roof on a RC concrete slab.

(OP)
The birds mouth shown in the picture is excessive, that is not the final design, i am changing that for sure.

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