kellez

Timber Roof on top of an RC Slab

7 posts in this topic

Hello everyone,

I am designing a timber roof on top of an RC slab, the space in middle needs to be usable space, the gable walls are constructed from clay bricks,

 

As you can see the roof structure is made up of rafters supported by the wall plate at the edges and by a stud wall further in the middle with the rest of the rafter left to cantilever in the middle of the roof, with a ridge beam in the middle. Rafters are spaced at 610mm centres

 

The concept model is shown below

 

 

MONASTIRI_v10_-_Structural_Model_-_33_-_

MONASTIRI_v10_-_Structural_Model_-_33_-_

 

MONASTIRI_v10_-_Structural_Model_-_33_-_

 

MONASTIRI_v10_-_Structural_Model_-_33_-_

Rafters - Wall plate connection

My main question now is, how shall I connect the rafters to the wall plate? I have talked with the roof contractor and we have different thoughts on how this should be done therefore i wanted to get some suggestions from experienced engineers. So what do you guys think>

 

Screen_Shot_2017-11-01_at_00.13.50_unja0

 

 

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Hello. 

 

So will you share your and your contractors' thoughts?

 

Reinis

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If I understand your question correct. If internal supports designed as pinned movable. Al least one or both of the edge supports should be pinned non-movable in the plane of the section you posted. So the roof structure not slide away from wind load action.

Edited by user277418

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On 11/17/2017 at 11:30 AM, ReinisGailitis said:

Hello. 

 

So will you share your and your contractors' thoughts?

 

Reinis

 

Ok, so the contractor said that he doesnt want to make a birdsmouth cut where the Rafter sits on the wall plate/sill plate but instead he said that he prefers to bevel the wall plate at an angle equal to the roofs and let the rafter sit on it, fixed with big screws on top. 

I disagree with this approach due to the fact that having the rafter sit on top of a beveled wall plate does not provide a firm base to the rafter to sit on, also the load path is not completely transferred to the wall plate where its supposed to be transferred. Instead most of the load I suppose is carried out by the screws that are holding the rafter down.

 

Please have a look at the image below in order to understand what i mean by saying that the load path in not transferred to the wall plate

1st image on the left shows the rafter with the birds mouth cut and the load path driven towards the wall pate

the other two images show the rafter without a birds mouth and with a beveled wall plate, as you can see the load path is not transferred to the wall plate but continues downwards, to me it looks like the rafter is left hanging by the screws and almost no support from the wall plate.

 

 

 

 

 

Screen Shot 2017-11-21 at 22.50.06.png

Edited by kellez

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Yes, you are correct with your assumptions about the situation. I wouldn't allow the situation contractor has proposed.

 

Cheers

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On 11/17/2017 at 2:20 PM, user277418 said:

If I understand your question correct. If internal supports designed as pinned movable. Al least one or both of the edge supports should be pinned non-movable in the plane of the section you posted. So the roof structure not slide away from wind load action.

Both of the edge supports will be designed as pinned non-movable, only allow rotations but no displacments

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8 minutes ago, ReinisGailitis said:

Yes, you are correct with your assumptions about the situation. I wouldn't allow the situation contractor has proposed.

 

Cheers

 

I thought so, thanks for the reply, i have already continued my design using a birds mouth.

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