What options to I have here? (residental building)

[n3jc]

Im dealing with a residental building with a ground storey + attic. All bearing walls are masonry 250 mm thick. RC slab above ground storey is 150 mm thick - but it wont be done across whole building area above ground storey! Above an attic there is another RC slab - roof slab that is 150 mm thick - it will be done acroos the whole buidling area.

Investor + arhitect made it clear that I have no freedom/other options regarding RC slabs - they have to be made as I described.

Since RC slab above ground storey is not across whole area - I do not have a diaphragm which take care of tension forces as a result of loads acting on a roof slab.

What are my options here? Im pretty sure I wont be able to design bearing walls as masnory walls since they are only good for vertical loads.

So I think I have 2 options here:
1) to make tension ties below roof slab which take care of tension forces so masonry walls only see vertical loads OR
2) to make RC (reinforce concrete) walls that can take out of plane (horizontal) loads. The problem is that without a slab in the middle my wall is 4,60 m high.

What would you do?

Thank you for suggestions/help.

 

[derim01]

You can solve it with RC beams along the perimeter (on top of the walls), so area 5 will be clear without tension ties.

 

[CANPRO]

Picture how the structure will deflection in section 2-2 - the ridge of the roof will deflect downwards, the angle between your two roof slopes will increase, and your walls will spread. You only need to restrict one of those movements to restrain the rest (or use some combination of them based on stiffness). So the way I see it you have 3 main options:

- Looks like you've drawn in a ridge beam of sorts. If you can make that stiff enough and have proper support, I think your problems are solved. I think this would probably be my first choice.

- You will have natural moment continuity at the ridge between the two sloping portions of your roof. This will help you, but my gut says it probably wouldn't be enough. Even with the ridge beam option, you'll have to evaluate the bending moment at this location.

- As you noted, you could add a tension tie. Maybe work that in architecturally. In this scenario, consider how this will be built - the tension tie will need to be in place before the formwork is released. The final procedure for this should be the work of the contractor, but you need a rough plan in place at the design stage to give him a chance at making this work.

- Also as you noted, you could design cantilevered walls. You'll still get some movement here so you'll need to check something with more stiffness doesn't fail along the way to the walls being engaged (cracking at the ridge for example).

 

[TheDW]

Agree with CANPRO - my first choice would be to design the ridge beam as an actual beam between the upper wall shown in 1-1 and the column in the center of the left wall.

 

[BridgeSmith]

Not sure whether it can be done with acceptable deflections and aesthetics, but a beam spanning horizontally along the walls at the slab level, from the edge of the slab to the end wall may be another option. It would be as if you had a full slab, but cut most of it out on one end.

 

[Klitor]

you have gable walls and,as it seems, an internal 'gable' wall between 1 and 2.
Without seeing the drawings, im guessing the thing remains standing even if you dont do anything

 

[MIStructE_IRE]

Ridge beam + RC band beams. No need for tension ties in my view. They’ll go slack with wind uplift anyway.

 

[Klitor]

Wind lifting up a 15cm thick cast-in-place concrete slab?

 

[MIStructE_IRE]

Oops! Didn’t read the RC Roof bit! I thought your concern was mainly the wall panels.

In that case, surely a well detailed ridge connection at the top (cranked slab), gives you a sufficient roof diaphragm. Like putting shear walls on the classic house of cards? It would be incredibly stiff essentially acting as its own ridge beam.

 

[molibden]

What's the attic floor plan? Do you have any interior walls?
Make roof slab a one-way slab parallel to ridge if you have interior walls perpendicular to ridge. In that case, walls will take horizontal forces and there is no need for tension ties.
Also make sure you have enough horizontal and vertical RC ties in exterior walls to make them stable and withstand wind. I would surely run a RC horizontal tie at slab level above ground floor.

 

[n3jc]

Thx for help everybody!

@molibden, this is roof plan:

 

[molibden]

Four short shear walls for seismic. Plus heavy roof. Plus double height walls on one end. Yay! I hope you go with a lot of RC in the walls.