Critique my design 1

[hobo_ist]

Not a permanent works engineer and do not specialise in home builds (I do temporary works for medium/large commercial/resi structures, mainly in RC).

This is a single storey house extension in the UK that I'm doing a rough conceptual design for, for a friend. Mainly to assess feasibility and rough costs. Attached is the flat roof structure in question. Sizing of beams/columns will be done later.

Over designed? under designed? Potentially unstable? Give me all you've got. Insult me if you have to. Community feedback will be helpful. Thanks

 

[GeorgeTheCivilEngineer]

That's a hot mess.

Subdividing the spans with beams (as you've done with the flitch) is the way to go. The cantilever arrangement I do not like at all - there will be excessive deflection unless you have large footings. The diagonal rafters are unnecessary and will add complication to construction.

Although it is a flat roof, it is large and so will need a reasonable fall from one end to the other. It also means deflection control (esp over the beams) will be critical to avoid ponding. For that reason I would avoid flitches and cantilevers if possible.

 

[hobo_ist]

Thanks for your input George. To eliminate the diagonals I would need to continue the joists from the flitch beams over the steels in both directions. Trouble then becomes figruing out a suitable way to support these overhangs at the corners. Tricky.

Unfortunately my friend has seen the below and wants something similar. The appeal here is the cantilever/floating corner.

I agree deflections are crucial here and the structural steel will be designed accordingly. The moment connection detail to the post will be fun.

The flitch beams are just a fag packet idea and will likely be swapped out for steels when doing actual checks.

https://files.engineering.com/getfi...eb4e-4f8a-bd47-f1985318a7de&file=IMG_9958.jpg

https://files.engineering.com/getfi...-86eb-022e44e8aeff&file=IMG_9985-1024x683.jpg

 

[GeorgeTheCivilEngineer]

I'm less worried about he connection than the column itself contributing to the deflection.

I would run the cantilever back into the building to make a it a true projecting beam and use the weight of the blockwork to keep it pinned down.

I'd look at something more like this:

Lot more steelwork but save a lot on timber and complex connections.

You'll need to consider lateral stability etc of course.

 

[hobo_ist]

That's an elegant solution that will require thickening of the internal wall (although, not a problem) and propping of the existing wall to the south which goes up a level (problem, as cracks will ruin the remodelled bathroom upstairs). Not sure what you would do at the canopy eaves based on your sketch.

Can't really see a huge issue by going with the steel columns if I'm honest if, like you say the foundation is sufficient. It'll be fixed to the blockwork to provide some additional support.

 

[Eng16080]

hobo_ist, Where you show the "Steel column tied to new blockwork wall" and the "Moment connection" there, I think an alternative solution would be better, like what George shows above. To get your concept to work, there's going to be a lot of complicated/expensive construction at the wall, footing, and connections. I would feel much better using a large cantilevered steel beam instead.

Is there a reason why there can't be a column at the outside corner? Doesn't there need to be something there anyway to attach the glass panels to?

 

[hobo_ist]

Hi Eng16080

The idea is to have a floating corner similar to the below:

 

[Eng16080]

Alright, thanks. That makes perfect sense then. I still think the cantilevered beam is the better approach (not that your idea can't work).