## Long Beam Span

With a steel roof there would be greater twisting generated every day by solar heating and expansion.

The traditional solution (provision of movement joints and allowance in the supports) to this problem would also alleviate other movement issues.

 Quote by Studiot With a steel roof there would be greater twisting generated every day by solar heating and expansion. The traditional solution (provision of movement joints and allowance in the supports) to this problem would also alleviate other movement issues.
I was not talking about the roofs twisting. I was talking of the RC beams and columns of one portion of building suffering torsions or rotations during seismic movement. It is said torsions contribute much to earthquake damages... something to do with center of mass and lateral movement and torsions.. you not familiar with this?

 Studiot, check this out: http://www.ijens.org/Vol_12_I_01/125...JCEE-IJENS.pdf I was trying to apply it to the lot we talked about. See if you can apply it.

No I am not familiar with the Turkish design code but it looks sensible

 If the columns are more rigid than the beams, ductile deformations will occur at the ends of the beams. Beams can absorb a lot of energy by ductile deformations without an important loss in the load carrying capacity. In this system, all the beam–column connections in the building have to fail before the collapse of the ground floor columns. Architects should know that strong column–weak beam design is not only advisable but also obligatory according to Codes

 Quote by Studiot No I am not familiar with the Turkish design code but it looks sensible This is your key phrase from that link.
There is a phrase that I just can't connect with the lot drawing I attached earlier. It said "The center of building mass is generally considered as the geometrical center of the building and the center of rigidity is considered as the center of vertical elements of the structural system. The center of rigidity of a building should coincide with the center of mass. When the center of a building mass does not coincide with the center of rigidity, torsion and stress concentrations occur in the building when it is subjected to seismic loads."

So in the drawing a few message away. Where is the center of mass and center of rigidity? Do they coincide in the drawing?

 There is some explanation and alternative coping strategies in this american document. http://richardpclarke.tripod.com/earthquakedesign.pdf The basic strategy is to find some dissipate the earthquake energy within the ductility of the members - obviously without causing structural collapse. You are now getting into significant detail which can't be answered without knowing the detailing of the structure particularly the connections as I said before. The devil is in the detail (pun intended)

 Quote by Studiot There is some explanation and alternative coping strategies in this american document. http://richardpclarke.tripod.com/earthquakedesign.pdf The basic strategy is to find some dissipate the earthquake energy within the ductility of the members - obviously without causing structural collapse. You are now getting into significant detail which can't be answered without knowing the detailing of the structure particularly the connections as I said before. The devil is in the detail (pun intended)
Ok. Thanks.

Anyway. A separate question. I'm researching about hip roofs. It seems they rely on the perimeter walls. I'm studying about one storey building with hip roofs. Since hip roofs rely on perimeter walls. Does it mean all one storey building with hip roofs doesn't have beams that cross in the middle (I'm not referring to perimeter beams but beams in between walls)? I wonder if this is a rule or whether for larger one storey building, there must be beams in the middle to hold it.. or can one consider the hip roof enough to hold the 4 walls of the one storey building irregardless of the sizes. What do you think?

 Quote by Studiot Rough guess 75 to 100mm concrete slab 240 kg per sq metre = 180*12*13 to 240*156 = 25tonne to 35 tonne
How did you get 180? 12? 13? and why 240 multiply by 156?

 metal sheet roof 6 - 10 kg /sqm plus 4 trusses at 750kg each = 3 to 5 tonnes
are you saying that for every square meter of metal sheet roof, there are 4 trusses used.. so the 3 to 5 tonnes are for each square meter of metal roof?

also you said concrete slab is 240 kg per sq meter, so metal sheet weights more than concrete slabs because of the trusses requirement per square meter? remember concrete slab is 240 kg per square meter while that of metal sheet is over 3000kg (4 trusses at 750 kg each for square meter).

 Concrete weighs around 2400 kg per cubic metre. for a slab that is 0.1 metres thick this is 0.1*2400 = 240 kg per square metre. I estimated the roof slab as 0.075 to 0.1 so at 0.075 this figure reduces to 180 kg per sq metre. The area of the roof is 12 * 13 sq metres. = 156 The sheet metal is around 6 to 10 kg per sq metre. So weight of sheet metal is 10 * 13 * 12 You have to add to this the weight of the roof trusses I estimated 4 at 0.75 tonne each. Strictly there will also be purlins to add; these are probably between 1 and 2 tonnes. Does this help?

 Quote by Studiot Concrete weighs around 2400 kg per cubic metre. for a slab that is 0.1 metres thick this is 0.1*2400 = 240 kg per square metre. I estimated the roof slab as 0.075 to 0.1 so at 0.075 this figure reduces to 180 kg per sq metre. The area of the roof is 12 * 13 sq metres. = 156 The sheet metal is around 6 to 10 kg per sq metre. So weight of sheet metal is 10 * 13 * 12 You have to add to this the weight of the roof trusses I estimated 4 at 0.75 tonne each. Strictly there will also be purlins to add; these are probably between 1 and 2 tonnes. Does this help?
Thanks for this. But you forgot to add the weight of the beams across the walls in the case of the concrete slabs. Remember in metal sheet. They use trusses so no beams across walls are necessary. So instead of concrete slab being 25 to 35 tonnes compare to metal sheet 3 -5 tonnes. Concrete slabs with the required beams across may reach about 100 tonnes. So I guess for roof requirements. Metal sheets are sufficient. But then I wonder why many use concrete slabs as roofs which is about 30 times heavier. Eh?

 Why would an rc roofslab require additional beams? It could easily be self supporting, I would certainly expect to design it that way. After all it is not a floor slab that has to withstand the superimposed loading from the users.

 Quote by Studiot Why would an rc roofslab require additional beams? It could easily be self supporting, I would certainly expect to design it that way. After all it is not a floor slab that has to withstand the superimposed loading from the users.
are you saying there are roofslabs that are 12 meters in length that are simply put on top of the rectangular perimeter without any beam across? first time for me to hear of this. in my place, the concrete slabs are usually poured or made on site after rebars were put in the beams with wooden support for the new poured concrete. Unless you are referring to precast slabs with rebars inside that is simply put on top of the perimeter just like trusses??

 If I were the contractor I would be wanting to construct (cast) the floor slabs and cross beams all in one go at each level. That makes for economic construction. Equally any concrete poured has to be lifted to roof level so you would want to minimise the quantity. A concrete roof has only itself to support plus maintenance access. You cannot expect to load every square metre with filing cabinets or shop shelves as you could with a floor slab. Don't forget that the physical form need not be a plain flat slab it can (would be?) be ribbed or otherwise strengthened by shape. And for that span it would certainly be reinforced. But like I said we don't construct many concrete roofs in the UK.

 Quote by Studiot If I were the contractor I would be wanting to construct (cast) the floor slabs and cross beams all in one go at each level. That makes for economic construction.
Duh, if that is so, how do you connect these precast floor slabs with build in beams to the perimeter. This won't be strong. A slight jolt can disengage the connection and have the whole floors come crashing down.

 Equally any concrete poured has to be lifted to roof level so you would want to minimise the quantity. A concrete roof has only itself to support plus maintenance access. You cannot expect to load every square metre with filing cabinets or shop shelves as you could with a floor slab. Don't forget that the physical form need not be a plain flat slab it can (would be?) be ribbed or otherwise strengthened by shape. And for that span it would certainly be reinforced. But like I said we don't construct many concrete roofs in the UK.
Why, what is the difference between roof slab and floor slab. Arent' they the same. I assume that by using concrete slabs as the roof material, this means it is like another floors and if the owner wants to use it as floor. Then it's no longer concrete roof and ful fldege floors.

Unless there are really thin rc slabs that are only put on roofs and not to be walked on and never to function as normal floor.. maybe has got to do with the thickness?

How thick are normal floor slabs and how thick are roof slabs?

 What is your objective? To understand the construction next door or to quibble? What did I say about imposed loadings on a floor and a roof?

 Quote by Studiot What is your objective? To understand the construction next door or to quibble? What did I say about imposed loadings on a floor and a roof?
Well. I plan to move in to the new building after it is finished so want to understand it.

Thanks for your distinctions. I was under the impression that all roof concrete are automatically roof deck. I even argued with the manager about this convincing him all roof slabs can be used as roof deck with future expansion to be used as floor, and I was convinced too so much that your previous statement escaped me. Now I understand.

Anyway, the original plan was to have 2-storey with roofdeck for provision up to third storey. But then only the ground floor will be important. So I suggest to him that he better just build 2-storey with metal sheet on his second floor roofing.

So you are saying a roof slab is about 35 tonne and a metal roof about 5 tonne. But then what I was asking you are not actually roof slab.. but additional floor slab or roof deck.. meaning the floor above it can have normal load for extension in the 3rd storey. This means the floor slab is not just 35 tonne but can be twice over.

Anyway. For completion of our discussions. If a roof slab is 35 tonne. How many times over are floor slab with loads of 60 lbs per square foot in the same 12 by 13 measurement you used?