Long Span Beam Design for Optimal Seismic Performance

[rodsika]

Are there any structural engineers here or know the stuff. I'd like to know this.

Can structural engineers design long span beam for example 12 meters reinforced concrete beam that is as good as standard 6 meter beam in seismic performance or are 6 meter beams always better compared to 12 meters beam even if the latter were designed for ultimate seismic performance?

 

[Studiot]

For vibration analysis you have to consider the whole structure (including the foundations), not just the individual components separately since the connections play a great part in the result.

So it depends what your 6m and 12m beams are connected to and how they are connected.

 

[rodsika]

For vibration analysis you have to consider the whole structure (including the foundations), not just the individual components separately since the connections play a great part in the result.

So it depends what your 6m and 12m beams are connected to and how they are connected.

I mentioned "seismic analysis", you wrote "vibration analysis". Unless they are identical? In what ways?

 

[Studiot]

Vibration is just the more general term as there are other sources of excitation besides seismic activity.

For example the beams of motorway bridges are subject to continual vibration and are quite a bit longer than 12m.

That is less important what is important is the connection conditions of your beams.

Another important factor is the natural frequency of vibration.

 

[rodsika]

Vibration is just the more general term as there are other sources of excitation besides seismic activity.

For example the beams of motorway bridges are subject to continual vibration and are quite a bit longer than 12m.

That is less important what is important is the connection conditions of your beams.

Another important factor is the natural frequency of vibration.

In a building with 12 meters frontage. How much generally is 12 meter beam span more expensive compared to two 6 meter span in the overall cost of the construction, materials? Is it like 20-30% more expensive?

 

[Studiot]

Two 6m span beams would require a centre support that is not present with a 12m one so would add to the cost.

You have been asking about reinforced concrete. This is usually priced at a rate per cu metre placed so the cost would likely be the same for the beams either way. I don't see that any heavier duty craneage would be required 12m is not such a significant increase in size.

 

[rodsika]

Two 6m span beams would require a centre support that is not present with a 12m one so would add to the cost.

You have been asking about reinforced concrete. This is usually priced at a rate per cu metre placed so the cost would likely be the same for the beams either way. I don't see that any heavier duty craneage would be required 12m is not such a significant increase in size.

Supposed you would be given a choice by the architect/engineer whether you want a 6 meter or 12 meters RC beam span in your house or office building? What would you choose? I'm still thinking of two 6 meters RC beam span because there are 3 connections instead of 2. In construction they don't do it perfect or sigma 5 accuracy. Hence with the 3 connections in the 6 meter span.. at least there are 3 connections instead of 2... in this latter case of 2 connections, if there is some subtle weakness in one of the beam-column joint.. it could fail. In your country. Do you commonly see 12 meter RC beam span houses or office? Isn't 6 meter the standard?

 

[Studiot]

Most building component sizes are decided by architectural considerations not structural ones. If a longer span beam is specified by the architect then the structural engineer will design a suitably stronger one.

Beams of 6m span are probably so lightly loaded that they are only just above what we call 'minimum steel'. That is the steel required to control cracking.

It is unusual to use RC beams in the UK in any case, steel is more normal.
Suspended floors are normally RC slabs or prestressed.

 

[rodsika]

Two 6m span beams would require a centre support that is not present with a 12m one so would add to the cost.

You have been asking about reinforced concrete. This is usually priced at a rate per cu metre placed so the cost would likely be the same for the beams either way. I don't see that any heavier duty craneage would be required 12m is not such a significant increase in size.

I think you forgot that the rebars of longer beam span would be quite larger and there will be much more rebars in 12 meter RC beam span versus two 6 meter RC beam span so the cost of the former would be higher.

 

[Studiot]

I think you forgot that the rebars of longer beam span would be quite larger and there will be much more rebars in 12 meter RC beam span versus two 6 meter RC beam span so the cost of the former would be higher.

No I didn't forget that.

I said that under standard building contracts reinforced concrete is priced at a rate per cu metre placed. That is the UK standards method of measurement.

Civil engineering contracts are priced differently so the reinforcement, concrete and formwork are all measured separately.

It is actually unlikely that there would be a greater number of reinforcing bars in the longer beam. Just that they would be larger diameter.

 

[rodsika]

No I didn't forget that.

I said that under standard building contracts reinforced concrete is priced at a rate per cu metre placed. That is the UK standards method of measurement.

Civil engineering contracts are priced differently so the reinforcement, concrete and formwork are all measured separately.

It is actually unlikely that there would be a greater number of reinforcing bars in the longer beam. Just that they would be larger diameter.

Consider a lot size of 12 meters frontage and 15 meters depth. Supposed they put 4 beams side to side and one girder at middle front to back. This means the front and back have columns at the center. Only two RC beams of 12 meters (without columns) are put at middle of lot with beam depth of 0.8 meters. Would they make the girder at middle (front to back) also with girder depths like 0.8 meters to support the whole 15 meters front to back lot size.. or would the girder be much smaller just to support the slabs? What is usually the case?

Note I'm not constructing any of this, of course but just want to understand the design of certain office near mine. Thanks.

 

[Studiot]

Do you mean something like this?

So the upper floor is divided into 4 slabs 6m x 6.5m.

 

[rodsika]

Do you mean something like this?

So the upper floor is divided into 4 slabs 6m x 6.5m.

Yes. But with the difference there are two 12-meter beams in the middle between the front and back making up 4 side to side beams (not 3 as your drawing shows).

Also the girder is not steel i-beam but RC beam and it has same level as the side to side RC beam.. meaning it is embedded like in normal column-beam joint so the definition of girder as something supporting smaller beams don't hold. I wonder if you also defined a girder as something just to support slabs and not exactly the beams (wherein the 12 meter RC beam can stand on their own).

 

[Studiot]

OK I think I understand.

There are 3 columns at the front, 3 at the back, and 4 down each side, making 10 in all. avoiding counting corners double). They are presumably evenly spaced. They are connected by a system of rc beams at the top. This network of beams divides the support for the upper floor into 3 strips 4.3m x 12m. When complete this makes an rc frame.

I would guess that the upper floor might be then constructed by lifting precast beams or floor units to span 4.3m onto the frame and screeding over the top. That would avoid slab formwork and 4 - 5 m precast beams are not that large and readily handleable by a small crane or even a digger arm. If the floor units can be cast off site or on site whilst the rc frame is being constructed there will not be some weeks waiting time for the floor concrete to cure.

 

[rodsika]

OK I think I understand.

There are 3 columns at the front, 3 at the back, and 4 down each side, making 10 in all. avoiding counting corners double). They are presumably evenly spaced. They are connected by a system of rc beams at the top. This network of beams divides the support for the upper floor into 3 strips 4.3m x 12m. When complete this makes an rc frame.

I would guess that the upper floor might be then constructed by lifting precast beams or floor units to span 4.3m onto the frame and screeding over the top. That would avoid slab formwork and 4 - 5 m precast beams are not that large and readily handleable by a small crane or even a digger arm. If the floor units can be cast off site or on site whilst the rc frame is being constructed there will not be some weeks waiting time for the floor concrete to cure.

Thanks. In my place. We don't use precast slabs. But simply pour fresh concrete into the floor support. Anyway. The building will be two storey. They plan to use metal hip roof on the roof of the second storey. Now do you know how heavy a hip roof is compared to using pure concrete slabs or roofdeck? Like is a hip roof with full truss support 1/2 heavier or 1/4 heavier than pure concrete roofdeck or same weight given the same building size?

 

[Studiot]

Rough guess

75 to 100mm concrete slab 240 kg per sq metre = 180*12*13 to 240*156 = 25tonne to 35 tonne

metal sheet roof 6 - 10 kg /sqm plus 4 trusses at 750kg each = 3 to 5 tonnes

 

[rodsika]

Rough guess

75 to 100mm concrete slab 240 kg per sq metre = 180*12*13 to 240*156 = 25tonne to 35 tonne

metal sheet roof 6 - 10 kg /sqm plus 4 trusses at 750kg each = 3 to 5 tonnes

Thanks. I read today that irregular size lots can experience torsion as a result of unequal lateral movement. Pls see attached picture. Supposed the lot we talked about has the left side 14.57 meters and the right side 17.29 meters.. that is the right side is 2.72 meters longer. How much torsions would be produce for earthquake or seismic movement that goes up and down, side to side and diagonal? Or can the size be considered rectangular approximations and torsions negligible? If not. Can you design columns and beams that would make the left and right side equal and preventing torsions?

 

[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.

 

[rodsika]

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?

 

[rodsika]

Studiot, check this out:

http://www.ijens.org/Vol_12_I_01/125001-9494-IJCEE-IJENS.pdf

I was trying to apply it to the lot we talked about. See if you can apply it.

 

[Studiot]

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

This is your key phrase from that link.

 

[rodsika]

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?

Really dumb about structural...

 

[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)

 

[rodsika]

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 regardless of the sizes. What do you think?

 

[rodsika]

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).

 

[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?

 

[rodsika]

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?

 

[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.

 

[rodsika]

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??

 

[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.

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.