Long Span Beam Design for Optimal Seismic Performance

[rodsika]

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?

 

[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?

 

[rodsika]

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 into 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?

 

[Studiot]

This is a two part post with information intended to help about construction practice, loadings and earthquake effects.

Firstly about the building.

UK practice we have

Ground floor
First Floor
Second floor
Roof

The floors are labelled differently in other countries, including USA.

Then we have what are known as Construction and Use regulations.

These specifiy loadings so the structure can actually be constructed as well as laodings for use.

So for an RC frame you have to provide support for the concrete before it sets and achieves its strength this support is needed for 1 to 4 weeks.

We call all the supports the falsework.

The falsework also has to support the construction plant and labour force duting construction and the 'formwork.'

The formwork is a temporary mould to hold the wet concete in shape until it sets.

Additionally the floor must be able to support the falswork and formwork for the floor aabove it until that floor has gained strength to support itself.
That is why precast units are preferred by contractors -they save the waiting time.

For loadings under the use heading we have

Floors are required to be able to support

Themselves
The weight of internal partitions etc.
The suspended ceilings etc
Possible the floor above during construction
A distributed load on every square metre of floor this allows for people, furniture shop counters etc.
A point load placed in the worst possible position on the floor.

Roofs are required to be able to support

Themselves
Snow on top of the roof
Uplift from wind forces
Maintenance staff walking on the roof, perhaps with crawler ladders.

The forces of an earthquake spread out from the source as two waves, one horizontal (P) wave and one vertical (S) wave.
These waves apply disturbing forces and movements to the building foundation points as the wave passes that particular point.
So if the building is a frame mounted on independent foundation pads or blocks the disturbance will occur at different times as the wave passes the individual pads.
this will introduce additional loading into the frame. You asked about torsion well torsion is bad because it can lead to very high stresses in directions the frame elements were not designed to be loaded and are thefore particularly weak.

Look at the sketch.

1) Is what happens when the wave arrives sqare-on ie parallel to the building grid.
As the wave passes AB at the front it lifts and drops the front pads and columns together so there is no side to dise twisting and no torsion.
So only additional bending forces aligned in the ordinary directions of the beams is introduced.

2) However there is only one direction paralle to the grid and many oblique to it so the wave is much more likely to arrive obliquely.
Immediately you can see that A is lifted before B (and CD_ so applies a twist to the frame as well as extra bending.
this is the dreaded torsion situation.

3) Shows the effect of the horizontal wave which moves the pads sideways. this applies a huge leverage on the joint at the top of the column.

It should be noted that the S and P waves travel at different speeds so arrive at different times unless the building is very close the the source.

 

[rodsika]

This is a two part post with information intended to help about construction practice, loadings and earthquake effects.

Firstly about the building.

UK practice we have

Ground floor
First Floor
Second floor
Roof

The floors are labelled differently in other countries, including USA.

Then we have what are known as Construction and Use regulations.

These specifiy loadings so the structure can actually be constructed as well as laodings for use.

So for an RC frame you have to provide support for the concrete before it sets and achieves its strength this support is needed for 1 to 4 weeks.

We call all the supports the falsework.

The falsework also has to support the construction plant and labour force duting construction and the 'formwork.'

The formwork is a temporary mould to hold the wet concete in shape until it sets.

Additionally the floor must be able to support the falswork and formwork for the floor aabove it until that floor has gained strength to support itself.
That is why precast units are preferred by contractors -they save the waiting time.

For loadings under the use heading we have

Floors are required to be able to support

Themselves
The weight of internal partitions etc.
The suspended ceilings etc
Possible the floor above during construction
A distributed load on every square metre of floor this allows for people, furniture shop counters etc.
A point load placed in the worst possible position on the floor.

Roofs are required to be able to support

Themselves
Snow on top of the roof
Uplift from wind forces
Maintenance staff walking on the roof, perhaps with crawler ladders.

The forces of an earthquake spread out from the source as two waves, one horizontal (P) wave and one vertical (S) wave.
These waves apply disturbing forces and movements to the building foundation points as the wave passes that particular point.
So if the building is a frame mounted on independent foundation pads or blocks the disturbance will occur at different times as the wave passes the individual pads.
this will introduce additional loading into the frame. You asked about torsion well torsion is bad because it can lead to very high stresses in directions the frame elements were not designed to be loaded and are thefore particularly weak.

Look at the sketch.

1) Is what happens when the wave arrives sqare-on ie parallel to the building grid.
As the wave passes AB at the front it lifts and drops the front pads and columns together so there is no side to dise twisting and no torsion.
So only additional bending forces aligned in the ordinary directions of the beams is introduced.

2) However there is only one direction paralle to the grid and many oblique to it so the wave is much more likely to arrive obliquely.
Immediately you can see that A is lifted before B (and CD_ so applies a twist to the frame as well as extra bending.
this is the dreaded torsion situation.

3) Shows the effect of the horizontal wave which moves the pads sideways. this applies a huge leverage on the joint at the top of the column.

It should be noted that the S and P waves travel at different speeds so arrive at different times unless the building is very close the the source.

After reading this. I googled for "does precast floor need beams" and after reading numerous pdfs. I'm still studying. Are you saying that it is possible to make 12 x 13 meter roof slab and put on top in the perimeter without beams across while for floor slabs.. maximum size seems to be 6 meters so there must be a 13 meter girder or beam at middle. Or are you saying it is also possible to make 12 x 13 meter floor slab and put on the perimeter without any beams in the middle? How. If not. So this means for 12 meter span... beams still need to be joined with the columns in poured Reinforced Concrete manner to suppor the 6 meter span precast floor.

In my place, precast buildings are so rarely used (maybe 1 in 10,000 buildings).

Btw.. as I originally mentioned (in the attachment earlier) the lot width is 12 meters and depth is 17.29 meters on the right and 14.57 on the left. But you mentioned "13" and we can use 13 for purpose of discussions.

I'll have to study the seismic angle later. Thanks.

 

[Studiot]

First thing you should know.

Nearly all building elements are 'controlled by' that is the 'determining factor is' deflection not strength. Most building elements are way too strong for the job.
This is quite important in building design.

I hope in reading the various posts you are gaining the impression the constructing a building is a blend of local availability vis a vis cost , labour, materials, lifting and other machinery, time etc. There is no one right answer.

You seem very worried about 6m limits on beam sizes. What is the maximum length of vehicles in your country.
Whatever size they are if they are made offsite they have to be delivered off loaded and subsequently handled on site.

On the other hand units made elsewhere can be erected immediately. There is no waiting time for concrete to gain strength.

Further under factory conditions you can manufacture prestressed units which have a considerably better strength/delection characteristics than those cast on site.

One thing you haven't said is how the foundations of your building were made. This also makes a difference to construction.
Does it have searate foundation pads at each column or one large foudation slab (we call it a raft foundation)?

 

[rodsika]

First thing you should know.

Nearly all building elements are 'controlled by' that is the 'determining factor is' deflection not strength. Most building elements are way too strong for the job.
This is quite important in building design.

I hope in reading the various posts you are gaining the impression the constructing a building is a blend of local availability vis a vis cost , labour, materials, lifting and other machinery, time etc. There is no one right answer.

You seem very worried about 6m limits on beam sizes. What is the maximum length of vehicles in your country.
Whatever size they are if they are made offsite they have to be delivered off loaded and subsequently handled on site.

On the other hand units made elsewhere can be erected immediately. There is no waiting time for concrete to gain strength.

Further under factory conditions you can manufacture prestressed units which have a considerably better strength/delection characteristics than those cast on site.

One thing you haven't said is how the foundations of your building were made. This also makes a difference to construction.
Does it have searate foundation pads at each column or one large foudation slab (we call it a raft foundation)?

In my place. We often used column footings below ground. In the building I'm leasing, they already have structural engineer to design the place and architect but I have the final word since I'd lease the place.

The design of the building is originally 2 storey with roofdeck (with provision up to 3 storey structural). After the architect and structure engineer already made the blue print for a month. I let the owner changed the design to 2 storey maximum with metal sheet as roofing in the second storey because I heard roofdeck always leaks and hard to maintain (I'd lease the ground floor and since there are limited parking spaces, the third storey is not important so I practially control the whole building). But then researching yesterday about metal sheet roof like garble and hip roof.. and gutters all around it. I realized metal sheet and gutter seems to need as much maintenance as roofdeck. What's your experience in this? The architect and structural engineer already charged us double for changing the plan.. do you have any reasons why roofdeck or concrete slab roof would be better in maintenance than metal sheet? But then for seismic consideration... metal sheet seems to be more advantageous, so for now metal sheet is still my choice.

 

[Studiot]

All foundations are below ground.

How about answering my question?

One thing you haven't said is how the foundations of your building were made. This also makes a difference to construction.
Does it have searate foundation pads at each column or one large foudation slab (we call it a raft foundation)?

 

[rodsika]

All foundations are below ground.

How about answering my question?

file attached is picture of the footer.

It is what we commonly use.

Why.. would precast slabs need different footings? But precast slabs are expensive so I don't consider this.

 

[Studiot]

Nice picture
So these are pad foundations and you also have the excavations for what we call a ground ring beam.

One purpose of the ring beam is to stiffen the edges of the ground slab (to prevent edge curling) and also to hold the whole thing together if there is local loss of support under the ground slab as the ground settles over the years.

I also do not see any significant seismic strengthening of the connection between the pads and the columns in your picture. In an earthquake (or even for building wind loading or thermal movement) the columns will pivot about their joints with the pads so these points should be regarded as pinned or ball joints.

One point I made earlier about the leverage of earthquake disturbance affects your roof.

I hope you understand leverage?

If you have a heavy lump waggling about on the end of a long lever you have a large effect.

The longest lever is up to the roof so a heavy (concrete) roof is most vulnerable to seismic activity. Further if it crashes down on the floor below it it will a great deal more damage than a light metal roof which might come off but could well save the structure and be easily replaced.

Please also remember this all is friendly advice, I am not acting as an unpaid structural consultant, nor would PF want me to.

 

[rodsika]

Nice picture
So these are pad foundations and you also have the excavations for what we call a ground ring beam.

One puropse of the ring beam is to stiffen the edges of the ground slab (to prevent edge curling) and also to hold the whole thing together if there is local loss of support under the ground slab as the ground settles over the years.

I also do not see any significant seismic strengthening of the connection between the pads and the columns in your picture. In an earthquake (or even for building wind loading or thermal movement) the columns will pivot about their joints with the pads so these points should be regarded as pinned or ball joints.

One point I made earlier about the leverage of earthquake disturbance affects your roof.

I hope you understand leverage?

If you have a heavy lump waggling about on the end of a long lever you have a large effect.

The longest lever is up to the roof so a heavy (concrete) roof is most vulnerable to seismic activity. Further if it crashes down on the floor below it it will a great deal more damage than a light metal roof which might come off but could well save the structure and be easily replaced.

Please also remember this all is friendly advice, I am not acting as an unpaid structural consultant, nor would PF want me to.

First. The picture is not mine. I just saw it at the net (and shared it to illustrate the idea of column footer). Second, the construction of the building I'd lease hasn't started. We haven't even started excavations yet because the final design of the building are still being done due to my changing the 3 storey to 2 storey. Anyway. Thanks for the many tips. I'll think and remember about them one by one. I'd also recommend the architect and structural engineer to use ground ring beam... as the original design is only one column footer at every corner. The picture I shared with you is from a person building a fence. I just shared it to illustrate a column footer. Thanks again for the tips. This will do for now.

 

[rodsika]

The forces of an earthquake spread out from the source as two waves, one horizontal (P) wave and one vertical (S) wave.
These waves apply disturbing forces and movements to the building foundation points as the wave passes that particular point.
So if the building is a frame mounted on independent foundation pads or blocks the disturbance will occur at different times as the wave passes the individual pads.
this will introduce additional loading into the frame. You asked about torsion well torsion is bad because it can lead to very high stresses in directions the frame elements were not designed to be loaded and are thefore particularly weak.

Look at the sketch.

1) Is what happens when the wave arrives sqare-on ie parallel to the building grid.
As the wave passes AB at the front it lifts and drops the front pads and columns together so there is no side to dise twisting and no torsion.
So only additional bending forces aligned in the ordinary directions of the beams is introduced.

2) However there is only one direction paralle to the grid and many oblique to it so the wave is much more likely to arrive obliquely.
Immediately you can see that A is lifted before B (and CD_ so applies a twist to the frame as well as extra bending.
this is the dreaded torsion situation.

3) Shows the effect of the horizontal wave which moves the pads sideways. this applies a huge leverage on the joint at the top of the column.

It should be noted that the S and P waves travel at different speeds so arrive at different times unless the building is very close the the source.

Hi Studiot,


I've been thinking something for half a day and needs your comment.

Please see the file attached picture. I'm deciding whether to add a column in the middle (marked in red). The architect wanted me to decide.. and I didn't have access to his structural engineer and he can't discuss with me anything structural because he said his specialty was architecture and not familiar with seismic loading or torsions.

Last I talked to him early today I wanted him to add the column at middle. He said it is ok. But then remember in the torsion article any unevenness in structural can introduce twist. Imagine "B" (in the picture) has column at middle while C doesn't. Imagine a seismic wave passing from left to right. This would make the C move differently from B and introduce torsions?

Also something perplexes me. The beam from top to bottom middle are not girder because they are same level as the horizonal beams.. meaning imbedded into each other. Supposed there was no column in the middle of "B". Would the load be distributed to the horizontal and vertical beams (in the picture)? Or would the horizonal beams be the main support and the vertical beam just to holds up the floor slabs? What is usually the case?

Note in the construction project. No precast slabs would be used. They would all be poured concrete and rebars.

So should I add the column in the middle of "B" or not? On one hand, I'm on tight budget and the architect said the "B" side beams would be smaller since they would be 6 metesr apart... and it would also support the 12 meter RC beams at "C" making cross beams. Would this make the loading at "C" distributed to the vertical and horizonal beams?

Actually one month ago. He told me it's nicer to have no columns in the middle. But now said it is ok because I'm on tight budget.

Now I'm quite undecided. I guess seismic considerations would be my primary concern in whether to put the middle column or not.. but then.. if "B" and "C" has no columns at middle... and "A" and "D" have them.. it would still introduce unbalance seismic loading for waves that come from left to right?

Lastly. The connections would be standard connections, the loading is simply one floor above or simply 2-storey with metal sheet in the 2nd floor roof.

What do you think? Column or no column at the middle of "B", that is the question?

Thanks.. this inquiry is my last question in this thread, don't worry :)

 

[Studiot]

I am not of the opinion that an extra column will do much for earthquake performance, in fact it may reduce it.

In post#21 I drew attention to the important key statement in your code.

The way to enhance earthquake performance is to make the horizontal elements more flexible. This way they can dissipate the quake energy input to the structure.

The whole matter depends upon the design of these things (again including the connections).
You should ask your structural engineer to explain.

 

[rodsika]

I am not of the opinion that an extra column will do much for earthquake performance, in fact it may reduce it.

In post#21 I drew attention to the important key statement in your code.

The way to enhance earthquake performance is to make the horizontal elements more flexible. This way they can dissipate the quake energy input to the structure.

The whole matter depends upon the design of these things (again including the connections).
You should ask your structural engineer to explain.

Problem with long span is the connections must be made perfect. Also in most homes, the distance between columns are 6 or less. It is unusual to find homes with distance between columns of 12 meters or more. Have you thought why.

Anyway. I let them added a column in middle yesterday making the span of one portion 6 meters instead of 12. This would save much steel and make the constructions cheaper.

Also for the 2-storey with metal roof. I let them design it for 3-storey foundationa and structural. I assume using 3-storey structural in 2-storey building would make the columns stronger and more earthquake proof. Any wrong with this idea? I know you would say to make it more ductile. But using 3-storey foundations and columns would add more steel making it stronger. What's the problem with this?