Raft Slab - Estimated Foundation Load

[Anonymous]

Hi,

I'm modelling a raft slab in Robot and am unsure of my inputs. I've had a look through past posts within the community with no luck. I've attached simplified what my situation is - a RC slab with majority of loads coming from load bearing walls on the perimeter. Worst case load is gravity plus the wall reactions from high wind loading. My questions are:

1. Using the Robot function to determine KZ - The estimated foundation load - should this be my total load on the slab divided by the entire slab area? Or should it be my worst case loading (ie w2 localised at the slab corners) divided by the area it acts over? It hugely effects my results.

2. Is using a raft correct? Or should I use a different slab type with different foundation elasticity relative to the different loading? Ie a different slab under the walls and then one internally ?

3. Locally I've got uplift at the wall edges. If I use the Robot function to calculate kz and therefore omit the uplift (for kz calculation - I will input it as a loading on the slab) - will this adversely effect my results? Keeping in mind the main issue I'm concerned with is ensuring my slab global deflection doesn't exceed limits I've set it.

4. Lastly to confirm something I've read - is it correct that the slab thickness/stiffness has little bearing on the raft displacements?

Many thanks in advance!

[Anonymous]

Hi,

Didi you manage to get someone from auto desk with answers to your question below? I have a similar problem with

Estimated Foundation Load input on Robot raft foundation. Do we have to input the estimated load plus the self weight of the rafter?

Does Robot calculate edge beam rafter of only flat slab rafter? What is the difference with both rafter on sandy low bearing capacity soils?

Please can someone kindly get back to our queries that has been raised in both the previous and present message?

Much Thanks,

[Artur.Kosakowski]

1. Using the Robot function to determine KZ - The estimated foundation load - should this be my total load on the slab divided by the entire slab area? Or should it be my worst case loading (ie w2 localised at the slab corners) divided by the area it acts over? It hugely effects my results.

The calculator is just a tool which gives KZ value for entered parameters. In reality there are number of load scenarios which would mean different KZ for each load case / combinations. As this cannot be done in Robot (unless you use phases which seems too big gun for the obtained effect) personally I'd assume the most typical (likely) load scenario (assuming loads as being vertical and using uplift to take into account horizontal loads when they cause the raft to be separated from the ground). You may also check other references and assume KZ value for given soil type instead.

2. Is using a raft correct? Or should I use a different slab type with different foundation elasticity relative to the different loading? Ie a different slab under the walls and then one internally ?

I'd rater do this if there are different soil types under the raft foundation.

3. Locally I've got uplift at the wall edges. If I use the Robot function to calculate kz and therefore omit the uplift (for kz calculation - I will input it as a loading on the slab) - will this adversely effect my results? Keeping in mind the main issue I'm concerned with is ensuring my slab global deflection doesn't exceed limits I've set it.

Yes, the result will be affected as springs will work in both directions.

4. Lastly to confirm something I've read - is it correct that the slab thickness/stiffness has little bearing on the raft displacements?

Unless you have a hole under significant part of it the uniform load applied to a slab with the same KZ all over it will just produce uniform vertical displacements regardless of the slab thickness. The difference may be for very large concentrated forces and relatively small thickness. 

[Anonymous]

Dear Artur,

So that means that Framed systems poured on grade would not work as a rafter foundation?

I did calculated all the loads and transfer it to the columns. Should I include the wall loads separately by a linear distributed force? Would this affect my calculations?

Please see attached work and note that Robot did not specify the dimensions and bars nor number of of beams in the raft foundation; why and what should I do?

Please note that the drawings are giving me a # symbol for the bars, is this the same as diameter symbol or is is the bar number?

Much Thanks

[Artur.Kosakowski]

So that means that Framed systems poured on grade would not work as a rafter foundation?

I'm not sure if I understand what you mean.

I did calculated all the loads and transfer it to the columns. Should I include the wall loads separately by a linear distributed force? Would this affect my calculations?

If they transfer load directly to the raft foundation then  think you should however not knowing your project i'm not in the position to decide on this.

Please see attached work and note that Robot did not specify the dimensions and bars nor number of of beams in the raft foundation; why and what should I do?

Again I'm not sure what beams do you mean as you haven't defined them in your model. If you assume that the program will generate them automatically under the lines of columns then it is not done.

Please note that the drawings are giving me a # symbol for the bars, is this the same as diameter symbol or is is the bar number?

# stands for diameter whereas the number in the circle is a position of a reinforcement bar.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

What I mean on " Framed systems poured on grade would not work as a rafter foundation?" Is that a slab on grade (slab with beams) would not work as a raft foundation? I am correct?

Yes I beams under the columns that are integrated with the raft foundation. Do I need to define this by dimensions in order to obtains the results of a entire raft foundations with beam?  Robot does not assume that raft foundation has beams under columns right?

Much Thanks,

[Artur.Kosakowski]

You need to define them as bar elements or panels with different thickness.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi,

Under the Drawing Parameter tab you explained that the # is for diameter bars. I am not sure if the l symbol is for length, but for what length? Is the e symbol for spacing of reinforcement bars center to center?

When I press the Provide reinforcement of RC Element TAB on The window of Calculation options ACI318-14 under General sub-tab gave me c1 and c2 = 0.75inch cover for the raft foundation. Is it because I have made the slab to shell rather than a floor slab foundation?

I know for a fact that it should have given me approximately 3" cover for these types of foundation.

Much Thanks,

[Artur.Kosakowski]

 I am not sure if the l symbol is for length, but for what length?

length of a reinforcement bar

Is the e symbol for spacing of reinforcement bars center to center?

Correct

When I press the Provide reinforcement of RC Element TAB on The window of Calculation options ACI318-14 under General sub-tab gave me c1 and c2 = 0.75inch cover for the raft foundation. Is it because I have made the slab to shell rather than a floor slab foundation?

It is because you declared such cover:

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

Yes the l under the parameter drawing setting; reinforcement description sub-tab. The one that comes out in the middle of # (diameter bar) and e (spacing of bars) on the final drawings of Robot. (Please see attached screenshot). The attached Robot generated drawings show l =29-10.5, and I have no clue what this means? (Please see page 2 of the attached PDF drawings).

What is D1 0.3 under the slab formwork drawing page? (Please attached PDF front page).

Yes the Calculation options, under the General sub-tab c1 and c2 could not changed. The box did not gave me option to change the reinforcement cover like your that is attached; I had no options to change anything under general sub-tab. I could not find the reason for this; why? While the other sub-tabs for concrete, reinforcement bars, etc gave me the option for making changes.

Can I have Robot generate a slab cross section drawing on the set of plans? How can this be done.

Thanks for the great help,

[Anonymous]

Hi,

Can someone from autodesk please respond to my previous email which relates to:

The attached Robot generated drawings show l =29-10.5, and I have no clue what this means? (Please see page 2 of the attached PDF drawings).

Thanks,

[HoshangMustafa]

Hi

The attached Robot generated drawings show l =29-10.5, and I have no clue what this means? (Please see page 2 of the attached PDF drawings)

29ft. 10.5inch