topic Strap foundation ( beam plus spread footings) in Robot Structural Analysis Forum

Strap foundation ( beam plus spread footings)

Refaat — Sun, 06 Sep 2015 08:13:18 GMT

Dear Colleagues

I have three spread foundation as shown in the attached capture. These foundations are connected by strap beam which have the same foundations’ height and rested directly on the ground.

I have modeled the case as shown in the capture (supports + beam with elastic modulus of subgrade).

Kindly, Could I know how to deal correctly with this case especially in the design calculation (the interaction between spread footings with continuous footing)?

Because , when I design the corner footing as a neighbor footing , I have gotten very large footing dimensions as its without strap beam .

Will be highly appreciated if you support your response with small video.

Thanks in advance

Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Tue, 08 Sep 2015 07:26:03 GMT

IMHO for a model you should allow for elastic rotations of supports. Otherwise you can use the indicated option directly in the foundation design module.

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Fri, 11 Sep 2015 07:12:10 GMT

Dear Artur

Many thanks for your guidance

1. Kindly, could you attach the file of your capture?

2. May I use bar elastic ground instead of dividing the ground beam to small parts?

3. How does (the moment transferred to the ground beam) consider in side RC-beam module when this beam to be designed?

Good wishes

Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Fri, 11 Sep 2015 07:30:29 GMT

1. Kindly, could you attach the file of your capture?

Here you are.

2. May I use bar elastic ground instead of dividing the ground beam to small parts?

I used unidirectional supports in nodes defined along bars without dividing them but you may use elastic soil instead.

3. How does (the moment transferred to the ground beam) consider in side RC-beam module when this beam to be designed?

It will be "exported" to the RC design module as calculated for a model.

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Wed, 16 Sep 2015 10:33:05 GMT

Dear Artur

Thank you for your clarifications. But still I have the following inquiries related to this case where I need your help to understand them.

1. I want to transfer e.g (60%) of the support moments to the ground beam. Pls, (take a look to the attached capture).
Could you show me how to apply this percentage to the ground beam and use it inside RC-continuous footing module because I suppose to use elastic bar as in the attach file.

2. May I know , what is the advantages of using (uplift Uz+) here ? . Where I compared the results (w/wo) (uplift) I didn't noticed any difference in the results (NTM and deformation) or something missing with me?.

Thank you for patience and help
Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Wed, 16 Sep 2015 14:51:49 GMT

Dear Refaat,

1. I want to transfer e.g (60%) of the support moments to the ground beam. Pls, (take a look to the attached capture).
Could you show me how to apply this percentage to the ground beam and use it inside RC-continuous footing module because I suppose to use elastic bar as in the attach file.

In the model the percentage is not to be "defined" but is calculated based on the properties of elements defined in the model and soil parameters (KZ values). I assume that what you refer to is to model a fixed support under a column (no ground beam) > export reactions to the foundation module > define 60% reduction there.

2. May I know , what is the advantages of using (uplift Uz+) here ? . Where I compared the results (w/wo) (uplift) I didn't noticed any difference in the results (NTM and deformation) or something missing with me?.

When a ground beam is "pressed" against the ground along its entire then none. This matters only when part of it would be "uplifted".

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Thu, 17 Sep 2015 06:22:49 GMT

Dear Artur

Once again thank you for all the assistance it is a great help.

If course the transferred moment percentage from foundation to the ground beam ‘‘ Strap beam “ is mainly dependent on the rigidity of the foundation and the connected ground beam . The increasing of the rigidity portion to the strap beam will be reduced the rotation of the exterior footings ( transfer percentage of the moments to the ground beam) as “ Joseph E. Bowles “ suggested in his book .

So, this percentage ( 60% for instance ) will be disregarded in the spread foundation design and it will be added to the ground beam which shall be taken into the design of it .

My enquiry is , what is the possibility ( workaround) to add this percentage to the ground beam when I model supports connected with the ground beam ?

Thanks
Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Thu, 17 Sep 2015 08:42:32 GMT

Personally I would do this at the RC design stage rather than in the model as I can't think of any easy way of obtaining this effect. I would use the reduction option in the spread footing design module and I would add corresponding bending moment load at the end of the beam for design of the ground beam.

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Tue, 22 Sep 2015 05:50:57 GMT

Dear Artur

Thank you for your suggestion

1. I have modeled the ground beam in design modules once as RC-beam and another as elastic bar respectively as appear in (capture 00). I applied (60%) of the total moment value at the beam ends in both cases of presentation. As well I have added the vertical reaction components. Also, I have used the same beam and soil properties which had been used in the main rtd. file ( attached to this post also ).

But, the results of internal forces for instance (moment & shear) were not identical between both cases of presentation (capture 01).

May I get an explanation of that?

2. Whereas I have (Mx) at the ends. Then (Torsion taken into account) option is activated.
But, I couldn't be able to get any results in both cases of presentation because of the error message (capture 02).
I tried to change the support type, but unfortunately didn’t work.

Could you help me to overcome this matter?

3. Since I have (Mx) at the beam ends then the soil stresses distribution per beam width should not be uniform (capture 03).

Is there any possibility in (ARS2016) to get a varying in stresses distribution per beam width in such case?

Regards

Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Wed, 23 Sep 2015 07:23:04 GMT

1. Why do you expect to have the same results? Mind that for RC beam you would need to apply additional load that corresponds to the stress distribution along the continuous foundation (see the attached picture) to make sure that both RC beam and the continuous footing deform in the same way.

2. To get the results you need to block torsional rotations on the supports

3. As I already explained (see http://forums.autodesk.com/t5/robot-structural-analysis/spread-footing-vs-continuous-footing/m-p/3945285 post 4) out of the plane bending transfer to soil is currently not supported in the RC Continuous footing module. On the other hand my understanding is that a ground beam is intended to deal with bending in its plane only (otherwise you would probably need it to be of T shape instead) whereas the bending in the perpendicular direction is handled by the isolated foundation itself (plus "another" ground beam defined in this direction).

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Mon, 28 Sep 2015 05:54:04 GMT

Dear Artur

Thank you again for your help .

I expect to have the same results because in the analysis model stage, I have got the same results (deformation and moments) for both cases of presentation. Please, see the attached picture.

That is why I expected to get the thing even in the design modules, is it?.

Knowing that I have applied the reaction of unidirectional supports in ( RC-beam module) as additional loads as same as in the analysis model stage to get the same deformation of continuous footing.

( Pls , see the file (Ground beam as RC-beam)) which was attached in my previous post .

Could you help me to clarify this issue ?

Thanks a lot
Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Mon, 28 Sep 2015 08:41:37 GMT

Mind that if you have nonlinear model (uplift) you cannot import simple load cases and them allow the RC Design module to create combinations based on the superposition of results of simple load cases but you need to import combinations instead. Otherwise you may use the linear model instead (no uplift).

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Wed, 30 Sep 2015 13:39:27 GMT

Dear Artur

Thank you for your advice .

Let me summarize and concentrate on main objective of this case (adding the corresponding bending moment at the ends of the beam for design of ground beam as you suggested in post 8).

Is there any possibility to add (the moment at the end of the beam) inside the design module after importing the beam to design module from structural model because I have noted the load definition icon is inactive?. (It will be simple and great).

Otherwise

Please, may I take from your time to review and correct the input data of my file let’s say (ground beam as continuous footing which included the moments at the end of the beam) specially the force values and the moment directions (based on the analysis results of main file) to be able to do the correct design calculation?

Thanks a lot
Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Thu, 01 Oct 2015 08:49:11 GMT

I'm not sure if I understand you correctly. As I view this situation there are two options:

Option #1:

You don't define the ground beam in a model but you assume that it is going to "take" e.g. 60 % of bending from an isolated foundation. You apply fixed support under the column > calculate model > export reactions to the RC Spread footing module and then you use the ground beam option in its parameters to define this 60 % reduction of moments. In addition you create a ground beam in the stand alone mode and you manually apply loads being this 60% of bending obtained for the foundation.

Option #2:

You include a ground beam in your model > calculate it > export results from the model to both RC Spread footing module and to RC Spread footing module (no need for any additional load as the design is entirely based on the defined model).

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.

Re: Strap foundation ( beam plus spread footings)

Refaat — Mon, 05 Oct 2015 05:44:01 GMT

Dear Artur

Greetings

1. I am sorry for lack of clarity in my last post. Hope it will be clear here.

I would like you to verify and correct the attached file (Ground beam as continuous footing) whether I added the moments at the ends of the beam in the right acting direction (either clockwise or anticlockwise) or not. Based on the obtained moment directions of supports in the main file (also attached here).

2. (Option #2) in my point of view even the ground beam is included in the structural model and the design is entirely based on the defined model. We need in some cases such as this case (Strap foundation) to transfer apart of moment from spread foundations to the connected ground beam. This transferred moment will lead to stiff beam and heavily reinforced which of course will reduce the rotations of external foundations and prevent relative settlement.

However, since this case (Strap foundation) is very common type of foundations especially when we can’t extend the foundation outside the building boundaries because of existence neighbors and besides have long distance between exterior and interior foundations. In this situation, we have to use this type of foundation (Strap foundation). we used to execute a lot of them in many construction projects.

I suggest here if can give priority in wish list to add (moment transferred from foundation option) in RC beam design module or build up individual module for this type of foundation (Strap foundation). It will be very useful in such situations.

Regards

Refaat

Re: Strap foundation ( beam plus spread footings)

Artur.Kosakowski — Mon, 05 Oct 2015 08:51:26 GMT

Dear Rafeet,

1. I am sorry for lack of clarity in my last post. Hope it will be clear here.

I would like you to verify and correct the attached file (Ground beam as continuous footing) whether I added the moments at the ends of the beam in the right acting direction (either clockwise or anticlockwise) or not. Based on the obtained moment directions of supports in the main file (also attached here).

I think they should be entered in the directions of the arrows on the picture below (mind that the signs next to values are from the model and may not be correct for the RC Continuous footing module stand alone).

2. (Option #2) in my point of view even the ground beam is included in the structural model and the design is entirely based on the defined model. We need in some cases such as this case (Strap foundation) to transfer apart of moment from spread foundations to the connected ground beam. This transferred moment will lead to stiff beam and heavily reinforced which of course will reduce the rotations of external foundations and prevent relative settlement.

My understanding of what you wrote is:

1. You define a fixed support under a column and ground beam in a model and calculate the model

2. You export calculated reactions to the RC Spread footing design module

3. You design the isolated foundation with 60 % reduction of bending with the use of the option inside the RC Spread footing module

4. You want to export the ground beam the RC continuous footing design module and you want to have 60% of bending moments from supports at the ends of the beam

if so you may:

1. You define a fixed support under a column and ground beam (as a chain of 3 bars for a span with releases defined for the middle one) in a model and calculate the model

1a. After checking the reactions you create new simple load cases with 60% bending moments at the ends of the middle bar of the ground beam (case #2 in the attached model)

1b. You calculate model again

2. You export calculated reactions to the RC Spread footing design module with original load cases exported (case #1)

3. You design the isolated foundation with 60 % reduction of bending with the use of the option inside the RC Spread footing module

4. You export the ground beam the RC continuous footing design module with all load cases exported (to include loads applied or transferred directly to the middle part of the ground beams).

Please post and vote for your idea at http://forums.autodesk.com/t5/Robot-Structural-Analysis/idb-p/93

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.

Re: Strap foundation ( beam plus spread footings)

Anonymous — Fri, 10 Jun 2016 18:54:08 GMT

Hello Refaat,

Could you send me the page ou document that show "(60%) of the support moments to the ground beam" in the case of calculation of strap footing pls?

Re: Strap foundation ( beam plus spread footings)

Refaat — Sun, 12 Jun 2016 00:47:45 GMT

Hello vantrung

It was only assumption to disregard apart of foundation moment while checking the condition for overturning. But it can be calculated based on rigidity ratio between ground beam and pad foundation (EI (ground beam) / EI (pad foundation)) as I mentioned in post 7 of this thread.
It is worth mentioning here that (Joseph E. Bowles) was recommended in his book (clause 9-4) (attached here) to use (I strap / I footing > 2) in order to transfer (100% of moment) to strap beam to control rotation of the exterior footing.
Therefore, you may calculate the rigidity of (ground beam and foundation) and then use interpolation based on hypothesis of Bowles (EI strap / EI footing >2 … = 100%).

I hope this helps
Refaat

Re: Strap foundation ( beam plus spread footings)

Refaat — Sun, 12 Jun 2016 01:05:39 GMT

Re: Strap foundation ( beam plus spread footings)

Anonymous — Mon, 13 Jun 2016 08:30:07 GMT

Thanks for your help!