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RELATION BETWEEN REDUCED LOADS FOR A WALL AND FOUNDATIONS LOADS(LINEAR SUPPORT).

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Message 1 of 15
patkson
776 Views, 14 Replies

RELATION BETWEEN REDUCED LOADS FOR A WALL AND FOUNDATIONS LOADS(LINEAR SUPPORT).

Hi, 

I have been trying to design a strip footing for a wall and upon a closer look at the 

foundation loads at the calculate note tab, I realised my axial loads for the static cases  is lesser than the reduced 

loads for the pannel at the base where the linear support where assigned. 

 

Again my moments under the seismic situation is entirely different from the foundation loads tab. and  far far lesser than that of the reduced loads for the same pannel.

Please can some explain this to me. how is this possible. I thaught the reduced forces for base pannel should atleast have some correlation with that transfered to the foundation directly. 

Pls!! help me figure this out.

 

Thank you,

Best Regards,

POK. 

14 REPLIES 14
Message 2 of 15
Artur.Kosakowski
in reply to: patkson

How did you create the strip footing? Based on selection of a supported node under a wall and its 'export' to the RC Spread Footing design module? 



Artur Kosakowski
Message 3 of 15
patkson
in reply to: patkson

I'm using the current implementation in the 2015 version. 

First I define a linear support under the wall and run anlaysis. I then select the linear support under the wall on plan view tab

and deselect the objects and go  to design provided reinforcment for rc. The program recognises this as continous footing 

alright but like indicated earlier the loads onto the foundation doesn't corespond with the reduced forces of the pannel under which the linear support 

was defined.

thank you.

Message 4 of 15
Artur.Kosakowski
in reply to: patkson

The option is the extension of the existing in the older versions of Robot strap footing mode of the RC Spread Footing module and is intended for gravity walls (compression and out of the plane bending) rather than shear walls (in plane bending). The import of load is from each node of the wall and the values of reactions are scaled to the unit length of the continuous footing based on the distance among the supported nodes.

 

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.



Artur Kosakowski
Message 5 of 15
patkson
in reply to: Artur.Kosakowski

Thank you Arthur, what it simply means is that this new implementation is going to useful those of us designing in seismic active areas, Well I hope robot extend this to include foundation for shear walls.
Message 6 of 15
patkson
in reply to: patkson

oops!! i mean not going to be useful those designing in seismic active regions
Message 7 of 15
Artur.Kosakowski
in reply to: patkson

Could you indicate some good reference that describes the design of continuous footing under shear walls ( I mean the approach for in plane bending)? Thank you.



Artur Kosakowski
Message 8 of 15
patkson
in reply to: Artur.Kosakowski

I read some suggestions from the forum : http://forums.autodesk.com/t5/Robot-Structural-Analysis/Wall-foundations/m-p/4353738#M15817
Just a quick comment on the above approach to wall foundation design,
First, placing the walls on elastic bars seem to me not quite a good assumption, the problem being that, you may end up with very flexible system and very long periods for your structure and this will will end up giving you relatively lower seismic forces.

Personally what I think is to fix all elements (walls, column) at foundation level , extract the reduces forces at the base and use it to design the foundation manually.
Message 9 of 15
Artur.Kosakowski
in reply to: patkson

Personally what I think is to fix all elements (walls, column) at foundation level , extract the reduces forces at the base and use it to design the foundation manually.

 

Do you assume a beam on elastic soil (as a separate 'model' ) and apply 'reduced' forces at half of its length? 



Artur Kosakowski
Message 10 of 15
patkson
in reply to: Artur.Kosakowski

Well I guess that might work to some extent. But for me Most foundation theories are developed on assuming rigid foundation behaviour. I am guessing introducing the foundations flexibility by bars on springs is a lot more of non-linear model behaviour. I don't no much about non-anlaysis but I'm sure it should be the best if done correctly.
Thank you.
Message 11 of 15
Artur.Kosakowski
in reply to: patkson

But for me Most foundation theories are developed on assuming rigid foundation behaviour.

 


I'm sorry to ask again  but I'd like to understand your approach correctly so that I could match it with the already existing options in Robot or create a proposal for new ones adding them to the wish list Smiley Happy

 

Assuming the strap foundation as rigid with in plane bending applied at its middle you can calculate value of stress in elastic soil / values of reactions along the bottom edge of the wall. I assume that for horizontal load we may assume something as on the picture below:

 

reractions and reduced moment.PNG

 

 

In other words the reduced moment of 385.15 kNm causes the distribution of reactions as indicated in the form of the red diagram. The point I have difficulty with is if you assumed your strap footing as rigid you will have no deformation in it and therefore no bending in the strap footing caused by the reduced moment (in other words theoretically no longitudinal reinforcement would be needed). Unless what I have just wrote doesn't make sense at all I'd like to ask you what is the way you deal with this issue making your hand calculations? Thank you.



Artur Kosakowski
Message 12 of 15
patkson
in reply to: Artur.Kosakowski

In other words the reduced moment of 385.15 kNm causes the distribution of reactions as indicated in the form of the red diagram. 

Yh.. the foundation being rigid does not mean there will be no bending??? or shear flows through the strib footing.. Are you saying that if I have 

an isolated footing of concentrated load xKN sitting on a rigid ground the ground presure(reaction) woudn't cause moments in the footing? Definitely No.

                                    x

plan.PNG y

 

                                   x

Please consider the two images below.

.   inplane pressure distribution                                                                                   . 

 

                                            Y-Y

                                                                                                                                                                                                                                X-X

         pressure distribution for walls with inplane bending                                                                                                Pressure distribution about 

 

If am right then on  a rigid base, wall with inplane bending will have a distribution looking like the above. This pressure coming will cause a two way moment distribution in both directions which can be used to design the bases.

 

And even in the case where your implane momen is very large we will have a distribution like below with possible uplifts:

probable uplift

 

I can send you spread sheet for design the wall base without uplift and with probable uplifts (ofcourse when reaction is outside the middle third of the base) if getting your email wouldn't be a problem. 

 

Thank you.

 

 

 

Message 13 of 15
Artur.Kosakowski
in reply to: patkson

Yh.. the foundation being rigid does not mean there will be no bending???

If you assume it as infinitely rigid then no as it is not going to deform but rotate as a rigid body instead. 

 

or shear flows through the strib footing.. Are you saying that if I have 

an isolated footing of concentrated load xKN sitting on a rigid ground the ground presure(reaction) woudn't cause moments in the footing? Definitely No.

 

My point is how you 'transfer' the value of concentrated bending moment into strip footing deformation. I assume that you actually 'separate' the strip footing design from the model and first you calculate reduced forces for a wall on 'rigid' supports and then you calculate a 'separate' model of a strip footing under the 'soil pressure' load based on the forces of reactions obtained for the whole model of a structure. This I can imagine for an isolated foundation where the position of the column is the support for calculation of a cantilever but I'm curious how you approach this for continuous footing under a wall defined along its entire length.

 

I can send you spread sheet for design the wall base without uplift and with probable uplifts (ofcourse when reaction is outside the middle third of the base) if getting your email wouldn't be a problem. 

 

I'll send it as  a private message. Thank you.

 

 

 

 



Artur Kosakowski
Message 14 of 15
patkson
in reply to: Artur.Kosakowski

 

My point is how you 'transfer' the value of concentrated bending moment into strip footing deformation.

 

 

I'm not sure I understand you clearly but if you asking how to translate an axial force and moment  into its equivalent 

ground pressure then is a very simple answer,  Ground pressure = P/A +/- M/Z ; where p=axial force, A= area of footing, M= moment and Z = Sectional Modulus.

 

This I can imagine for an isolated foundation where the position of the column is the support for calculation of a cantilever but I'm curious how you approach this for

 

continuous footing under a wall defined along its entire length.

 


The bending moment is actually calculated at the face of the support and not at the middle as you seem to suggest.  

Thus after establishing the ground pressure both design moments Mxx and Myy is calculated as cantiliver moment for each orthogonal direction.

 

Thank you.

Message 15 of 15
Artur.Kosakowski
in reply to: patkson

Let me show what I mean on the picture (my understanding of your approach for in-plane bending moment calculations is illustrated in the black color).

 

strip foundation.png



Artur Kosakowski

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