Waqas Haider

When ever we say Allowable bearing capacity, It can be either Gross allowable bearing capacity or net allowable bearing capacity. And it is the most important thing to decide when to add over burden, when to subtract and when not to. 
More over keep this equation before you 
for equilibrium
q(all.gross) > P/A +Y.Df
where 
q(all.gross) = gross bearing capacity
Y.Df = Backfill load
P/A = Applied pressure
 
q(all.net) = q(all.gross)-Y.df = (P/A +Y.Df) - Y.Df 
hence for equilibrium
q(all.net) > P/A
where 
q(all.net) = net allowable bearing capacity
Y.Df = Backfill load
P/A = Applied pressure
  So gross bearing capacity is capacity available to bear pressure of existing soil as well as applied loads and net bearing capacity is capacity available to bear just applied pressures because pressure caused by existing soil has already been deducted.  
Case when NOT to deduct over burden
 
Lets say in my geotechnical report i have been provided by NET bearing capacity of 1 tsf at a depth of 5 ft. Now it means at 5 ft depth, soil is capable to bear pressure of 1 tsf IN EXCESS of existing soil. Because it is NET allowable bearing pressure and pressure of existing soil has already been subtracted/balanced from bearing capacity. Now only extra available bearing capacity is reported as net. The soil can bear 1 tsf of EXTERNAL/APPLIED pressure which includes load of above structure as well as load of footing. Load of footing is also external and applied. In this case, you need not to deduct over burden because over burden is already deducted and you are provided with NET bearing capacity.
But If you have bearing capacity at 5 ft depth but in return you have plan to backfill upto 7 ft, then obviously extra 2 ft soil is applied pressure. which you need to either add in applied pressure or you should deduct it from the bearing capacity.
 
Case when to deduct over burden
But if my geotech report says me that at 5 ft depth, i have 1.25 tsf GROSS allowable bearing capacity. Now it means my soil is capable to bear 1.25 tsf pressure including pressure caused by existing soil. Because it is gross allowable bearing pressure and i have not subtracted pressure of existing soil. The soil can bear 1.25 tsf of total pressure which includes EXTERNAL/APPLIED pressure caused by load of above structure including load of footing, and pressure caused by existing soil . In this case, for the sake of sizing of footing, you need to to deduct weight of over burden soil so that so may find out net bearing capacity available to resist applied load of structure and footing. Note that, after deducting the pressure caused by existing soil i.e. Y.Df = 110*5 = 550 psf = 0.25 tsf, we again get the same 1tsf bearing capacity available for external/applied loads.
 
Case when to ADD over burden
Now Let say you are given with any of capacity. Let say Net allowable capacity of 1tsf at 5 ft depth. But you know that you are not going to backfill the foundation (in case of basement etc), then NOT ONLY this 1 tsf is available for applied pressure but also that 0.25tsf available which was going to be consumed by existing soil backfill. Now you can add this 0.25tsf to your net bearing capacity to get bearing capacity, available for external loads and you need to find out size of foundation according to this available bearing capacity.

Why dont you simply assign insertion point to model this eccentricity? If you are having a frame, You can assign these insertion points to either the beam or the column a per situation? You can offset any end of member (Column or Beam) by this method to model eccentricity. If you are having a frame, and want to model the eccentricity at a specific joint, select the member at that joint (the member which is offest from center lines) and go to assign manu--> Assign insertion Point--> and assign desired offset nd desired axis. But dont forget to uncheck DON'T TRANSFORM STIFFNESS otherwise only geometry will be offset instead of stiffness i.e. required in calculations by the software.
If you having only one column, You can also o the same, You want to provide eccentricity. You can either offset your load point as said by sir Rana, or you can offset your column centroidal axis by applying required offset in insertion points in desired axis. again dont forget to uncheck DON'T TRANSFORM STIFFNESS.

Kindly clarify details of section? What diameter and thickness you are using for pipe sections? And sheet thickness you mentioned is sheet of billboard? What exactly structural system you are using?  

Why dont you simply assign insertion point to model this eccentricity? If you are having a frame, You can assign these insertion points to either the beam or the column a per situation? You can offset any end of member (Column or Beam) by this method to model eccentricity. If you are having a frame, and want to model the eccentricity at a specific joint, select the member at that joint (the member which is offest from center lines) and go to assign manu--> Assign insertion Point--> and assign desired offset nd desired axis. But dont forget to uncheck DON'T TRANSFORM STIFFNESS otherwise only geometry will be offset instead of stiffness i.e. required in calculations by the software.
If you having only one column, You can also o the same, You want to provide eccentricity. You can either offset your load point as said by sir Rana, or you can offset your column centroidal axis by applying required offset in insertion points in desired axis. again dont forget to uncheck DON'T TRANSFORM STIFFNESS.

Waqar Saleem Bhai, I would suggest u read this document. last month i was also very much confused regarding this spring option in softwares. This document helped me much. I wil upload other documents too when i will find them. But this is good... In last topic of this document, u will get to know how to calculate stiffness of soil spring using bearing capacity...
 
 
 
Correlation_BC_and_K.pdf

Thanks a lot. Keep it up. You are serving engineering community a lot... 

Why dont you simply assign insertion point to model this eccentricity? If you are having a frame, You can assign these insertion points to either the beam or the column a per situation? You can offset any end of member (Column or Beam) by this method to model eccentricity. If you are having a frame, and want to model the eccentricity at a specific joint, select the member at that joint (the member which is offest from center lines) and go to assign manu--> Assign insertion Point--> and assign desired offset nd desired axis. But dont forget to uncheck DON'T TRANSFORM STIFFNESS otherwise only geometry will be offset instead of stiffness i.e. required in calculations by the software.
If you having only one column, You can also o the same, You want to provide eccentricity. You can either offset your load point as said by sir Rana, or you can offset your column centroidal axis by applying required offset in insertion points in desired axis. again dont forget to uncheck DON'T TRANSFORM STIFFNESS.

Waqar Saleem Bhai, I would suggest u read this document. last month i was also very much confused regarding this spring option in softwares. This document helped me much. I wil upload other documents too when i will find them. But this is good... In last topic of this document, u will get to know how to calculate stiffness of soil spring using bearing capacity...
 
 
 
Correlation_BC_and_K.pdf

Hello Naqash, 1st of all welcome to this forum. And also try to spread this forum in your area/circle as much as possible, so that maximum people can take benefit of it. 
Naqash if you remember, in manual design of roof slab or footing slab, we use to consider a unit strip and design this unit strip for output results. Then we provide the steel and section calculated for this unit strip, to whole of footing or slab.
Similarly, in Safe foundation, you are to define and assign design strip of unit width to let software show you output i.e. forces and steel etc for this unit design strip. you can also go for any width of design strip. Results will be accordingly but design will be same. Mostly a unit strip i.e. for MKS system 1 meter strip and for FPS 1 ft strip is defined and assigned. 
After running the model, you have two options to check your results of forces and reinforcement. 
1) Based on Finite Elements
In this method reinforcement and forces for each and every element, in which structure has been divided / meshed, will be reported. This is a bit different than our usual manual practice of designing.
2) Based on Design strips
In this method reinforcement and forces are reported on strip based. This is quite easy to understand. You can check both methods for better idea. Another important thing is that we define 2 design strips "Strip A" and "Strip B" to read results in X and Y axis or in Shorter and Longer directions respectively. If you provide strip A in both X and Y directions, Then you will not be able to read results in X and Y axis, separately. display of results will be mixed. For this purpose we provide 2 different strips.

I am confused regarding this approach of reducing torsional modifier to such great extent. According to ACI 11.5.2.2
In a statically indeterminate structure  where reduction of the torsional moment in a member
can occur due to redistribution of internal forces upon cracking, the maximum Tu shall be permitted to be
reduced to the values given in (a), (b), or (c), as applicable:
(a) For nonprestressed members, at the sections
described in 11.5.2.4
φ4λ Sqrt(fc ′) A2cp / Pcp
 
It says that we can reduce torsional moment upto a specified limit. Not to zero or 0.001.  In its commentary it says, 
 
For this condition, illustrated in Fig. R11.5.2.2, the
torsional stiffness before cracking corresponds to that of
the uncracked section according to St. Venant’s theory. At
torsional cracking, however, a large twist occurs under an
essentially constant torque, resulting in a large redistribution
of forces in the structure.11.34,11.35 The cracking torque
under combined shear, flexure, and torsion corresponds to
a principal tensile stress somewhat less than the
quoted in R11.5.1.
When the torsional moment exceeds the cracking torque, a
maximum factored torsional moment equal to the cracking
torque may be assumed to occur at the critical sections near
the faces of the supports. This limit has been established to
control the width of torsional cracks.
Also according to Nislon, this distribution is only possible after extensive cracking as highlighted in below pic. So I doubt the approach used to neglect torsion upto 0.001 level
What i got from the 2nd attachement of Zain Saeed the author is dividing Tcr with Tu to find how much reduction in Tu is needed to reduce torsion upto Tcr which is, as mentioned above, is necessary to keep torsional crack widths in control. and hence using the modifier for each section defined for beam. It might be a bit lengthy task to evaluate for each type of section ( most loaded members of a type of beam may be checked only), but the approach seems more realistic. 
Kindly comment as I think if even we reduce to 0.001, still this redistribution in torsion is not possible without large twisting which is not possible without excessive cracking. So reucing upto such a low value does not seem good. 

This is what i found in "Structural Concrete theory and Design by Nadeem Hussen"
 
Here it also reports the same thing that we should design for torsion for atleast 
φ4λ Sqrt(fc ′) A2cp / Pcp
Which is also specified by ACI that we can reduce our torsion upto phi.Tcr i.e. cracking torsion and not below this. Even if we neglect the remaining torsion for compatibility, we should atleast perform design for phi.Tcr or should at least provide minimum longitudinal and transverse reinforcemet for torsion. This is to control crack width to satisfy servicability. ACI, Nilson and Nadeem hussen all quote the same thing.
Hence reducing the modifier upto such a little value is not good at start. At start we should go for a value of 1, 0.3 or any other suitable value which user think is small enough to release majority of compatibility torsion and will sustain only smaler torsional moments. After designing, if still some indeterminate beams are being failed, then for the specific beams we can reduce value unless we get Tu equall to phi.Tcr or a bit larger than that. Because putting Torsional modifier to 0.001 would not report any torsional reinforcement and hence adding no torsional reinforcement at all would cause excessive crack widths affecting serviceability of the structure.

Last month, i design a structure with shear walls. I will attach more documents. What i know/remember i will share with you.
In simple, Pier is similar to column and Spandrel is similar to beam. Shear wall is modeled in etabs as shell element. You are to assign it either pier label or spandrel label (Depending on situation of shear wall) to design a shear wall. If you want to design a shear wall, it mostly needs peir label to be assigned. But for the portions of shear wall above openings, as its behavior is somewhat like beam resting on two supports, you need to assign it a spandrel label. I have attached screen shots of my model where you can see i have assigned pier label to the vertical elements and i have assigned spandrel labels to the elements spaning horizontally above openings. The reason behind is detailing based. The detailing in vertical elements of shear wall is similar to wall (or column) and its design is based on considering it as an element resisting majorly AXIAL FORCES +  MOMENTS along with inplane shear forces. (Theoratically it acts as a cantilever beam spaning vertically which again represent behavoiur similar to column). I have also attached screen shot of design detail of pier which reports total vertical steel in wall with respect to section area for resisting moments, horizontal steel for resisting shear and some times boundry elemtns (i.e. special detailing at ends of shear wall exactly as in column i.e. shear rings confining vertical bars). I have also attached screen shot of design detail of spanderal which reports top steel, bottom steel, to resist moments similar to beam and vertical steel to resist shear along with DIAGONAL reinforcement some times needed to account for reversal of forces.
I will attach more documents throwing detailed light on shear wall design i found few weeks ago. Also my model is attached (it is in seismic zone 4 from where you can get idea about pier labeling and spanderal labeling)
One important thing is that, pier label and spanderal label also meant to integrate forces. i.e. if you assign same pier label to all walls in a floor connected with each other, the software will report results only for one and critical wall because all walls were assigned only one pier label. If you want to have multiple outputs at multiple walls, you should assign them different pier label.
in image number 5, note that i have not assigned any pier to the selected portion of wall. hence no reporting of results is done.
 





Etabs Center Portion.e2k

Kindly mention names of members as we are unfamiliar with faces. Thanks..

Top reinforcement is needed in isolated footing majorly for two reasons.
1) Due to negative pressure under some or whole part of footing. In part of footing where there is positive pressure, the footing is in complete contact with soil and tension is in bottom side of footing. But in part of footing where there is negative pressure the footing is no more in contact with ground. Either it is designed as it is (and reduced contact area is used for calculations) or it is made to become in contact with ground by help of over burden loads. In either case, the bending of footing is in such a way which causes tension of footing at top of foundation demanding top reinforcement in footing. Mostly top reinforcement is less than bottom reinforcement but for simplicity, if it is not affecting economy much same reinforcement can also be used for top and bottom.
2) Due to temperature and shrinkage control. Code says we can provide temperature based minimum steel either in one layer at center or at any face or we can divide total steel in two layers i.e. half at top face and half at bottom. According to Zahid Ahmad Siddique (Professor at UET Lahore) in his book concrete structures mentions it is better to provide temperature steel in two layers if thickness of footing increases 18 inch.

If you are having a geotechnical report with you, there must be clearly stated value of spring constant with the name Modulus of Subgrade Reaction. Incase soil investigation have not been done on site for any reasons, you can use a tentative value for k by following relation.
k = 3 x qa x 12 (in FPS in k/ft3)
k = 3 x qa x 40 (in SI in KN/m3)
where qa = allowable bearing capacity. and 3 is Factor of safety applied on soil while determining bearing capacity of soil.  For the reference of this formula, please read the attached document.
Correlation_BC_and_K.pdf

Top reinforcement is needed in isolated footing majorly for two reasons.
1) Due to negative pressure under some or whole part of footing. In part of footing where there is positive pressure, the footing is in complete contact with soil and tension is in bottom side of footing. But in part of footing where there is negative pressure the footing is no more in contact with ground. Either it is designed as it is (and reduced contact area is used for calculations) or it is made to become in contact with ground by help of over burden loads. In either case, the bending of footing is in such a way which causes tension of footing at top of foundation demanding top reinforcement in footing. Mostly top reinforcement is less than bottom reinforcement but for simplicity, if it is not affecting economy much same reinforcement can also be used for top and bottom.
2) Due to temperature and shrinkage control. Code says we can provide temperature based minimum steel either in one layer at center or at any face or we can divide total steel in two layers i.e. half at top face and half at bottom. According to Zahid Ahmad Siddique (Professor at UET Lahore) in his book concrete structures mentions it is better to provide temperature steel in two layers if thickness of footing increases 18 inch.

Last month, i design a structure with shear walls. I will attach more documents. What i know/remember i will share with you.
In simple, Pier is similar to column and Spandrel is similar to beam. Shear wall is modeled in etabs as shell element. You are to assign it either pier label or spandrel label (Depending on situation of shear wall) to design a shear wall. If you want to design a shear wall, it mostly needs peir label to be assigned. But for the portions of shear wall above openings, as its behavior is somewhat like beam resting on two supports, you need to assign it a spandrel label. I have attached screen shots of my model where you can see i have assigned pier label to the vertical elements and i have assigned spandrel labels to the elements spaning horizontally above openings. The reason behind is detailing based. The detailing in vertical elements of shear wall is similar to wall (or column) and its design is based on considering it as an element resisting majorly AXIAL FORCES +  MOMENTS along with inplane shear forces. (Theoratically it acts as a cantilever beam spaning vertically which again represent behavoiur similar to column). I have also attached screen shot of design detail of pier which reports total vertical steel in wall with respect to section area for resisting moments, horizontal steel for resisting shear and some times boundry elemtns (i.e. special detailing at ends of shear wall exactly as in column i.e. shear rings confining vertical bars). I have also attached screen shot of design detail of spanderal which reports top steel, bottom steel, to resist moments similar to beam and vertical steel to resist shear along with DIAGONAL reinforcement some times needed to account for reversal of forces.
I will attach more documents throwing detailed light on shear wall design i found few weeks ago. Also my model is attached (it is in seismic zone 4 from where you can get idea about pier labeling and spanderal labeling)
One important thing is that, pier label and spanderal label also meant to integrate forces. i.e. if you assign same pier label to all walls in a floor connected with each other, the software will report results only for one and critical wall because all walls were assigned only one pier label. If you want to have multiple outputs at multiple walls, you should assign them different pier label.
in image number 5, note that i have not assigned any pier to the selected portion of wall. hence no reporting of results is done.
 





Etabs Center Portion.e2k

Brick masonry can not be designed in these software because brick masonry is neither the frame elements which can be modelled in these software nor are the simple isotropic type materials which can be modelled easily like area elements. You will have to do design for brick masonry manually. But you can take help from these software to read results of different type of analysis u need while doing manual design. There might be other softwares which can do this perhaps. But regarding these two softwares, i hold this opinion. 

Top reinforcement is needed in isolated footing majorly for two reasons.
1) Due to negative pressure under some or whole part of footing. In part of footing where there is positive pressure, the footing is in complete contact with soil and tension is in bottom side of footing. But in part of footing where there is negative pressure the footing is no more in contact with ground. Either it is designed as it is (and reduced contact area is used for calculations) or it is made to become in contact with ground by help of over burden loads. In either case, the bending of footing is in such a way which causes tension of footing at top of foundation demanding top reinforcement in footing. Mostly top reinforcement is less than bottom reinforcement but for simplicity, if it is not affecting economy much same reinforcement can also be used for top and bottom.
2) Due to temperature and shrinkage control. Code says we can provide temperature based minimum steel either in one layer at center or at any face or we can divide total steel in two layers i.e. half at top face and half at bottom. According to Zahid Ahmad Siddique (Professor at UET Lahore) in his book concrete structures mentions it is better to provide temperature steel in two layers if thickness of footing increases 18 inch.

Top reinforcement is needed in isolated footing majorly for two reasons.
1) Due to negative pressure under some or whole part of footing. In part of footing where there is positive pressure, the footing is in complete contact with soil and tension is in bottom side of footing. But in part of footing where there is negative pressure the footing is no more in contact with ground. Either it is designed as it is (and reduced contact area is used for calculations) or it is made to become in contact with ground by help of over burden loads. In either case, the bending of footing is in such a way which causes tension of footing at top of foundation demanding top reinforcement in footing. Mostly top reinforcement is less than bottom reinforcement but for simplicity, if it is not affecting economy much same reinforcement can also be used for top and bottom.
2) Due to temperature and shrinkage control. Code says we can provide temperature based minimum steel either in one layer at center or at any face or we can divide total steel in two layers i.e. half at top face and half at bottom. According to Zahid Ahmad Siddique (Professor at UET Lahore) in his book concrete structures mentions it is better to provide temperature steel in two layers if thickness of footing increases 18 inch.

From engineering point of view, it is really very amazing to see a complete city builtup in water ways, in a lagoon. How did they make the foundation system of buildings completely all the time underwater? Really interesting article here is. It also explains some modern challanges to the city i.e. flooding due to raised levels of sea and an inteseting solution to this problem.
https://sites.google.com/site/engineeringvenice/
 

From engineering point of view, it is really very amazing to see a complete city builtup in water ways, in a lagoon. How did they make the foundation system of buildings completely all the time underwater? Really interesting article here is. It also explains some modern challanges to the city i.e. flooding due to raised levels of sea and an inteseting solution to this problem.
https://sites.google.com/site/engineeringvenice/
 

I use etabs version 9.7.4 so i can tell u about importing plan from cad to etabs in this version.
Define a layer in CAD for example GRID LAYER. then draw ur whole grid in this cad layer.. after you finish marking grid in cad drawing, save the file as DXF FILE instead of DWG extension. Please note that the insertion units of cad drawing (you can see insertion units by UN command) and in etabs while importing this grid must be same. One u saved drawing as dxf file. open new etabs model, delete existing grid, import file as DXF file, set units before importing the file in drop down manu. Select layer of Grid and import this layer. U will get your grid import. since there are a lot of grids usually in a cad drawing, it is better to shift whole grid before savin as dxf file into a new cad drawing with origin set 0,0,0. Copy grid from old drawing as a block into new drawing in desired layer, open new cad drawing, paste and put insertion coordinates as 0,0,0 Then explode the block and save file as dxf and import this file as said above.  More over Perpendicular an inclined grids can be made in etabs but i prefer to go for importing grid because it is less time consuming and easy. 

I use etabs version 9.7.4 so i can tell u about importing plan from cad to etabs in this version.
Define a layer in CAD for example GRID LAYER. then draw ur whole grid in this cad layer.. after you finish marking grid in cad drawing, save the file as DXF FILE instead of DWG extension. Please note that the insertion units of cad drawing (you can see insertion units by UN command) and in etabs while importing this grid must be same. One u saved drawing as dxf file. open new etabs model, delete existing grid, import file as DXF file, set units before importing the file in drop down manu. Select layer of Grid and import this layer. U will get your grid import. since there are a lot of grids usually in a cad drawing, it is better to shift whole grid before savin as dxf file into a new cad drawing with origin set 0,0,0. Copy grid from old drawing as a block into new drawing in desired layer, open new cad drawing, paste and put insertion coordinates as 0,0,0 Then explode the block and save file as dxf and import this file as said above.  More over Perpendicular an inclined grids can be made in etabs but i prefer to go for importing grid because it is less time consuming and easy. 

In manual calculation we do 2D analysis of structures using different method of analysis than stiffness method. So somewhat difference will always be there while you compare results of manual vs Etabs in Systems.. For individual elements, they are 100% same.... More over, be sure to make modifiers of slab to 0.0001 to nullify slab stiffness because in manual analysis, might be u are assuming only rectangular beam and applying line load to this beam. but in etabs you are modeling slab with beam making system a bit stiffer due to slab stiffness. If you will reduce slab stiffness to negligible value, than results will be near to each other. Still there will be some difference because we might be using force methods idealizing 2D structure while etabs performs 3D analysis using direct stiffness method.