Waqas Haider

Assalam o alaikum.
According to 21.5.1, width of beams must satisfy the following
1. Clear span ln ≥ 4 effective depth General
2•Width to depth ratio bw/h ≥ 0.3
3.Width bw ≥ 10 in.
It means, i can provide 10 inch thickness in zone 4, if it is satisfying above 3 clauses. Is there further limit?

Assalam o alaikum,
 
What are load cases for which i need to check my deflection.
 
ACI table 9.5 ( b ) there need to check two types of deflections.
 
i) Immediate deflection due to live load 
 
ii) Total deflection [which is sum of long term deflection due to sustained loads [sustained load means majorly dead load and some portion of minimum live load which is permanently applied]  and immediate deflection due to live load) 
 
And long term deflection can be calculated as immediate deflection due to sustained loads (sustained load means majorly dead load and some portion of minimum live load which is permanently applied) multiplied by 2. 
 
So equation goes some what like this
 
i) for immediate deflection,
 
Deflection = immediate due to live
 
 
ii) For Total Deflection
 
Total Deflection = immediate + long term
 
where immediate deflection due to Live
 
Long term deflection = 2 x Immediate deflection due to dead             2 is taken for simplicity in case of five years
 
so
Total deflection = immediate deflection due to Live  + 2 x immediate deflection due to sustained load (dead majorly)
 
Kindly varify.

Waqas,
I will just add that for beam/ coulmns sizing its always a good idea to do a quick model and check all B/C Ratios in the beginning of the project. Also, construction guys are more used to beam size multiples of 3-probably because of formwork. So you may want to provide a beam width/ depth accordingly.
Thanks.

Assalam-o-alaikum!
 
Dear colleagues!
 
While carrying out structural steel design according to AISC code, we need to specify materials to be used for construction of various structural elements, with reference to relevant ASTM standards.
 
Which material we use, will depend upon loading conditions to be considered, structural framing system adopted and other structural design requirements.
 
But the question is, are we specifying these structural steel materials correctly? The following article, published in Modern Steel Construction, throws light on this issue and helps us understand how to specify materials for structural steel design per AISC properly.
 
http://msc.aisc.org/globalassets/modern-steel/archives/2015/02/steelwise.pdf
 
 
Regards.

Thank You so much For such a detailed Answer. Kindly also explain me about Maximum Allowed reinforcement in slabs/footings.

*SEFP Consistent Design*
*Torsion: Reinforced Concrete Members *
*Doc No: 10-00-CD-0001*
*Date: May 24, 2013*
 
Torsional forces, generally speaking, occur in combination with flexural and transverse shear forces. From a design perspective, we need to understand difference between two torsion types:             
Compatibility Torsion Equilibrium Torsion Compatibility Torsion
Compatibility Torsion is when a member twists to maintain deformation compatibility; its induced in structural members by rotations (twists) applied at one or more points along the length of member. The twisting moments induced are directly dependent on the torsional stiffness of the member. These moments are generally statically indeterminate and their analysis necessarily involves (rotational) compatibility conditions(click on the image to enlarge).
 

 
For the floor beam system shown above, the flexure of the secondary beam BD results in a rotation ǾB at the end B. As the primary (spandrel) beam ABC is monolithically connected with the secondary beam BD at the joint B, deformation compatibility at B implies an angle of twist, equal to ǾB at spandrel beam ABC, and a bending moment will develop at the end B of beam BD. The bending moment will be equal to, and will act in a direction opposite to the twisting moment, in order to satisfy static equilibrium. The magnitude of ǾB and the twisting/ bending moment at B depends on the torsional stiffness of the beam ABC and the flexural stiffness of beam BD.
 
Now here is the fun part, the torsional stiffness of a reinforced concrete member is significantly reduced by torsional cracking. So, if you don’t design your spandrels for compatibility torsion, they will crack, increasing ǾB and reducing the induced twisting moment. To paint the same picture while using ETABS, set your torsional stiffness of the main beam to zero. This will also increase the amount of flexural reinforcement in your secondary beams.
 
Moreover, considering design practice in Pakistan (since we never design beams without shear reinforcement), compatibility torsion can be ignored for regular structures, as minimum shear reinforcement in most cases would stand up to cracking torque. From ACI 318 commentary R11.6.1,
 
Do note that there are some situations (such as circular beams supported on multiple columns) where both equilibrium torsion and compatibility torsion coexist. Also, eccentrically loaded beams, member curved in plan, and member of space frames will be subjected to torsion. See the attached “Timesaving-TorsionDesign-IA.pdf” as a go-by. Timesaving-TorsionDesign-IA.pdf
 
 
Equilibrium Torsion
In simplest words, Torsion is a limit state in this condition; a structure is subjected to equilibrium torsion when it can maintain equilibrium only by resisting the torsion.  In such a case, torsional moment cannot be reduced by redistribution of internal forces since the torsional moment is required for the structure to be in equilibrium. From ACI- 318 (click on the image to enlarge).
 
 
 

 
Moreover, see the structures below that defy gravity when subjected to different kind of loads by standing up to equilibrium torsion.
 
Overall Building Torsion
For overall building torsion, the torsional effects can be minimized by reducing the distance between the center of mass and center of rigidity. Center of Mass is the point where the mass of an entire story is assumed to be concentrated. The center of mass is crucial as the location of seismic force at a particular level depends upon it.  The distance between the Center of Mass and the Center of Rigidity should be minimized, but may not be possible due to building geometry. Invariably, effects of torsion are present in at all buildings although analysis may show that in some buildings torsional effects are negligible. 

Dear Zain,
 
I don't know the origin of document,you have uploaded for calculating torsional constant,but the methodology given therein is incorrect.As "Tcr" and "Tu" given therein are indeed threshold torsional strength and ultimate torsional stresses respectively, and are both design properties  not analysis properties. (See ACI 318-11 section 11.5.1).
 
Whereas the torsional constant, ETABS asks in "analysis property modification factors" is simply the torsional moment of inertia (J) used to determine torsional stiffness of a member (JG/L) i.e something else.
 
As long as its value is concerned,then in building structures it is a general practice to use a negligible value like .001 to nullify beam's torsional stiffness.In this way, the torsional stresses (if arising due to compatibility of deformation i.e compatibility torsion ) are transferred via alternate load path (i.e redistribution of torsional moments occurred), considering that beam is unable to provide torsional restraint and in other condition if torsional stresses in beam is required to satisfy equilibrium of structure (where redistribution is not possible) then torsional stresses in beams remains independent of whatever value of "J" you have selected as equilibrium equations are necessarily satisfied independent of stiffness as "Compatibility is optional and equilibrium is essential".
 
This approach of minimization of "J" economize beam sizes that arise from stringent combined shear and torsion requirement of building codes,but consequently beam sections designed in this way will start developing internal horizontal cracks (hairline cracks not affecting functionality of structure) due to torsional stresses and their torsional strength will continuously degrade till the design condition is achieved i.e negligible torsional strength of beam.But as the structure is designed to be stable without torsional stiffness of beam so it remain stable after this condition is achieved.However, the beam member itself cracks that doesn't affect the functionality of structure in any way.
 
A very descriptive and clarifying description is available in "Reinforced concrete design by Arthur Nilson".
 
As long as authentication of this approach is concerned then it is allowed by building codes as,
 
1, ACI-318-11 section 11.5.2.1 & 11.5.2.2.
 
2, UBC97 section 1911.6.2.1 & 1911.6.2.2
 
3, BS 8110-1 1997 section 3.4.5.13
 
Keeping in view above mentioned, it is a general practice to nullify torsional constant of beams in building structures and it is not required to use any iterative process to derive torsional constant of each beam section that is indeed not practical as there will be thousands of beam span in large structures.

For interpreting the ETABs Results see the 1st picture in Attaachment
As far as choosing a torsional constant for a member see the second doc. in attachmetns

Torsional Constant for Beams in ETABs .docx

Assalam o alaikum.
I have been doing calculations of foundations manually. I m new user of SAFE. I have not used it for any practical project yet so i m not sure whether i m doing correct modelling or not. Kindly correct me if I am doing wrong/missing some step.
 
1) I made an ETABS model with its based as fixed. (because my site is in Quetta zone 4 (SDC E) so i cant model base as pinned because lateral drift exceeds limits)
2) After running model, I exported base of project by EXPORT MODEL---> TO SAFE V.12 F2K ---> Selected Story is BASE and Selected option is LOAD FROM AND ABOVE ALL FLOOR and selected load cases ALL (because i needed all)
3) Then I imported model into safe. It imported all load cases, along with all load combos. 
4) The after defining material, soil properties and slab frames (as footing), I assigned slabs to relevant points.
5) I assigned design strips in X and Y direction to the slabs and I assigned SOIL SUPPORT to the slabs in the way that SELECT SLAB-->ASSIGN SUPPORT DATA-->SOIL SUPPORT--->SELECT SOIL----OK
6) Now I dont need to assign any thing further because SAFE is meshing slabs automatically at max size of 4ft.
7) I just did RUN the project and then RUN DETAILING option.
8) Only step left is to read results of analysis and design.
 
My confusions:
a) Whether am i missing some steps?
b ) Are, in this way, Load sizes (i.e. column sizes for shear checks and bending moment etc) also imported?
c) Does soil support value matters in determining soil pressure under footing?? Or it just comes to importance when determining deformations under footing?? 
d) my bearing capacity is 0.75 tsf = 1650 psf  so tentative value of my Modulus of sub grade is 1650x12x3 = 59400 pcf.... Am I right?? here 3 is FOS to convert allowable bearing capacity to ultimate bearing capacity. 12 is to convert inch to ft. so my final value is 59400 pcf. Am i right?
e) If I model two slabs with ONE EDGE of both matching. Will SAFE automatically treat these two slabs as one and will mesh them??
 
Thanks.

Assalam o alaikum.
I have been doing calculations of foundations manually. I m new user of SAFE. I have not used it for any practical project yet so i m not sure whether i m doing correct modelling or not. Kindly correct me if I am doing wrong/missing some step.
 
1) I made an ETABS model with its based as fixed. (because my site is in Quetta zone 4 (SDC E) so i cant model base as pinned because lateral drift exceeds limits)
2) After running model, I exported base of project by EXPORT MODEL---> TO SAFE V.12 F2K ---> Selected Story is BASE and Selected option is LOAD FROM AND ABOVE ALL FLOOR and selected load cases ALL (because i needed all)
3) Then I imported model into safe. It imported all load cases, along with all load combos. 
4) The after defining material, soil properties and slab frames (as footing), I assigned slabs to relevant points.
5) I assigned design strips in X and Y direction to the slabs and I assigned SOIL SUPPORT to the slabs in the way that SELECT SLAB-->ASSIGN SUPPORT DATA-->SOIL SUPPORT--->SELECT SOIL----OK
6) Now I dont need to assign any thing further because SAFE is meshing slabs automatically at max size of 4ft.
7) I just did RUN the project and then RUN DETAILING option.
8) Only step left is to read results of analysis and design.
 
My confusions:
a) Whether am i missing some steps?
b ) Are, in this way, Load sizes (i.e. column sizes for shear checks and bending moment etc) also imported?
c) Does soil support value matters in determining soil pressure under footing?? Or it just comes to importance when determining deformations under footing?? 
d) my bearing capacity is 0.75 tsf = 1650 psf  so tentative value of my Modulus of sub grade is 1650x12x3 = 59400 pcf.... Am I right?? here 3 is FOS to convert allowable bearing capacity to ultimate bearing capacity. 12 is to convert inch to ft. so my final value is 59400 pcf. Am i right?
e) If I model two slabs with ONE EDGE of both matching. Will SAFE automatically treat these two slabs as one and will mesh them??
 
Thanks.

Assalam o alaikum.
I have been doing calculations of foundations manually. I m new user of SAFE. I have not used it for any practical project yet so i m not sure whether i m doing correct modelling or not. Kindly correct me if I am doing wrong/missing some step.
 
1) I made an ETABS model with its based as fixed. (because my site is in Quetta zone 4 (SDC E) so i cant model base as pinned because lateral drift exceeds limits)
2) After running model, I exported base of project by EXPORT MODEL---> TO SAFE V.12 F2K ---> Selected Story is BASE and Selected option is LOAD FROM AND ABOVE ALL FLOOR and selected load cases ALL (because i needed all)
3) Then I imported model into safe. It imported all load cases, along with all load combos. 
4) The after defining material, soil properties and slab frames (as footing), I assigned slabs to relevant points.
5) I assigned design strips in X and Y direction to the slabs and I assigned SOIL SUPPORT to the slabs in the way that SELECT SLAB-->ASSIGN SUPPORT DATA-->SOIL SUPPORT--->SELECT SOIL----OK
6) Now I dont need to assign any thing further because SAFE is meshing slabs automatically at max size of 4ft.
7) I just did RUN the project and then RUN DETAILING option.
8) Only step left is to read results of analysis and design.
 
My confusions:
a) Whether am i missing some steps?
b ) Are, in this way, Load sizes (i.e. column sizes for shear checks and bending moment etc) also imported?
c) Does soil support value matters in determining soil pressure under footing?? Or it just comes to importance when determining deformations under footing?? 
d) my bearing capacity is 0.75 tsf = 1650 psf  so tentative value of my Modulus of sub grade is 1650x12x3 = 59400 pcf.... Am I right?? here 3 is FOS to convert allowable bearing capacity to ultimate bearing capacity. 12 is to convert inch to ft. so my final value is 59400 pcf. Am i right?
e) If I model two slabs with ONE EDGE of both matching. Will SAFE automatically treat these two slabs as one and will mesh them??
 
Thanks.

W.Salaaam Waqas !
 
1) No ! you are not missing anything....
2) choose the 3rd option in Loads to export ...(have a look to the attached screenshot)
3) Yeah Soil support value matters in soil pressure because high modulus of  sub grade value means more stiffer is the soil and hence more will be the soil pressure.
4) Your value is OKKY but I usually take the safty factor 2.5 (average of 2 & 3) because safty factor is 2 for granular soil  & 3 for cohessive soil.
    (0.75x2.204x1000x2.5x12=49590 pcf)
5) i would model it as a single footing slab, so it will act as combined footing....

Assalam o alaikum.
I have been doing calculations of foundations manually. I m new user of SAFE. I have not used it for any practical project yet so i m not sure whether i m doing correct modelling or not. Kindly correct me if I am doing wrong/missing some step.
 
1) I made an ETABS model with its based as fixed. (because my site is in Quetta zone 4 (SDC E) so i cant model base as pinned because lateral drift exceeds limits)
2) After running model, I exported base of project by EXPORT MODEL---> TO SAFE V.12 F2K ---> Selected Story is BASE and Selected option is LOAD FROM AND ABOVE ALL FLOOR and selected load cases ALL (because i needed all)
3) Then I imported model into safe. It imported all load cases, along with all load combos. 
4) The after defining material, soil properties and slab frames (as footing), I assigned slabs to relevant points.
5) I assigned design strips in X and Y direction to the slabs and I assigned SOIL SUPPORT to the slabs in the way that SELECT SLAB-->ASSIGN SUPPORT DATA-->SOIL SUPPORT--->SELECT SOIL----OK
6) Now I dont need to assign any thing further because SAFE is meshing slabs automatically at max size of 4ft.
7) I just did RUN the project and then RUN DETAILING option.
8) Only step left is to read results of analysis and design.
 
My confusions:
a) Whether am i missing some steps?
b ) Are, in this way, Load sizes (i.e. column sizes for shear checks and bending moment etc) also imported?
c) Does soil support value matters in determining soil pressure under footing?? Or it just comes to importance when determining deformations under footing?? 
d) my bearing capacity is 0.75 tsf = 1650 psf  so tentative value of my Modulus of sub grade is 1650x12x3 = 59400 pcf.... Am I right?? here 3 is FOS to convert allowable bearing capacity to ultimate bearing capacity. 12 is to convert inch to ft. so my final value is 59400 pcf. Am i right?
e) If I model two slabs with ONE EDGE of both matching. Will SAFE automatically treat these two slabs as one and will mesh them??
 
Thanks.

Is this a MUST check to perform? What If I just provide ratios obtained from ETABS and dont satisfy it?
 
From Etabs I got much lesser required rations i.e. 2 legged #3 @ 4 inch is satisfying my etabs required ratio. But ETABS MANUAL of concrete frame design CFD-ACI-318-08, Topic 3.4.4.3 says
  "The column shear reinforcement requirements reported by the program are based purely on shear strength consideration. Any minimum stirrup require- ments to satisfy spacing considerations or transverse reinforcement volumetric considerations must be investigated independently of the program by the user."    
ACI 318-08 R21.6.4.4 says
"Instead, Eq. (10-5) and (21-4) are required, with the intent that spalling of shell concrete will not result in a loss of axial load strength of the column. Equations (21-3) and (21-5) govern for large-diameter columns, and are intended to ensure adequate flexural curvature capacity in yielding regions."

The document presented by Sir Umer clearly says that this stress increment should not be taken even in case of earth quack loads if 0.75 reduction factor has been already used in load combinations.
 
More over, in sizing of footing, we go for ASD approach because bearing capacity given by geotechnical engineer is based on the concept that full factor of safety of 2-3 is applied on soil strength and no loads are increased. This is ASD methodology. So to be compatible, we also go for ASD while sizing of foundation. So in my opinion, if you are using load combinations which are carrying 0.75 reduction factor, you can not increase soil stress by 33% even not for earth quack loads. but if you are using the load combinations which are not having 0.75 reduction, then you can increase this stress as 33% but just for those cases where two or more than two transient loads are acting simultaneously.

What is meant by SW waseem bhai?

I want to get real moment forces in shells too. For the time being, positive moments are less and negative are more because of the reason that etabs has modelled slab-wall connection as monolithic. But when this connection will be released for moments, positive moments in slab and wall will increase. I want to model this in such way.
Thanx.

Assalam o alaikum,
I am to model a basement in ETABS. Basement has 4 side walls and a slab at top. If I model slab as it as at top of walls, it behaves as partially fix support and produces moment at connections between slab and walls. But i want to release this moment.I want my slab to behave as Simply Supported where it connects wall and as monolithic where slab connects with beam and also I want my walls acting as Propped Cantilevers instead of both ends fixed. Also suggest me how to provide same connection in real by detailing of reinforcement. 
Jazak Allah.
Thanks.

Dear Khalid !
 
i go through your model and upto my understanding your model is OK but 2 corrections are required.
 
1) As you mention that the zone of the building is ZONE 3.So you must have to check the Beam Column Joint Ratio which you are saying that it is not done.it is because you have overwrite the sway property from Sway Special to Sway Intermediate.So change your sway type from Sway Intermediate to Sway Special then you will get the required B/C Ratio.
 
2) In defining the slab you have define slab as a MEMBRANE and membrane takes in-plane stresses only. And in reality the slab also take out of plane stresses by deflecting/bending. So define slab as a SHELL THIN then it will take both in-plane and out-plane stresses.
 
Have a look to the snaps and the model with the two corrections i have made.


Sari Pul_V13.rar

There are two noticeable references mentioned in this thread.
 
One is specific to Load Combinations(ASD stuff) and subsequent 1/3rd reduction, the other is about C12.13.4 of ASCE 7-05: 25% reduction of seismic overturning moment for the case of Equivalent Static Lateral Force Procedure. For some reason, all the replies have been focused on ASD Load Combinations and subsequent 1/3rd reduction. Anyways, ASD is old in a matter of few years would be completely wiped out of the scene for foundations. The current practice in industry is to use LRFD for foundation design. Moreover, make sure you touch base with the Geotechnical Engineer.  Sometimes geotech reports specify what could be done or not with regards to thread subject.
Thanks.

The document presented by Sir Umer clearly says that this stress increment should not be taken even in case of earth quack loads if 0.75 reduction factor has been already used in load combinations.
 
More over, in sizing of footing, we go for ASD approach because bearing capacity given by geotechnical engineer is based on the concept that full factor of safety of 2-3 is applied on soil strength and no loads are increased. This is ASD methodology. So to be compatible, we also go for ASD while sizing of foundation. So in my opinion, if you are using load combinations which are carrying 0.75 reduction factor, you can not increase soil stress by 33% even not for earth quack loads. but if you are using the load combinations which are not having 0.75 reduction, then you can increase this stress as 33% but just for those cases where two or more than two transient loads are acting simultaneously.

yes Waqas you got it right..No increase in 0.75load combinations as in ASCE/ACI

The document presented by Sir Umer clearly says that this stress increment should not be taken even in case of earth quack loads if 0.75 reduction factor has been already used in load combinations.
 
More over, in sizing of footing, we go for ASD approach because bearing capacity given by geotechnical engineer is based on the concept that full factor of safety of 2-3 is applied on soil strength and no loads are increased. This is ASD methodology. So to be compatible, we also go for ASD while sizing of foundation. So in my opinion, if you are using load combinations which are carrying 0.75 reduction factor, you can not increase soil stress by 33% even not for earth quack loads. but if you are using the load combinations which are not having 0.75 reduction, then you can increase this stress as 33% but just for those cases where two or more than two transient loads are acting simultaneously.

I just came across this. Here is an article by AISC that addresses that. Its not allowed anymore and reasons are in the article.
13rd stress increase AISC.pdf

thanks for your question and answer.
that's useful suggestion for me also.