Expert: Babak Esmailzadeh Hakimi - 2/2/2012

Question
QUESTION: Dear sir
First of all I thank you for clearing my confusions over lateral load distribution of a model.(in case of non twisting structures load application is same but the analysis is different which take care of the software itself

1) in case of area elements why it is not allowed modify the weight (weight modifier) EVEN THOUGH this facility is available

2) Can you give me rough idea no of division in case of meshing to get accurate result

ANSWER: Dear Priyantha

You are welcome. If you apply your wind loading like what happens actually and you arrange carefully your wind forces you will not need to find the center of your diaphragm. ( take a look at an image i sent you)

1- No idea about why no weight modifier is allowed.

2- The engineering criteria is you have to divide so that in the case of doubling the number of elements the accuracy of your calculation doesn't change by more than 5%. Generally you are constrained to your analysis and design requirements . I suggest you a minimum of 10 elements in each direction. Another idea is the element size being at least equal to thickness of your wall.

Regards
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear Sir
METHOD-1
I use BS 8110- 1985 for my RC design. According to table 3.16 there are shear force coefficients(ß) for uniformly loaded(n) rectangular panels supported on four sides with provision for torsion at corners
Loads on supporting beam= ß n lx  (uniform load)
Lx=short span length
This distributes ( .75l) of the beam supporting a two way spanning slab
To idealized this situation  followed this procedure using SAP
A)   Modeled the structure
B)   In case of slabs set weight modifier closer to zero (add new section->stiffness modifier->weight modifier-> zero)
C)   No other uniform loads apply to slabs
D)   Instead uniform line load(ß lx) apply to beams
E)   Uniform load(n) can be manipulated using factors in  “ load pattern definition
METHOD-2
Further  this model  was analysed  by normal method (mesh the slab and apply the load)
METHOD-3
Further I tried  above method -1 using ETABS also

Question
1)   Basically load transfer of slabs to beams affect  moments/shear of columns beams and service load on foundations etc..( Hence I feel this going to be the most  critical part that we have to handled  don’t know whether correct or not)
I  observe that there are significant differences between results of method 1 & 2 can this be happen
2)   Further above method -1 cannot  adopt in ETABS DUE TO FOLLOWING REASONS
*There is no “load pattern “ definition in ETABS
*in ETABS it is not allowed make the weight modifier zero (define wall section->set modifiers->analysis stiffness modification factors- weight modifier) it is automatically disabled in the programe
Sir can you explain this

ANSWER: Dear priyantha

1- The first method is based on an mixed analytical and experimental method, which takes into account probability of loading concentration instead of uniform loading and also to have some loading arrangement to reach to maximum intensity for positive and negative moments and shears. These types of design are usually more conservative and in general situation where you don't know how exact your loading might be it is recommended. However you can use a method matching your own situation.

2- Instead of that option you can use a specific concrete material with near zero weight and assign it specially to your slabs.

Regards
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear Sir
"probability of loading concentration instead of uniform loading"
In all types of designs we assumed live loads as uniform loads eventhough they are not so which codes take care of.

1).Sir I feel in case of software we got real analytical back ground.However other factors like assuming load conditions ,end conditions etc. are assumptions.(eventhough load transfer from slabs to beam is real the load on slab is assumption). Therefore codes has provisions (safety factors etc..)  to cater this situation(both analysis and design).Sir what do you think about it
2) Earlier I raised a question with regard to column and foundation connection . here some opinion I got what do you think about it
"
The column may be rigidly attached to the footing slab, but the footing is
not fixed to the ground. Almost all columns on isolated footings can't stand
by themselves without props or interconnections. A column on an isolated
footing is statically unstable except for marginal case.

Some designers assume partial fixity of a column base because the footing
can resist bending moment coming on the column. The footing resisting the
bending moment doesn't mean the column base is fixed. It only means that
the footing along the column tilt and leading to an unequal compression of
the soil below the footing. The fact there could be variable soil reaction
pressure below a footing indicates rotation of the footing. The footing and
column interconnection by a rigid joint should not be considered as a rigid
support to the column. The beams and column connections are also rigid but
the columns are not considered fixed at such locations.

Deep foundations such as pile and well foundations are good examples of the
fixed bases. The raft foundation can also be considered as fixed support to
the columns resting on them.  An individual footing fixed on rock with
proper dowel bars having full development length and shear can also be
considered as fixed base. Combined footing in the length direction can
provide fixity to the column bases. The assumption of an isolated footing as
a hinge support to column is more realistic and on safer side.
Partial fixity / hinged support assumption leads to unsafe design in case
such partial restraint from the foundation is not available.
"

Answer
Dear priyantha

1- Structural design is an art : to use loadings which are not known exactly, in a combination which is not known exactly , to analysis of structure assuming a simple model instead of the real building located on the soil without knowing exactly how this model would match with the real one and to  size members with materials having unknown parameters , strength and behaviors but to perform all these in a manner that no one has any doubt about the efficiency of your calculations. The science of deriving design methods from this jungle of uncertainties is : Reliability

2- Your vision is perfect in my opinion


Regards

Babak E HAkimi