Infill Wall modelling - (calculation)

[haran5441]

Dear Sir,Mam,
Please Help me, Following calculations I have done for Masanory Infill wall is correct or not.

Initial Youngs modulus, Em = 2400 N/mm2 (Kaushik et.al.)
comprssive Strength of masanory , fm = 4.1 N/mm2 ( Kaushik et.al.)
Tensile Strenght, ft = 0 (assumed to be Zero)
Shear bond strength = 0.3 Mpa
Friction coefficient = 0.7
Maximum shear = 1 MPa

Strain at maximum stress = 0.0035 ( Kaushik et.al.)
Ultimate strain = 0.0059 ( Kaushik et.al.)
Closing strain = 0.0059 ( Kaushik et.al.)

Strut Area1 reduction - e1 = 0.0036 (assumed)
Strut Area2 reduction - e2 = 0.005 (assumed)

column size= 0.35m x 0.35m
Beam size = 0.3m x 0.375 m
full infill panel net length, lw = 4.65m
full infill panel net heigth, hw = 2.825m
thickness of wall, tw = 230 mm
Youngs modulus of concrete, Ec = 25000 N/mm2
Moment of Inertia of column, Ic = 1250520833 mm4

Horizontal offset, xo = ((4.65- (o.35/2))/4.65)x 100 = 3.76 %
Vertical offset, yo = ((2.82565- (o.375/2)))x 100 = 6.63 %

lambda = 1.07E-03 mm-1 (calculated according to the formula)

Z = pi /(2 lambda) = 1.46e3 mm
hz = Z/hw x 100 = 1.46/2.825 x100 = 51.7% restricted to 50%

Diagonal lenght, dm = sqrt (4.65^2 + 2.83^2) = 5.44 m
width of strut = 522 mm ( w = 0.175 (lamda.h)^-0.4 lw ) (gives reasonable results)
Area of strut, A1 = w.t = 522 x 230 =120077 mm2
A2 = 10% of A1 ( Assumed)

Remaining Default values are used.

When I am running Dynamic time history analysis for 2 storey infilled frames. Maximum interstorey drift I am getting as 20mm for PGA of 1g, where as for bare frame it gives reasonable results.

Please help whether any data I have calculated for masonry properties is wrong or not. I will be very much thankful to you.

Thanks
Haran

[huffte]

Hi haran5441.

I cannot claim any exceptional proficiency in detailed calculations for infilled frames, but I would direct you to the SeismoStruct Verification Manual, pages 111-136, which includes 5 example problems dealing with the modeling of infill frames.

Pages 228-232 of the SeismoStruct Users Manual would also be a valuable resource for you as details of the infill element theory are discussed there.

I would also ask what makes you believe that the 20mm drift is unreasonable? Even though the load may be 1 g, the frequency content may still be such that very little damage ensues. And the frequency content which would damage the bare frame is certainly different that that which would damage the infilled frame.

And, of course, there is always the possibility that you anre correct and that there is a problem somewhere.

I hope the manuals help haran5441. Best of luck.

[Stelios_Antoniou]

In addition to the very detailed response of huffte, I would complement
that 20mm of drift in 2.83m high storeys is around 0.7-0.8%, which is not unreasonable in the presence of infill panels. Do you have significant inelastic behaviour of failure of the infills? (for this you have to check the infill panel output in the post-processor). If not, then I would not expect drifts higher than 1% even for very high earthquake loading.
By the way, what is the max. drift in the analysis without the infills?

[haran5441]

Dear Huffte,
Thanks for your Kind reply, Sure I will read it once more and try to do it.

Thanks

[haran5441]

Dear Stelios Antoniou,
Thanks for you reply,
I have done Eigen value, Push over analysis and Time history analysis for 2storey and 4 Bays 2D building.
I am happy of Eigen value and Push over analysis for Bare frame (BF) and Fully infilled frame (FF).In all the case, Ground storey Drift is maximum. And I am getting very low interstorey drift for FF frames as shown below. Below, I am showing some of the results.

1) Eigen Value
FF (s) BF (s)
1st natural time period 0.2042 0.603
2nd natural time period 0.087 0.222

2)Push Ovr Analysis (Uniform Loading)
FF (kN) BF (kN)
Maximum Base Shear 3600 550

3) Time History Analysis

PGA FF (mm) BF (mm)


0.25g 2.8 25
0.5g 5.6 61
0.75g 8.5 132
1g 11.8 160


Please suggest me the values I am getting for Infilled frames are very low, Is it right or I am some where wrong in Modelling. Kindly help

Thanks

[seismosoft]

Dear Haran,

It is not necessarily easy for one to readily assert the dynamic analysis results that you have obtained (2.5% drift for bare frame and 0.2% drift for infilled frame) as being wrong or right, since it all depends on the details of your structure, on the frequency content of the accelerogram, etc.

All we can suggest if for you to carry out some hand-calculations and/or sanity checks, in order to gain confidence (or lack of) on the results that you have obtained.

Seismosoft Support

[Stelios_Antoniou]

Dear Haran,
The fact that in pushover the FF base shear is 7 times larger than the BF one indicates that you have extremely strong infills (or extremely weak RC columns and obviously absence of RC walls). A typical ratio would be between 1,5 and 2.
It is up to you to decide if the modelling is correct or wrong (e.g. if you have 20x20 RC columns and thick infill panels with very good mortar the results make sense).

Stelios Antoniou
SeismoSoft Developer