Abstract
The presence of infill walls in a building increases the stiffness and mass of the building leading to significant changes in the fundamental period. If this increase in stiffness is not considered, it may cause severe damages to the buildings during the earthquakes. This paper presents an analysis of the fundamental period of buildings considering the effect of infill walls. For this, a computer program was created to generate buildings with different configurations and calculate the fundamental period. It was found that besides the geometric parameters: number of stories and the height of the building, the bay span also had a significant effect on the fundamental period. Thus, a formula that takes into account those parameters was formulated for infilled reinforced concrete (RC) frames. The building codes must take parameters like bay span and the presence of infill walls into account for estimating the fundamental period.
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Code availability
The code of the program used for fundamental period calculation is available at a Github repository (https://github.com/bpanthi977/fundamentalperiod) of the author. Another repository has the code used for regression analysis and exploration of generated data. (https://github.com/bpanthi977/fundamentalperiodworkbook).
References
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No funding was received to assist with the preparation of this manuscript.
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KBT conceived of the presented idea. PS carried out sample verification of buildings for member sizes using SAP2000. BP and PD wrote programming languages in LISP and MATLAB. BP made programs for the regression analysis, sensitivity analysis, and curve fitting in LISP. PD and PS conducted curve fitting and regression analysis in MATLAB and Excel. KBT supervised the findings of this work. BP, PD and PS wrote manuscript with the support from KBT. KBT reviewed the final manuscript and made subsequent corrections.
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Appendices
Appendix
Sample calculation performed by the computer program
To elaborate on the method that the computer program uses, a sample calculation is presented here:
In the following steps, the fundamental period of the building with the belowmentioned geometry will be calculated.
Geometry
Number of stories (\(N = 3\)), story height (\(h = 3\)), bay span (\(B_{s} = 5\)) and two bays along each axis (\(B_{x} = 2,B_{y} = 2\)).
Member sizes and strength
Column size 0.3 m by 0.3 m, Beam Depth 0.500 m, Beam Width 0.300 m, Wall Thickness 0.230 m, Slab thickness 0.150 m, Column Elasticity \(5000\sqrt {25} {\text{MPa}} = 2.500*10^{10} {\text{Pa}}\), Wall Elasticity \(2.65*10^{9}\) Pa, Concrete Unit Weight 25 kN, Masonary Unit Weight 20 kN or Concrete Density = 2548.420 kg/m^{3} and Masonry Density = 2038.736 kg/m^{3}.
Mass matrix
Mass calculation for Floor 1 and 2.
Total extra load of (3.5 kN/m^{2} = 356.779 kg/m^{2} (live load + floor finish)) on the floor is added on slab and subtracted from column and wall area.

1.
Each column = 2548.420*0.300*0.300*2.850 − 356.779*0.300*0.300 = 621.560 kg

2.
Columns total = 621.560*9 = 5594.04 kg

3.
Each beam span = 2548.420*4.700*0.300*0.350 = 1257.645 kg

4.
Beams total = 1257.645*12 = 15,091.74 kg

5.
Each wall span = 2038.736*4.700*0.230*2.500 − 356.779*4.700*0.230 = 5124.006 kg

6.
Walls total = 5124.006*12 = 61,488.072 kg

7.
Slab = 2548.420*10.300*10.300*0.150 + 10.300*10.300*356.779 = 78,404.944 kg
Total mass = 160,578.796 kg.
And from the distribution of mass from the floor plan, the centroids and the mass moment of inertia about the centroid is calculated to be:
XC  5.000  Xcoordinate of centroid 
YC  5.000  Ycoordinate of centroid 
MASS  160,578.798  Total mass 
IXX  1,734,595.484  Moment of inertia along Xdirection from the centroid 
IYY  1,734,595.484  Moment of Inertia along Ydirection from the centroid 
IPC  3,469,190.968  Polar Moment of Inertia 
Mass calculation for 3rd floor

Each column = 2548.420*0.300*0.300*1.350 = 309.633

Each beam span = 2548.420*4.700*0.300*0.350 = 1257.645

Each wall = 2038.736*4.700*0.230*1.000 = 2203.874

Slab = 2548.420*10.300*10.300*0.150 + 10.300*10.300*0.000 = 40,554.283
MASS  84,879.207 
IPC  1,838,611.448 
Finally, we get the mass matrix:
160,578.798  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0 
0.0  160,578.7981  0.0  0.0  0.0  0.0  0.0  0.0  0.0 
0.0  0.0  3,469,190.968  0.0  0.0  0.0  0.0  0.0  0.0 
0.0  0.0  0.0  160,578.7981  0.0  0.0  0.0  0.0  0.0 
0.0  0.0  0.0  0.0  160,578.7981  0.0  0.0  0.0  0.0 
0.0  0.0  0.0  0.0  0.0  3,469,190.968  0.0  0.0  0.0 
0.0  0.0  0.0  0.0  0.0  0.0  84,879.2070  0.0  0.0 
0.0  0.0  0.0  0.0  0.0  0.0  0.0  84,879.2070  0.0 
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  1,838,611.448 
Stiffness matrix

Area moment of inertia of a column \(I_{c} = \frac{1}{12}0.3^{4}\) = 6.75*10^{–4}

K of column = \(12\frac{{E_{c} I_{c} }}{{h^{3} }}\) = 75*10^{5}
Infill wall (length l = 5–0.3 = 4.7 m, height = 3–0.5 = 2.5 m).

\(L_{ds}\) = 5.323

\(\theta\) = 0.489

\(\alpha\) = 3.289

\(w_{ds}\) = 0.579

\(t_{ds} = \max \left( {\frac{h}{12},\frac{l}{12},\;b = 0.23} \right)\) = 0.392

Strut’s stiffness \(K = \frac{{t_{ds} w_{ds} E_{w} }}{{l_{ds} }}\cos^{2} \theta\) = 8.794*10^{7}
For a single frame in Xdirection (single floor).

Stiffness from walls = 8.794*10^{7}*2 = 17.58*10^{7}

Stiffness from columns = 75*10^{5} * 3 = 2.25 * 10^{7}

Total stiffness = \({k}_{1}={k}_{2}={k}_{3}=19.84*{10}^{7}\)
Local stiffness matrix (\(K_{lx1}\)).
396,755,259.2  − 198,377,629.6  0.0 
− 198,377,629.6  396,755,259.2  − 198,377,629.6 
0.0  − 198,377,629.6  198,377,629.6 
Local coordinate to the global coordinate transformation matrix \({A}_{x1}\) (for the first frame along Xaxis).
1.0  0.0  5.0  0.0  0.0  0.0  0.0  0.0  0.0 
0.0  0.0  0.0  1.0  0.0  5.0  0.0  0.0  0.0 
0.0  0.0  0.0  0.0  0.0  0.0  1.0  0.0  5.0 
Finally, Stiffness matrix is obtained \(K = \sum \left[ {A_{xi}^{T} K_{lxi} A_{xi} + A_{yi}^{T} K_{lyi} A_{yi} } \right]\).
1,190,265,777.6  0.0  − 0.0000009537  − 595,132,888.8  0.0  0.00000047684  0.0  0.0  0.0 
0.0  1,190,265,777.6  0.0  0.0  − 595,132,888.8  0.0  0.0  0.0  0.0 
− 0.0000009537  0.0  39,675,525,920  0.00000047684  0.0  − 19,837,762,960  0.0  0.0  0.0 
− 595,132,888.8  0.0  0.00000047684  1,190,265,777.6  0.0  − 0.0000009537  − 595,132,888.8  0.0  0.00000047684 
0.0  − 595,132,888.8  0.0  0.0  1,190,265,777.6  0.0  0.0  − 595,132,888.8  0.0 
0.00000047684  0.0  − 19,837,762,960  − 0.0000009537  0.0  39,675,525,920  0.00000047684  0.0  − 19,837,762,960 
0.0  0.0  0.0  − 595,132,888.8  0.0  0.00000047684  595,132,888.81  0.0  − 0.0000004768 
0.0  0.0  0.0  0.0  − 595,132,888.8  0.0  0.0  595,132,888.81  0.0 
0.0  0.0  0.0  0.00000047684  0.0  − 19,837,762,960  − 0.0000004768  0.0  19,837,762,960 
Once we have the stiffness and mass matrix we can solve the eigenvalue problem \(\left( {K  M\Omega^{2} } \right)\Phi = 0\).

Eigenvalues \(\left\{ {\omega_{i}^{2} } \right\}\) = 13,587.9, 7273.8, 974.4, 20,947.9, 11,212.6, 1502.0, 974.4, 13,587.9, 7273.8

Timeperiods (sorted) \(T_{i} = 2\pi /\omega_{i}\) = 0.2012, 0.2012, 0.1621, 0.0736, 0.0736, 0.0593, 0.0539, 0.0539, 0.0434,
Hence, the fundamental period is 0.2012s.
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Panthi, B., Dahal, P., Shrestha, P. et al. Fundamental period of RC buildings with infill walls in Nepal. Asian J Civ Eng 22, 983–993 (2021). https://doi.org/10.1007/s4210702100359y
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Keywords
 Fundamental period
 Seismic design
 Infilled frames
 Masonry wall
 Nepali buildings
 Computer modeling