Full structural analysis and design of commercial building project - Modeling and design (part - 3)

Modeling and Analysis

1. Structural Modeling of Al-Nijmah Commercial Center 
A three dimensional structural model of Al-Nijmah Commercial Center building established using ETABS software. The model built using preliminary cross sections taken from the introduction to graduation project for columns and beams. Solid slab of 35 cm thickness on (50cm X75 cm) beams is used for all floors so the beam depth of 75 cm serves as preliminary dimension for stiff beam as required by tabulated coefficients method.

After analysis, sections of beams and columns changed such that they resist applied loads.

Figure 5. 1: Three Dimensional Structural Model of the Commercial Center

Figure 5. 2: Three Dimensional Shear Wall System the Commercial Center


The structure will be designed to resist both gravity and lateral loads, therefore the slab must be defined as rigid diaphragm such that it supports the gravity load, transfer lateral loads to the vertical structural elements such as shear walls and moment resisting frames according to their relative stiffness.

It's important that the model in the ETABS or any structural analysis software that the model includes the stiffness of the elements which resist the lateral load, and this can be achieved by considering cracked section properties of the structural elements defined in ETABS model by reducing moment of inertia as specified in ACI-10.10.4.1 as follows:

➧ Compression members: 
Columns …. 0.7 Ig
Walls-Uncracked ………0.7 Ig
Cracked ……….0.35 Ig

➧ Flexural members: 
Beams…….0.35 Ig
Flat Slabs and flat plates…….0.25 Ig
Where: Ig is the gross moment of inertia of the element cross section.


2. Static and Dynamic Analysis of Al- Nijmah Commercial Center
5.2.1
Definition of equivalent static force procedures done according to UBC97 code. So two load patterns defined in both X and Y direction. Different parameters needed for analysis are shown in figure 5.3.

Figure 5. 3: Definition of Equivalent Static Lateral Force in the X-Direction in ETABS

It is obvious from figure 5.3 that story range defined from basement 1 to top floor so that the free height of the building will be only considered in the calculation of base shear.


2. Dynamic Analysis Definition in ETABS 
As mentioned in chapter 4, dynamic lateral force procedure will be used as an analysis method which takes into consideration dynamic properties of structure and earthquake.

Dynamic lateral force procedure can be carried out using either of the two approaches: 

➧ Time history analysis
➧ Response spectrum

In this project the dynamic lateral force procedure performed using response spectrum analysis as a dynamic analysis which is permitted by UBC1657.5.3 for soil profiles SA, SB, SC, or SD.

Response Spectrum Analysis:
Definition of response spectrum function in ETABS is shown in figure 5.4.

Figure 5. 4: Definition of Response Spectrum According to UBC-97

Definition of Response Spectrum Load Cases
As the response spectrum function has been defined, two load cases in the X and Y directions are defined as shown in figures 5.5 and 5.6 respectively. 

Figure 5. 5: Definition of Response Spectrum Load Case in the X-Direction

Figure 5. 6: Definition of Response Spectrum Load Case in the Y-Direction

3. Scaling of Dynamic Analysis Results
For the response spectrum analysis, UBC97 requires in sections 1631.5.4 and 1659.7.3 that if base shear determined by response spectrum is less than that determined by equivalent static lateral load procedure; it should be scaled up to the base shear determine by equivalent static lateral load procedure. Therefore, the initial scale factor for response spectrum is (I×g)/Rwhere g is the acceleration due to gravity (9.81 m/sec2), R =5.5 for building frame system and R=6.5 for dual system and I =1. After the analysis, base shear determined by response spectrum in both directions must be scaled to base shear from equivalent static method.

Therefore, after the analysis static base shear founded to be greater than the dynamic one and the scale factor modified so the response spectrum base shear is scaled up to the static shear.


Direction Static Analysis base shear (KN) Response Spectrum base shear (KN)
Before scaling x 4883 3228
y 4830 3210
After scaling x 4833 4833
y 4830 4830

4. Load Combination 
Load Combinations generated by ETABS in accordance with ACI-318 and UBC97 and checked manually depending on ACI-9.2.1 and UBC 2211.4 Part I, section3. Load combinations are:

Combination 1= 1.4 D.L

Combination 2= 1.2 D. L + 1.6 L. L

Combination 3= 1.3 D. L + 1 L. L+ (1 RSx + 0.3 RSy)

Combination 4= 1.3 D. L + 1 L. L+ (0.3 RSx + 1 RSy)

Combination 5= 0.8 D. L + (1 RSx + 0.3 RSy)

Combination 6= 0.8 D. L + (0.3 RSx + 1 RSy)

Orthogonal effects that can be noted in load combinations are to be considered in designing structural elements for 100 percent of the design seismic forces in one direction plus 30 percent in the perpendicular direction as stated in UBC 1633.1.


Design and Detailed Drawings

1. Analysis and Design of Slabs 
Analysis and Design of the Floors Slabs
Each slab within the structure was exported to be analyzed and designed using SAFE software. Slab divided into strips in both directions, each strip has a width of one meter. Then analysis and design done for these strips.

Figure 6. 1: Middle and Column Strips Distribution Along X-Direction


Figure 6. 2: Middle and Column Strips Distribution Along Y-Direction

Analysis and Design of Ramps
The ramps within the basements floors designed as simply supported one-way solid slabs of 28 cm thickness. They will support a live load of 8 KN/m².

Analysis and Design of Staircase Slab
Staircase slab designed as one way simply supported solid slab of 22 cm thickness to carry a live load of 5 KN/m2 and its own weight.

2. Design of the Structural Frames
Design of Columns 

As the analysis results of the three dimensional model had been accepted as shown in chapter 3, a column has been chosen to be designed manually and the result compared to that given by the ETABS to give approximately the same results. So, design of all columns within the structure taken from the software.

The following table summarize columns cross section for the fifth basement floor as can be also seen from the detailed structural drawing for columns.

Table 6. 1: Columns Cross Section in the Fifth Basement Floor 
Column Label Column Cross Section Basement floors Reinforcement Reinforcement Ratio (%)
C1 30X30 8 Φ 16 1.79
C2 60X30 12 Φ 16 1.34
C3 55X55 16 Φ 18 1.34
C4 60X60 32 Φ 18 2.26
C5 65X65 24 Φ 20 1.78
C6 70X70 24 Φ 20 1.82
C7 75X75 24 Φ 20 1.34
C8 80X80 24 Φ 20 1.18
C9 90X90 28 Φ 28 2.13
C10 100X100 28 Φ 25 1.37
C11 120X120 32 Φ 25 1.09
C12 120X120 40 Φ 30 1.96
C13 130X130 40 Φ 28 1.46
C14 130X130 44 Φ 32 2.09


Design of Beams
Design of beams was taken from ETABS after a sample calculation done to verify ETABS analysis results. The preliminary cross section of 50 cm X 75 cm changed for some beams and remains the same for others. The following table summarize beams cross sections for all floors as can be also seen from floors plans drawings and the detailed structural drawing for beams.

Table 6. 2:Beams Cross Sections 
Beams Cross Sections
35X35 (hidden)
40X50
50X75
60X80


Transfer Beam:
The transfer beams are members that will transfer the axial load of the tension and compression columns, to adjacent columns in order to eliminate some columns in the 6th and 7th floors for architectural purposes.

These beams are resting on columns of dimensions ranges from 60cm x 60cm to 100cm x 100cm. Depth of transfer beam is larger ordinary one because of high shear resulted from concentrated axial load of column.

For beams details see drawings of (B8, B9, B12, B13 and B14) In the 6th floor.

3. Shear Walls and Boundary Elements Design
Shear walls designed using ETABS for shear forces, axial and bending effects. Manual check done for design results.

Shear wall system within the structure is assumed to carry almost the lateral load from the earthquake forces so they should be designed as special shear walls according to ACI21.9.

A check for the need of boundary elements done and results shown that no need for boundary elements. Boundary elements can be defined as a compression member to be provided along boundaries of walls. Check can be done by two methods which are displacement based and stress based methods. Stress based method used based on ACI21.9.6.3. Manual check and detailing of all shear walls done according to ACI21.9.2 -21.9.6.


4. Analysis and Design of Mat Foundation 
Mat foundation analysis results exported from ETABS to SAFE and designed using SAFE.

Depth of mat foundation determined to avoid punching shear, for huge columns drops used to avoid punching shear. Punching shear checked using SAFE and checked manually.

Figure 6. 3: Punching shear check for a) column does not need a drop. b) column needs a drop c) The same column in (b) after drop added

Value of 0.93 in Figure 6.4 (a) is the ratio between punching shear and concrete capacity. If the value less than 1 then the depth of mat foundation is safe for punching shear.

Check for stresses done so that all stresses are less than bearing capacity of soil and no existence for uplift forces. Mat foundation designed by SAFE, checked and detailed manually.

Φ 25/15 cm is used in both directions at bottom and top of mat foundation and an additional bars used where needed.


5. Analysis and Design of Basement and Retaining Walls 
Basement walls modeled and analyzed using SAP2000 program, and designed manually, just the same as one-way solid slab design.
The sample basement wall that is shown in the figure below has a total height of 17.75 m and extending through 7 floors, and it has a total thickness of 45 cm.

Figure 6. 4: Sample basement wall model

Basement wall modeled as shown in the figure simply supported at the level of each slab, and it is subjected to horizontal earth pressure due to the backfill behind it and to vertical load due to building loads (dead and live). The thickness of the wall is determined based on concrete shear strength, which must be at least equals the ultimate applied shear.

Flexural reinforcement in both directions calculated for 1m strip as done in solid slabs design.


Conclusion

In this project, a general study of the floors, columns, shear walls, foundations, basement walls stair case and ramps is performed. Serviceability requirements for the deflection of slabs and beams are considered.

All columns are checked for slenderness and sway, resulting in short and non-sway columns, for which no magnification of moment is to be needed. Reinforcement of columns is about 1-3% of gross area, this is to ensure a ductile behavior of columns.

Shear walls added to the east building to carry lateral loads. Shear walls through earthquakes effects are extensively investigated to determine design criteria and forces. Shear walls design is based on shear failure and checked for flexure.

Basement walls thickness determined based on shear failure. The main reinforcement in basement walls is in the vertical direction and the secondary reinforcement is placed in the horizontal direction.

In the west building, building Frame system used in which shear walls resist lateral loads and frames resist vertical loads while in the east one dual system used in which both shear walls and frames resist lateral loads and vertical loads carried by frames.

Mat foundation is used to support the stresses of the superstructure. The bearing stress of the soil due to loadings was between 120 and 450 kN/m2. The depth of the foundation for most columns is 1200 mm and some critical columns needed a larger depth and hence drops used. Minimum reinforcement controls the design of mat foundation.


References
➧ Wang, Chu, and Charles G. Salmon. Reinforced Concrete Design. 7th ed. Hoboken, NJ: John Wiley & Sons, 2007. Print.
➧ Uniform Building Code. 1997 ed. Whittier, CA: International Conference of Building Officials, 1997.
➧ PIQUE, J. & BURGOS, M. Effective Rigidity of Reinforced Concrete Elements In Seismic Analysis And Design. 14th World Conference on Earthquake Engineering October, 2008. (12-17).
➧ Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary. Farmington Hills, MI: American Concrete Institute, 2011. Print.
➧ Qudaimat, Musa. Seismic Design of Concrete Structures, 1st ed. Babel Printing Press. 2012. Print
➧ Nazzal S. Armouti, and International Code Council., Earthquake Engineering: Theory and Implementation. 2nd ed. Country Club Hills, IL: International Code Council, 2008.
➧ Das, Braja M. Principles of Foundation Engineering, SI. 7th Ed.; SI ed. Stamford, CT: Cengage Learning, 2011. Print.

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