Project 1 - MODELLING, ANALYSIS & DESIGN OF TALL BUILDINGS IN ETABS

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a. Modelling – Key Highlights & Workflow

1. Understand the Architectural Plan

• Identify:

  • Grid layout

  • Typical floor plan

  • Location and size of beams, columns, cores, and walls

  • Staircases, shafts, and elevator zones

  • Load-bearing and non-load-bearing elements

2. Import the Architectural Floor Plan

• Use DXF / DWG file from the architect.

• In ETABS:

  • Go to Plan View > File > Import DXF.

  • Import as background drawing for tracing.

  • Align the grid if necessary.

3. Set Up Grids and Story Levels

• Define:

  • Number of floors

  • Floor-to-floor heights (e.g., podium, typical, terrace)

• Use "Edit Grid Data" to create:

  • Orthogonal or radial grids

  • Custom names for levels (B1, G, L1–L30...)

4. Define Materials

• Concrete (e.g., M30, M40)

• Steel (Fe500, structural steel if composite sections used)

• Rebar properties (as per IS 456 or design standard)

• Go to Define > Materials and input:

  • Compressive strength

  • Modulus of elasticity

  • Poisson’s ratio

  • Unit weight

5. Define Section Properties

• Slabs – Thickness (150–200 mm typical), one-way or two-way

• Beams – Size based on architectural constraints and span

• Columns – Rectangular or circular, different for core/periphery

• Walls – Shear walls, lift walls

• Use Define > Section Properties > Frame/Wall/Slab

6. Model the Structural System

• Use:

  • Shell elements for slabs and walls

  • Frame elements for beams and columns

• Model:

  • Typical floor plate first (copy to upper levels later)

  • Core walls and lift shafts (accurate to architectural drawing)

  • Staircase areas as slab openings or separate modeling

b. Loadings & Load Combinations in ETABS

1. Define Load Patterns (Gravity Loads)

Go to: Define > Load Patterns

Add the following basic Gravity Load Patterns:

Load Name	Type	Self Weight Multiplier
DEAD	Dead	1.0
LIVE	Live	0.0
SUPERDEAD	Dead	0.0
FLOOR_FINISH	Dead	0.0
TERRACE_LIVE	Live	0.0

ETABS automatically applies self-weight based on section definitions if multiplier = 1.0.

2. Define Load Patterns (Lateral Loads)

Seismic Loads

• Add: EQX, EQY

• Type: Seismic

• Direction: X or Y

• Select appropriate seismic code: IS 1893 (Part 1): 2016

Enter key seismic parameters (from IS 1893):

Parameter	Value (example)
Zone Factor (Z)	0.16 (Zone III)
Importance Factor (I)	1.5 (hospital)
Response Reduction (R)	5 (SMRF)
Soil Type	II (Medium)
Damping Ratio	5% (default)

Wind Loads

• Add: WINDX, WINDY

• Type: Wind

• Use code: IS 875 Part 3

• Define wind pressure profiles manually OR use automated wind load generation.

Example Parameters:

Parameter	Value (example)
Wind Speed (Vb)	50 m/s (Chennai)
Exposure Category	C
Terrain Category	3
Height	15+ storey building
Topography Factor	1.0

ETABS will compute wind pressure profile based on height.

3. Define Load Combinations (IS 456:2000 – Limit State Method)

Go to: Define > Load Combinations > Add Default Combo > IS 456:2000

Key Load Combos (Ultimate & Service):

Ultimate Limit State (ULS)

• 1.5 (DL + LL)

• 1.2 (DL + LL ± EQ)

• 1.5 (DL ± EQ)

• 1.2 (DL + LL ± Wind)

• 1.5 (DL ± Wind)

Serviceability Limit State (SLS)

• 1.0 (DL + LL)

• 1.0 (DL ± EQ)

• 1.0 (DL ± Wind)

You can customize combos for different cases (e.g., terrace live, long-span beams).

4. Define Mass Source for Earthquake Analysis

Go to: Define > Mass Source

Choose either:

• From self and additional loads

• Or specify manually

Typical setup:

Source	Included in Mass	Included in Weight
Self Weight	Yes	Yes
Super Dead / Floor Finish	Yes	Yes
Live Load (25% as per IS 1893)	Yes (0.25)	No

This ensures correct base shear calculation for seismic forces.

Summary Checklist

• Gravity loads defined (DL, LL, etc.)

• Lateral loads (Seismic, Wind) set as per IS codes

• Load combinations created for ULS & SLS

• Mass source correctly defined for seismic analysis

c

Shear Force and Moment Diagrams for Floor Beams & Columns

For Floor Beams:

• Step 1: Apply Loads – Start by applying dead and live loads to the beams. These loads can be calculated from floor slab loads, partition walls, and other building elements.

• Step 2: Identify Boundary Conditions – Define the boundary conditions of the beams, including fixed, pinned, or roller supports, and ensure the load application is realistic.

• Step 3: Use Equilibrium Equations – Calculate reactions at supports using static equilibrium equations:

  $$\Sigma M = 0, \quad \Sigma F_y = 0, \quad \Sigma F_x = 0$$

  where $M$ is the moment, and $F$ is the force.

• Step 4: Shear Force and Moment Diagrams – Use the equilibrium equations to create shear force and moment diagrams for each beam. The shear force diagram will be piecewise linear, and the moment diagram will be piecewise quadratic (for uniform loads).

For Columns:

• Step 1: Apply Axial Loads – The column will experience axial loads from the beams and slabs.

• Step 2: Use Equilibrium for Vertical Forces – For each column, calculate the axial load based on the load distribution from the floor beams. Use the same static equilibrium principles for reactions at the column base.

• Step 3: Moment Distribution – For columns subject to moments, use the moment-curvature relationship to calculate bending moments at the top and base.

2. Check the Gravity Load Path

Gravity Load Path Verification:

• Step 1: Define Load Distribution – Ensure that the gravity loads (dead load, live load, etc.) are distributed through the slabs, beams, and columns correctly.

• Step 2: Trace the Path – Confirm that the gravity load path follows the intended route from the floor slabs to the foundation. This means ensuring that loads from slabs go to beams, beams to columns, and columns to the foundation.

• Step 3: Ensure Proper Load Transfer – Check that the load transfer is efficient and that the load distribution through the structural system follows the designer’s intent.

3. Perform Quick Hand Calculations for Base Reactions

Quick Base Reaction Calculation:

• Step 1: Identify Loads on the Structure – Identify the dead load, live load, and any additional loads on the beams and columns.

• Step 2: Use Static Equilibrium for Base Reactions – Apply static equilibrium equations for the structure as a whole. For a simple structure (e.g., a single beam with two supports), you can calculate the reaction forces by taking moments about one support and solving for the other.

  $$R_A = \frac{\sum M_B}{L}, \quad R_B = \frac{\sum M_A}{L}$$

  where $R_A$ and $R_B$ are the reactions at the supports, $\sum M_B$ and $\sum M_A$ are the moments about the respective supports, and $L$ is the span length.

• Step 3: Verify Results – Quickly check these hand calculations with a structural analysis software like ETABS, STAAD Pro, or similar, ensuring that the reactions from the software match the hand-calculated values.

d.

1. Select Appropriate Parameters for the Design of the Members

Beam Design Parameters:

• Material Properties:

  • Concrete Strength (f'c): Typically taken from the structural specifications (e.g., 30 MPa, 40 MPa).

  • Steel Yield Strength (fy): Common values are 415 MPa or 500 MPa depending on the region and code.

• Cross-Sectional Shape: Choose the type of beam cross-section, like rectangular, T-beam, I-beam, or hollow section, depending on the loading conditions and architectural layout.

• Beam Length (L): The span length between supports.

• Live Load (LL) and Dead Load (DL): Define these based on the floor usage (e.g., residential, office, etc.).

• Beam Supports: Whether the beam is simply supported, fixed, or continuous, affecting the bending moment and shear force distribution.

Column Design Parameters:

• Column Type: Short or slender, which will influence the buckling behavior and slenderness ratio.

• Concrete Strength (f'c) and Steel Yield Strength (fy): Similar to beams.

• Column Dimensions (d, b, h): Define the column cross-section dimensions (e.g., square, rectangular, circular).

• Load Type and Magnitude: Identify the axial loads and bending moments (if any) acting on the column.

• Column Height (h): The height of the column influences the slenderness ratio and buckling strength.

Slab Design Parameters:

• Slab Thickness (t): Based on span and loading.

• Live and Dead Loads: Applied uniformly or point loads, depending on the layout and function of the floor.

• Support Conditions: The slab can be simply supported, continuous, or fixed at the edges.

• Reinforcement Type and Grade: Use appropriate rebar grade based on local building codes.

2. Interpret the Design Results

Once you have input all the parameters into a structural design tool like ETABS or STAAD Pro, the software will provide detailed design results. Here's how to interpret the output:

For Beams:

• Bending Moment (M): The maximum bending moment and its location should be checked to ensure the beam is adequately designed to resist bending.

• Shear Force (V): The shear capacity at critical points (e.g., supports, mid-span) should be verified.

• Deflection: Check that the deflection does not exceed allowable limits, usually $L/360$ for beams under live load.

• Utilization Ratio: The ratio of the required capacity to the provided capacity (e.g., $f_{cd}$ for concrete, $f_{yd}$ for steel).

For Columns:

• Axial Load Capacity: For short columns, compare the axial load to the column's axial capacity. For slender columns, the interaction of axial load and bending should be checked using the slenderness ratio.

• Slenderness Ratio (λ): Check if the column is prone to buckling. For slender columns, the buckling capacity should be calculated according to the relevant code.

• Utilization Ratio: Ensure that the column’s utilization ratio is within safe limits (typically ≤ 0.85).

For Slabs:

• Bending Stress: Check the maximum bending stress at critical sections (mid-span or edges).

• Shear Stress: Ensure shear stress is within the design limits for the slab thickness.

• Reinforcement Requirements: Verify if the slab requires distribution and main reinforcement to resist bending, shear, and torsion.

3. Re-Design and Restrict Member Utilization Within 0.9

After reviewing the initial design results, you may find that some members are over-utilized, meaning they are working beyond their capacity (e.g., utilization ratio > 1). To optimize the design and restrict member utilization within a safe limit (commonly ≤ 0.9):

• Increase Member Size: Increase the dimensions of beams, columns, or slabs to reduce the utilization ratio.

• Enhance Material Strength: You can opt for higher strength concrete or steel to improve the member’s capacity.

• Change Reinforcement: Adjust the amount or type of reinforcement (e.g., increase the number of bars, change the rebar grade, or adjust spacing).

• Optimize Loading or Layout: Redesign the load distribution to reduce moments and shear forces at critical locations.

4. Identify the Reinforcement Requirements for Each Structural Member

For each member (beam, column, slab), reinforcement is designed to resist bending moments, shear forces, and axial loads. Here's how to determine the reinforcement:

Beam Reinforcement:

• Flexural Reinforcement: The primary reinforcement will be placed in tension zones (usually the bottom of the beam). Use the bending moment to calculate the required steel area using:

  $$M_{u} = A_s f_y (d - a/2)$$

  where $A_s$ is the area of steel, $f_y$ is the yield strength of the steel, $d$ is the effective depth of the beam, and $a$ is the distance from the compression face to the neutral axis.

• Shear Reinforcement: Stirrup or shear links are designed based on the shear force at critical sections (near supports and mid-span).

Column Reinforcement:

• Axial Load Reinforcement: Column reinforcement depends on the axial load and the interaction with bending. Use the axial load capacity and bending moments to calculate the necessary reinforcement using the interaction equation from the relevant code.

• Longitudinal Reinforcement: Based on the column size and axial load, calculate the number of longitudinal bars required.

• Transverse Reinforcement: Provide lateral ties or spiral reinforcement to resist buckling, especially for slender columns.

Slab Reinforcement:

• Main Reinforcement: Located at the bottom of the slab for positive bending and at the top for negative bending (over supports).

• Distribution Reinforcement: Generally placed perpendicular to the main bars to resist shear and reduce cracking.