Distribution of Forces in Lateral Load Resisting Systems

Transcription

1 Distribution of Forces in Lateral Load Resisting Systems Part 1. Vertical Distribution and Load Paths IITGN Short Course Gregory MacRae Many slides from 2009 Myanmar Slides of Profs Jain and Rai 1

2 Gravity Forces

3 Gravity Forces Gravity forces are from MASS going through a vertical acceleration (gravity) Modified from Rai, Murty and Jain

4 Gravity Load Path Consider a box on the slab The box has mass so it is subject to gravity force If that force is not resisted, the box will fall to the ground

5 Gravity Load Path We can say that: The box is subject to force from gravitational attraction by gravity causing bearing slab and is supported by

6 Gravity Load Path We can say that: We must provide the slab with enough bending/shear strength. Reinforcing steel The slab is subject to force from the box by bearing It acts in bending/shear and is supported by beams

7 Gravity Load Path We can say that: We must provide the beams with enough bending/shear strength. Reinforcing steel The beams are subject to force from the slab by bearing They act in bending/shear and are supported by columns

8 Gravity Load Path We can say that: We must provide the columns with enough compressive and bending/shear (b/s) resistance. Reinforcing steel P M The columns are subject to force from the beams by shear/bending. They acts in compression & b/s and are supported by footings

9 Gravity Load Path We can say that: We must provide the footing with enough steel to resist the bending and shear. Reinforcing steel P M V The footing is subject to force from the columns by compression and b/s. It acts in bending/shear soil. and is supported by

10 Gravity Load Path We can say that: We must provide the footing with enough area so that the bearing capacity of the soil is not exceeded. The ground is subject to force from the footings by shear/compression. It acts in compression and is supported by deeper soil.

11 Building Structures Structural Systems Rai, Murty and Jain

12 Tooth-pick columns, if gravity designed!! Rai, Murty and Jain

13 Seismic Forces Points

14 Gravity Load Path Consider a box on the slab The box has mass so it is subject to gravity force If that force is not resisted, the box will fall to the ground

15 Vertical Loads: EQ versus Gravity Factor of safety on gravity load design Earthquake induced vertical inertia forces are generally small!! Vertical component of earthquake-induced inertia force Gravity Load Rai, Murty and Jain

16 Earthquake-Induced Force EQ shaking generates inertia forces At every location where mass is (e.g., at floors, in beams, walls, contents, etc) F = ma Lateral accelerations are the primary concern. Load paths are needed. Rai, Murty and Jain

17 Earthquake-Induced Force EQ inertia forces tend to be greater with height Wind forces Earthquake Forces

18 Lateral Load Transfer Path Rai, Murty and Jain

19 EQ Induced Horizontal Inertia Force Significant mass (and hence inertia force) is at floor level Rai, Murty and Jain

20 Lateral Load Path We can say that: We must provide the slab with enough in-plane enough bending/shear strength. Depends on slab stiffness Reinforcing steel The slab is subject to force from the earthquake by shaking. It acts in in-plane bending/shear and is supported by frames

21 Lateral Load Path We must provide the slab-frame (SF) connection with enough shear strength. Drag struts needed? The SF connection is subject to force from the slab by shear. It acts in shear and is supported by frame

22 Lateral Load Path We must provide the frame with enough bending/shear reinforcement. The frame is subject to force from the SF connection by shear. It acts in moment frame action and is supported by footings

23 Lateral Load Path We must design the footing to transfer forces to soil. The footing is subject to force from the slab by shear, axial/bending. It acts in moment frame action and is supported by footings

24 Building Structures Structural Systems Rai, Murty and Jain

25 Challenges of Steel Structures Tooth-pick columns, if gravity designed!! Seismic columns need more bracing To develop plastic moment hinges Rai, Murty and Jain

26 Bracings In Steel Structures Seismic structures Large amount of bracing required Rai, Murty and Jain

27 Bracing In Steel Structures Seismic structures Bracing needed even in low rise buildings Columns are very flexible Rai, Murty and Jain

28 Bracings In Steel Structures Seismic bracing Not slender Much like structural members Rai, Murty and Jain

29 Bracing In Steel Structures Seismic bracing Can be architectural features Rai, Murty and Jain Rai, Murty and Jain

30 Thank you!! 30