Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

1

Chapter 3: Categories of Limit States for Tall

Building Structures

o Load Factor and Material Factor o Ultimate Limit State and Ultimate Strength of Tall Building o Ultimate Limit State and Ultimate Strength of Shear Wall o Instability of Tall Structures o Serviceability Limit States of Tall Buildings o Survival Limit States of Tall Buildings o What About Earthquake o Ultimate Strength of Shear Walls Structures

INTRODUCTION o The design of tall building structures may be considered at various

limit states. o The limit states represent the performance during the intended life

under design loads with appropriate factor of safety and under extreme loading events and misused.

o For design purposes, these limits may be termed as serviceability, ultimate and survival limit states.

o Each force actions imposed on the structures may contribute in combination to analysis at the above limit states.

Note on Code:

ULS loading

PLS design

SLS design check

Survival misuse

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

2

1. LOAD FACTORS and MATERIAL FACTORS o There are two components to the safety factor concept in limit state

design, namely load factor, f and material factor, m.

o The load factor is for each form and combination of loading, F. Material factor is applied to the characteristic value of material strength,

S. The relation between these two is such that,

o Both factors vary with limit states according to various codified

sources o Direct comparison between codes may be misleading due to different

treatment of load and material factors in overall factor of safety, S/F.

Note on Design Stress Block:

sections CP110 CP114 BS8110/EC2

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

3

2. ULTIMATE LIMIT STATE and ULTIMATE STRENGTH

OF TALL BUILDINGS Ultimate limit state represent the attainment of limiting strength of the structure subject to the provision of the way the analysis of the structures being made.

o Although force actions on the structure are based on ultimate (factored loads) and the capacity of the individual elements is based on their quantifiable ultimate strength, the forces within the structure may be determine elastically or plastically, depending on the capacity for redistribution of internal forces.

o Because of the uncertain or incalculable post elastic behaviour of

some forms of tall building, elastic method are usually employed. Ultimate limit state is less restrictive than serviceability limit state to measure the ability of the structure to return to normal working condition before plasticity is reached at any location. In design of each element, various load combinations may be required at the ultimate limit state. For instance, three load combinations should be considered in the design of walls and foundations leading to maximum tensile and compressive forces. Wall may be designed in compression by conventional means with various modes of failure of planar walls as shown.

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

4

3. ULTIMATE LIMIT STATE and ULTIMATE STRENGTH

OF SHEAR WALLS

In coupled shear walls, onset of plasticity in the lintels is not in itself an ultimate limit state criterion, due to considerable reserve of strength against overall collapse exist.

o By analyzing the structure elastically, the potential for internal force redistribution can be assessed before the ultimate strength of the wall is attained.

o If the lintels do not contribute significantly to the structural actions, they may be neglected at the ultimate limit state, providing they behave satisfactory in service.

Recent research lead to the mode of failure and ultimate strength of the shear wall structure assessment has been made analytically. The mode of failure of walls element are highly dependent on the behaviour and modes of failure of the coupling beams.

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

5

Failure of Solid Slender Wall

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

6

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

7

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

8

flexible (racking) stiff (double curvature) cantilever

Failure of Shear Wall with Single Band of Openings

Failure of Shear Wall with Two Bands of Openings

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

9

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

10

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

11

90% 50% 30% 0% reserved

strength P

displacement

P

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

12

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

13

Punching shear

Wall

Floor slab

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

14

Associate Professor Dr A K Marsono Chapter 3: Categories of Limit States for Tall Building Structures

15