Masonry Code Of Practice Amp

Dr. A. Meher PrasadDr. A. Meher Prasad

Department of Civil EngineeringDepartment of Civil EngineeringIndian Institute of Technology MadrasIndian Institute of Technology Madras

email: [email protected]

ACI 530-02 / ASCE 5-02 / TMS 402-02

Minimum requirements for structural design and Construction of masonry unit.

Allowable Stress Design (ASD)

Limit State Design (LSD)

IBC 2000

NZS 4230: Part 1: 1990

Eurocode 6: Design of Masonry Structures

IS: 1905 - 1987

Masonry Codes of PracticeMasonry Codes of Practice

Design PhilosophiesDesign Philosophies

Working Stress Method (WSM)Working Stress Method (WSM)

Ultimate Load Method (ULM)Ultimate Load Method (ULM)

Limit States Method (LSM)Limit States Method (LSM)

“Limit states design” is supposed to be the most rational design

philosophy. Why?

Design PhilosophiesDesign Philosophies……

Design PhilosophiesDesign Philosophies• Working stress method (WSM)

- Behaviour under ‘service loads’ - All uncertainties accommodated in ‘factors of safety’ applied to material strengths

• Ultimate load method (ULM)

- Behaviour under ‘ultimate loads’ - All uncertainties accommodated in ‘load factors’

WSM attempts to ensure adequate safety under service loads, while ULM attempts to ensure adequate safety under extreme loads.

WSM does not investigate behaviour beyond service loads, ULM does not guarantee serviceability under service loads.

WSM and ULMWSM and ULM

WSM attempts to ensure adequate safety under service loads, while ULM attempts to ensure adequate safety under extreme loads.

WSM does not investigate behaviour beyond service loads, ULM does not guarantee serviceability under service loads.

A limit state is a state of impending failure, beyond which a structure ceases to perform its intended function satisfactorily, in terms of either strength or serviceability; i.e., it either collapses or becomes unserviceable.

Unlike WSM, which bases calculations on service load conditions alone, and unlike ULM, which bases calculations on ultimate load conditions alone, LSM aims for a comprehensive and rational solution to the design problem, by considering safety at ultimate loads and serviceability at working loads.

LSM is described as a ‘semi-probabilistic’ method or a ‘Level I reliability’ method

Limit States Method (LSM)Limit States Method (LSM)

Strength Design ModelStrength Design Model

pdf

Sn Rn

LOAD EFFECT S

RESISTANCE R

Load and Resistance variables 0

R and S are independent random variablesR S Structure will survive!

R < S Structure will fail!

Probabilistic approach:

nominal / characteristic values (deterministic)

Moral:

There is always a risk of failure. No structure is 100% safe!

Fs = Rn / Sn

Pf = Prob [R < S]

Reliability = Probability of survival

= 1 - Probability of failure

pdf

Sn Rn

LOAD EFFECT S

RESISTANCE R


Deterministic measure of safety:

Probabilistic measure of safety:

fS (s)

fR (r)

dsdrrfsfPS

RSf

00)()(

0 0)()(1 drdssfrfP

S

SRf

SSR 0= Prob

Rk

Sk

Load and Resistance Design (LRFD) Format

Design Resistance Design Load Effect

Sn Rn

LOAD EFFECT S

RESISTANCE R


Sn = Rn

Load factor > 1

(‘overloading’)

Resistance factor < 1

(‘understrength’)

SR

nn

(WSM format)

R Sn n

(ULM format)

Partial load and material safety factorsPartial load and material safety factors

Ultimate limit states – partial load factors:UL = 1.5 (DL + LL)UL = 1.5 (DL + QL) or (0.9DL + 1.5 QL)UL = 1.2 (DL + LL + QL)

c

Note: It is not correct to apply 33.3% increase in allowable stress in WSM when only DL and QL are involved!

Ultimate limit states – material safety factors:Concrete: = 1.5 Steel: = 1.15

c

s

Design PhilosophiesDesign Philosophies

Empirical Design

• Formulae for the design developed by experience

• Not a design analysis for sizing and proportioning masonry elements

• For simple structures still being continued in ACI 530 – 02 and to some extent in IBC 2000

• Imposes severe limitation on building height proportions

• IS 1905 – mixes empirical with allowable stress design

Design Philosophies ...

Allowable Stress Design

• All uncertainties are accommodated in Factor of safety applied to material strength

• Under working loads, the stress developed in a member must be less than the permissible stress

• For URM, tensile stress in masonry is less than allowable limits and for RM, tensile stress neglected

• ACI follows this for URM and RM

• In IS code applies only to URM

• Does not find place in Eurocode, NZS 4230

Design Philosophies . . .

Limit State Design

Adopted by ACI, IBC 2000 and New Zealand codes

Proportion masonry members such that

Design strength ≥ Required strength

where, Design strength = Nominal strength φ

φ is the strength reduction factor

Required strength is computed from design load combinations of building code

Eurocode - 6

Limit state design for Collapse and Serviceability

Partial safety factors for loads and materials are specified

separately instead of strength reduction factor

Partial safety factor for loads depends on the load combinations

Partial safety factor for materials depends on the type of masonry unit and the failure mode

Assumptions in LSD and LRFD

Strain continuity between reinforcement, grout and masonry

Max Compressive strain εmu ≈ 0.0035 for clay material

≈ 0.0025 for concrete (CMU)

≈ 0.008 confined masonry ( NZ)

Stress in steel, fs = Esε ≤ fy

= fy for ε ≥ εy ( = fy/ Es)

Flexural strength is assessed by neglecting tensile strength of masonry

But deflections assessed by including tensile strength

Masonry stress uniform over hatched block

0.8 fm (ACI) 0.85fm (IBC)

Assumptions in LSD and LRFD

Equivalent rectangular masonry stress distribution Equivalent rectangular masonry stress distribution for confined masonry according to NZS

ACI

(fa / Fa) 1

fa = Calculated compressive stress

Fa = Allowable compressive strength

= 0.25 fm R Capacity reduction factor for slenderness

Accounts for material uncertainties

Axial Compression - ASDAxial Compression - ASD

R = 1 – (h/40t)2 for h/t ≤ 29

R = (20t/h)2 for h/t > 29

P ≤ ¼ Pe where

32

2

I 21m n

e

E eP

h t

Slenderness can affect capacity either as a result of inelastic buckling or because of additional bending moment


IS 1905 :1987: a stress reduction factor ks which depends on slenderness ratio and eccentricity of load (Table 9 of Code)

Slenderness effects on axial compression


Image is Image is not clearnot clear

The The previous previous fig is the fig is the same….same….

Slenderness ratio for different EccentricitiesSlenderness ratio for different Eccentricities

Reinforced MasonryReinforced Masonry


ACI 530-02

IBC 2000, NZS:4230: Part 1, , Eurocode 6, IS: 1905-1987

No Provisions have been givenNo Provisions have been given

Maximum h/t RatioMaximum h/t Ratio

There is no directly specified limit to the h/t ratio. It is There is no directly specified limit to the h/t ratio. It is indirectly specified through the check against Euler’s indirectly specified through the check against Euler’s buckling formulabuckling formula

ACI 530-02

IS: 1905-1987

Maximum h/t ratio depends on the storey height and Maximum h/t ratio depends on the storey height and type of mortar used.type of mortar used.

IBC 2000, NZS:4230: Part 1, , Eurocode 6


Flexural stress due to

eccentricity of axial loading

application of horizontal loads such as wind and earthquake loads

ACI: A member subjected to pure flexure only

Allowable bending compressive stress, Fb = 0.33 fm

Calculated fb Fb

Axial compression with flexure: ASD

Interaction formula

1a b

a b

f f

F F Very conservative



Significance of M/Vdv factor

IS 1905:1987

Bending compressive and tensile stresses

Permissible value for bending compressive stress is increased by 25% and then reducing it for eccentric loading causing flexure.

Permissible loads for 3 eccentric values

(a) e < t/24

(b) t/24 < e < t/6

(c) e > t/6

Applied moment converted into equivalent eccentricity


e < t/24

t/24 < e < t/6

e > t/6


Axial compression with flexure

Flexure and Axial Wall Loading Interaction Diagram


Design for Shear – ASDDesign for Shear – ASD

ACI 530-02

Shear stress shall not exceed either of Shear stress shall not exceed either of

0.125 , 0.83MPa or v+0.45N0.125 , 0.83MPa or v+0.45Nvv/A/Ann



IS: 1905-1987

Permissible shear stress is given by:Permissible shear stress is given by:FFvv = 0.1 + /6 < 0.5MPa = 0.1 + /6 < 0.5MPa

Un Reinforced MasonryUn Reinforced Masonry

If shear reinforcement is not providedIf shear reinforcement is not provided

ACI 530-02

For flexural members,For flexural members,

For shear walls,For shear walls,

a. M/Vda. M/Vdvv < 1, < 1,

FFvv = 0.028[4-M/Vd = 0.028[4-M/Vdvv] < (0.55-0.31M/Vd] < (0.55-0.31M/Vdvv)MPa)MPa

b. M/Vdb. M/Vdvv > 1, > 1,

FFvv =0.083 < 0.24MPa =0.083 < 0.24MPa

FFvv = 0.083 < 0.35MPa, = 0.083 < 0.35MPa,

IBC 2000, NZS:4230: Part 1, , Eurocode 6, IS: 1905-1987




ACI 530-02

For flexural members,For flexural members,

Fv = 0.25 < 1.03MPaFv = 0.25 < 1.03MPaFor shear walls,For shear walls,

a. M/Vda. M/Vdvv < 1, < 1,

Fv = 0.042[4-M/Vdv] < (0.82-0.031M/Vdv)MPaFv = 0.042[4-M/Vdv] < (0.82-0.031M/Vdv)MPa

b. M/Vdb. M/Vdvv > 1, > 1,

Fv = 0.125 < 0.52MPaFv = 0.125 < 0.52MPa

If shear reinforcement is providedIf shear reinforcement is provided



IS: 1905-1987

Permissible shear stress is given by:Permissible shear stress is given by:Fv = 0.1 + /6 < 0.5MPaFv = 0.1 + /6 < 0.5MPa


No Provisions have been given ASD formatNo Provisions have been given ASD format

If shear reinforcement is providedIf shear reinforcement is provided

Design for Shear – ASDDesign for Shear – ASDReinforced MasonryReinforced Masonry

IS 1905:1987: Shear

Masonry load bearing walls also act as shear walls to resist in plane lateral loads.

Shear failure in URM are: (3 modes)

Diagonal tension cracks from through mortar and masonry units


vF 0.125 mf


Sliding occurs along a straight crack of horizontal bed joints

While specifying Mohr coulomb type failure criterion

Stepped cracks form, alternating from head joint to bed joint depends on bond pattern of masonry

tandc

IS 1905:1987: takes care of sliding failure by specifying permissible shear stress URM

0.16d

vF

0.5MPa

d Average axial stress not more than 2.4 MPa

Allowable shear for reinforced walls

Capacity Design for strength of flanged wallCapacity Design for strength of flanged wall

Effective Shear AreasEffective Shear Areas

LSDLSD

AXIAL COMPRESSION - LSDAXIAL COMPRESSION - LSD

Un Reinforced Masonry:Un Reinforced Masonry:

ACI 530-02

IS: 1905-1987, NZS:4230: Part 1

IBC 2000

Eurocode 6

Charactersistic Compressive strength of Masonry isCharactersistic Compressive strength of Masonry is

Maximum compressive strain is limited to 0.002Maximum compressive strain is limited to 0.002

No Provisions are givenNo Provisions are given

Reinforced Masonry:Reinforced Masonry:

ACI 530-02

Eurocode 6, IS: 1905-1987


AXIAL COMPRESSION - LSDAXIAL COMPRESSION - LSD

IBC 2000

NZS:4230: Part 1

AXIAL COMPRESSION WITH FLEXURE - LSDAXIAL COMPRESSION WITH FLEXURE - LSD


ACI 530-02, IBC 2000, NZS:4230: Part 1, IS 1905- 1987 ACI 530-02, IBC 2000, NZS:4230: Part 1, IS 1905- 1987


Eurocode 6

1. Design Md equals1. Design Md equals

2. In case of vertical load,2. In case of vertical load, increases toincreases to

3. Lateral resistance3. Lateral resistance



ACI 530-02

2. For walls with factored axial stress < 0.2f2. For walls with factored axial stress < 0.2fmm,,

1. For walls with factored axial stress < 0.05f1. For walls with factored axial stress < 0.05fmm,,

and SR > 30 shall be designed as above with walls having tand SR > 30 shall be designed as above with walls having tminmin=150mm.=150mm.

3. at extreme fiber is 0.0035 for clay masonry and 3. at extreme fiber is 0.0035 for clay masonry and

0.002 for concrete masonry.0.002 for concrete masonry.



IBC 2000

1. For wall design against out-of-plane loads, all values are 1. For wall design against out-of-plane loads, all values are same as of the ACI code, exceptsame as of the ACI code, except

2. is the same as in the ACI code.2. is the same as in the ACI code.

NZS:4230: Part 1NZS:4230: Part 1

1. is 0.0025 for unconfined concrete masonry and1. is 0.0025 for unconfined concrete masonry and0.008 for confined concrete masonry0.008 for confined concrete masonry

Eurocode 6Eurocode 6

For singly reinforced rectangular c/s subjected toFor singly reinforced rectangular c/s subjected tobending onlybending only

IS 1905- 1987IS 1905- 1987

No provisionsNo provisions

DESIGN FOR SHEAR - LSDDESIGN FOR SHEAR - LSD


ACI 530-02ACI 530-02

Nominal shear strength can be obtained from code section 3.3.4Nominal shear strength can be obtained from code section 3.3.4

IBC 2000

Nominal shear strength can be obtained from code section 2108.10.4.1Nominal shear strength can be obtained from code section 2108.10.4.1

Eurocode 6Eurocode 6

1. F1. Fvv = 0.1 +0.4 = 0.1 +0.4

2. V2. Vnn = F = Fvvtltlcc/ /

IS: 1905-1987, NZS:4230: Part 1

No ProvisionsNo Provisions

DESIGN FOR SHEAR - LSDDESIGN FOR SHEAR - LSDReinforced Masonry:Reinforced Masonry:

ACI 530-02

1. V1. Vnn = V = Vmm + V + Vss

2. When M/Vd2. When M/Vdvv < 0.25, < 0.25, VVnn < (0.083) 6A < (0.083) 6Ann

When M/VdWhen M/Vdvv > 1.00, > 1.00, VVnn < (0.083) 4A < (0.083) 4Ann

For M/VdFor M/Vdvv value between 0.25 and 1.00, V value between 0.25 and 1.00, Vnn may be interpolated may be interpolated

IBC 2000

1.1. Nominal shear strength is same as in the ACI code.Nominal shear strength is same as in the ACI code.

2. If shear wall failure mode is in flexure, M2. If shear wall failure mode is in flexure, Mnn shall be at least 1.5M shall be at least 1.5Mcrcr

for fully grouted wall or 3Mfor fully grouted wall or 3Mcrcr for partially grouted wall. for partially grouted wall.

DESIGN FOR SHEAR - LSDDESIGN FOR SHEAR - LSDReinforced Masonry:Reinforced Masonry:

NZS:4230: Part 1

1. Shear strength is given in section 7.3.2.11. Shear strength is given in section 7.3.2.1

2. Minimum value of A2. Minimum value of Avv = 0.15 b = 0.15 bwws/fs/fyy

3. 3.

4. Required area of shear friction reinforcement is4. Required area of shear friction reinforcement is

Eurocode 6

1. Ignoring shear reinforcement:1. Ignoring shear reinforcement:

2. Taking shear reinforcement into account:2. Taking shear reinforcement into account:

IS: 1905-1987No ProvisionsNo Provisions