Panos Kloukinas, Viviana Novelli Innocent Kafodya, Ignasio ... · chemical bonds Brittle mechanical...

UK - Malawi Disaster Research Workshop MUST, 6 -10 August 2018

Mechanical Characteristics of Masonry,with Reference to the buildings in Malawi

Panos Kloukinas, Viviana NovelliInnocent Kafodya, Ignasio Ngoma

Day 4: Structural and Earthquake Engineering


Consisting elements of masonry:


the micro-mechanics of masonry materials Granular structure/

particles cemented together with chemical bonds

Brittle mechanical behaviour / failure at small deformation

Strong in compression, very weak in tension


External actions and internal forces MomentsMoments

are the internal stresses, expressing the intensity of force per unit area (N/m2)

ForcesForces


Behaviour of the “composite” material

brickbrick

interfaceinterface

mortarmortar

Strength: governed by the weaker material

Deformation: governed by the softer material


Example: tensile cracks in block in compression


Characteristics of Malawi Local Clay Bricks:Factors affecting strength

Uncontrolled raw material / soil consistency

Uncontrolled / non-uniform firing conditions


Other deficiencies from the production process:Pre-existing cracks, voids, organic material, stones etc


Other deficiencies from the production process:

Geometry issues:

Irregular shape, variable size, etc


How can we say if a brick is strong or not?Simplified Quality Control

method for local bricks

Malawi Government – Department of Lands, Housing and Urban Development (2016)


Bending Beam theory

Where I is the area moment of inertia:

Stress along the section is:

Maximum stress at outermost point, at:


Experiment: 3-point bending (flexure) test

Brick A Brick B

b (mm)h (mm)l (mm)F (N)

F

Which one is stronger? Brick A or B?


3-point Bending test – maximum tensile stress (indirect tensile strength)

Bending moment diagram

,

F

,

N/mm2 or MPa =106 N/m2 (Pa)

(kN/m2 = kPa= 10-3 MPa)


Experiment: calculations & conclusion

Brick A Brick Bh (mm)F (N)σtf,b


5 different areas,40 sampling sites

14 random samples per site- 6 / 14: in-situ stepping test- 6 / 14: 3 – point flexure test

Experimental results from field testing


Experimental results from field testing:Stepping test vs flexural strength


Mortar characteristics for Malawian buildingsMortar: either mud, or a mixture of sand and cement

Recommended: not less cement than 1_6 (1 cement to 6 parts of sand)In practice: could be 1_8, 1_10 or

whatever the owner can afford

Other factors affecting mortar quality: Sand not clean from clay Environmental conditions –initial water loss & curing period

No use of lime


How cement mortar builds up strength?

Cement + H2O 28 days!

Hydration mechanism!


How a lime material builds up strength?Carbonation mechanism!

When lime is used in the mixture: Increased water retention

Improved bonding with the bricks, i.e. stronger interfaces!

Other factors affecting bonding: Water absorption from bricks

Not clean brick surface, but covered by a coating of clay or dust


Experiment – interface in tension1_4F 1_4U 1_8F 1_8U M

b (mm) 85 85 85 85 85F (N) 420 250 220 92 33

,∗ ∗

* In the above force measurement we need to add the weight of the

hanging brick: ~ 16N

Mortars: Mud, 1_8 & 1_4 cement to sandF, U (favourable and unfavourable bonding


Experiment – interface in shear1_4F 1_4U 1_8F 1_8U M

b (mm) 85 85 85 85 85L (mm) 195 195 195 195 195T (N) 2800 1950 1300 650 385


How the strengths we measured relate to the resistance of a wall element?


Numerical example: free-end wall, out-of-plane

Unit weight of the wall:17000 N/m3

(Kg/m3) (m/s2)

Distributed horizontal load (N/m):

𝜎6𝑤𝑙

2𝑏𝑡3𝑎 𝛾 𝑙

𝑡𝑀

𝑤𝑙2

𝑤 𝑎 𝑥 𝛾 𝑥 𝑏 𝑥 𝑡Stress due to bending:


Numerical example: restrained wall, out-of-plane

Unit weight of the wall:17000 N/m3

(Kg/m3) (m/s2)

Distributed horizontal load (N/m):

𝜎6𝑤𝑙

8𝑏𝑡3𝑎 𝛾 𝑙

4𝑡

𝑤 𝑎 𝑥 𝛾 𝑥 𝑏 𝑥 𝑡

As before, stress due to bending:


Numerical example: restrained wall, out-of-planeWhich are the critical acceleration values for these cases?

𝛾 𝑙 𝜎 ,3𝑎 , 𝛾 𝑙

𝑡

𝜎3𝑎 𝛾 𝑙

𝑡

𝛾 𝑙 𝑜𝑟 𝑙/2

𝑜𝑟 3𝑎 𝛾 𝑙

4𝑡

We need to take also into account the pre-existing axial compressive stress due to the gravitational load, i.e.

𝛾 𝑙/2 𝜎 ,3𝑎 , 𝛾 𝑙

4𝑡

Constrained wall:

Free-end wall:


Walls under out-of-plane loading, collapse Cracking does not mean collapse! Overturning moment of

the seismic action, i.e.ah x 𝑊 x 𝑙/2

>Stabilising action of wall

vertical load, i.e.𝑊 x tw/2

ah x

Critical ratio: tw / 𝑙 !


Walls in out-of-plane loading, collapse

Post cracking:Free body rocking motion -resistance and stiffness of the system is decreased

Collapse:Governed by kinematics,

when the wall rocking exceeds certain drift limits


The actual 3D behaviour in out-of-plane


Visualisation of the out-of-plane behaviour


Walls under out-of-plane loading, conclusions

Strong connections with side walls and restraint at the top (ring beam, rigid roof truss on wall plate etc) will increase the wall resistance!

Long walls unsupported or high & slender walls are more vulnerable in out-of-plane!

Wall thickness is critical for both: a) the increase of wall strength (before cracking) and b) the wall block stability (post cracking)!


Walls under in-plane loading


,

Walls under in-plane loading, base shear failure

Horizontal force:

Base shear stress:

Critical acceleration for shear failure:


, ,

Walls under in-plane loading, tension failure

Horizontal force:

Tensile stress on the diagonal:

Critical acceleration for tensile failure:


Observations and Conclusions Walls are weaker in out-of-plane than in-plane

The most critical parameter for both in-plane and out-of-plane is the tensile strength!

~ out-of-plane: tension generated because of the bending ~ in-plane: tension on the diagonal because of the shear

Even if the material strength is low, we can increase the resistance of our structure by adding simple geometrical components that can decrease the stresses in the walls.

i.e. increased wall thickness, abutments and buttresses, wall connections and restraints, ring beams etc


How all these combine together on a whole building subjected to earthquake loading?

Panos Kloukinas, Viviana Novelli Innocent Kafodya, Ignasio ... · chemical bonds Brittle mechanical...

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