Chapter-09 Masonry Structures under later loads Siddharth shankar Department of Civil(structure) Engineering Pulchowk Campus
Earthquake
Earthquake cause shaking of ground, so a building resting on it will experience motion at its base. The roof has a tendency to stay in its original position and the roof experiences a force, called inertia force. Inertia force is the multiplication of the weight and the acceleration, so larger the weight of the building more the earthquake shaking. F
Engineering representation of earthquake force
Masonry Structures Masonry
is brittle and tensile and shear strength is very low. Due to Large mass of masonry structures, heavy weight attracts large amounts of seismic forces. Wall to wall connection and roof connection is generally weak. Stress concentration occurs at the corners of windows and doors.
Failure Modes of a Masonry buildings
Out of plane failure In plane failure Diaphragm failure Connection Failure Failure due to opening of wall Pounding Non-structural component failure
Out of Plane Failure The Earthquake force is perpendicular to the plane. The wall tends to overturn or bend. This causes the partial or full collapse of the wall. This is due to Inadequate anchorage of wall and roof , long and slender wall, etc. Characterized by vertical cracks at corner, cracks at lintel, roof level and gable wall, etc.
In Plane Failure
The Earthquake force is parallel to the plane The wall is shear off or bend X- cracks occurs Characterized by vertical cracks at wall intersection, separation of corners of two walls, spalling of materials, etc
Diaphragm Failure Lack of anchoring produce a push of diaphragm against the wall. Absence of good shear transfer between diaphragms and reaction wall accounts for damage at corner of wall Rare phenomenon in the event of seismic motion Separation of wall and diaphragm cause collapse of buildings
Connection failure For given direction of earthquake, wall A acts as a shear wall and B acts as flexure wall. If the walls are not tied together wall B overturn (out of olane) and wall A slides (in plane) and collapse occurs. Masonry units should tied properly
Failure due to opening in walls
Opening will obstruct the flow of forces from one wall to another. Large opening in shear wall reduces the strength of wall against the inertia forces. Results diagonal cracks in the areas of masonry between opening and cracks at the level of opening. Thus, openings should small and away from corners.
Pounding
When the roofs of two adjacent buildings are at different levels, during earthquake, two buildings strike against each other is called pounding. Pounding results into cracking of the wall.
Non Structural components failure
Falling of plaster from walls and ceiling. Cracking and overturning of parapets, chimneys, etc. Cracking and overturning of partition walls. Cracking of glasses. Falling of loosely placed objects.
Ductile behaviour of reinforced & unreinforced masonry
It is the capacity of an element or structure to undergo large deformation without failure. Masonry is brittle in nature. Ductility of masonry structure is governed by the ductility of masonry units & properties of mortar. Unreinforced masonry cannot withstand tension so cracks develops. In-plane & out-of-plane failure is also due to ductility of masonry. To improve ductility reinforcing bars are embedded in the masonry, called reinforced masonry which can resist the seismic force more than unreinforced masonry.
Brittle and Ductile force-deformation behavior
Force
Brittle
Ductile
Δy
Deformation
Δu
1. Walls tend to tear apart.
2. Walls tend to shear off diagonally in direction.
3. Failure at corners of walls
4. Walls tend to collapse
5. Failure at corners of openings
6. Hammering/pounding between two adjacent buildings
7. Separation of thick wall into two layers
8.Separation on unconnected wall at junction
9.Seperation of wall from roof
Major causes of failure of masonry buildings
Non-integrity of wall floor and roof. Configuration – irregularity of building causes torsional effect. Large opening of the building. Inappropriate position of opening. Lack of cross wall in large length of wall. Lack of reinforcement make the masonry building brittle. Pounding effect. Lack of anchoring element between two walls.
Elements of Lateral Load Resisting Masonry System
Horizontal bands for integrity Connecting peripheral walls for structural robustness and integrity Plinth
band Lintel band Roof band Gable band
Roof structure Light
and strong roof is desirable. Secure
tiles/slates or use GI
sheets. Good
jointing in trusses
Concrete floors in 1:2:4 concrete with reinforcement in both directions and bend up near supports.
Overall arrangement of masonry structure
Chapter-10 Testing of masonry elements siddharth shankar Pulchowk Campus Department of Civil Engineering
Compressive Strength of Bricks and wall
Testing of Wall in compression
Diagonal Shear Test
Normally carried out: 1. Periodically to evaluate the performance of building 2.To gather information on old building in order to ascertain the methods of repair or to demolish 3. To ascertain the strength of concrete if cube tests failed.
NON DESTRUCTIVE TEST (NDT)
Elastic wave tomography Rebound Hammer / Schmidt Hammer Ultrasonic Pulse Velocity Impact Echo Test X-Rays Flat Jack Test
Elastic wave tomography
Technique used for locating shallow delaminations, cracks, and voids. Elastic wave tomography is based on two basic principles from heat transfer: conduction and radiation. Sound materials with no voids, gaps, or cracks are more thermally conductive than materials that are delaminated or contain moisture. This allows rapid areal mapping of internal conditions. It should be noted that the IT method is most useful for the detection of shallow defects and flaws. Tests For: Voids, Cracks, Moisture.
Rebound Method
Can be used to determine the in-place compressive strength of concrete within a range of 1500 – 8000 psi (10-55MPa) A quick and simple mean of checking concrete uniformity. Measure the distance of rebound of a spring-loaded plunger after it struck a smooth concrete surface. Results of the test can be affected by factors such as smoothness of concrete surface, size, shape, rigidity of specimen, age & moisture condition. Type of coarse aggregate & the carbonation of the surface.
Nondestructive Test Re-bound hammer Method
Nondestructive Test Methods
Rebound Hammer Tests Schmidt Hammer
Rebound Method Using Rebound Hammer
Ultrasonic Pulse Velocity
It uses measurement of the speed of ultrasonic pulses through the concrete to correlate concrete strength to standard strength. Allows the determination of compressive concrete strength and location of cracks. It will identify non homogenous condition in the structure such as honeycomb, voids and cracks. Size of cracks can also be determined.
Ultrasonic Pulse Velocity
Flat Jack Test
Flat jack testing is a nondestructive test of evaluating existing masonry structure. It does not require removal of masonry units - only the removal of small portions of mortar is enough. The flat jack test uses small, thin, hydraulic jacks to apply a force to a section of an existing masonry wall, and the method uses measuring devices to determine the resulting displacement of the masonry.
Flat jack testing has many useful applications:
It can be used to determine masonry compressive modulus, which is the stress/strain relationship of the masonry, or axial stress by applying axial load and measuring resulting axial deformation. It can be used to estimate compressive strength and measure the shear strength. If the destruction of the masonry units is acceptable, it can be used to directly measure the compressive strength by testing the masonry to failure.
Flat-Jack Test
Push Shear Test
Push Shear Test
Prepare the test location by removing the brick, including the mortar, on one side of the brick to be tested. The head joint on the opposite side of the brick to be tested is also removed. Care must be exercised so that the mortar joint above or below the brick to be tested is not damaged. The hydraulic ram is inserted in the space where the brick was removed. A steel loading block is placed between the ram and the brick to be tested so that the ram will distribute its load over the end face of the brick. The dial gauge can also be inserted in the space.
Push Shear Test
The brick is then loaded with the ram until the first indication of cracking or movement of the brick. The ram force and associated deflection on the dial gauge are recorded to develop a forcedeflection plot on which the first cracking or movement should be indicated. A dial gauge can be used to calculate a rough estimate of shear stiffness
