Home Trust Development Ltd.

30/01/2019

12/06/2013

1.0 STRUCTURAL ANALYSIS & DESIGN OF NEW BUILDINGS

The engineering design will be based on selection of suitable and economic structural system, loading, analysis and design of structural elements, selection of suitable construction materials, and incorporation of requirement of architectural considerations.
All components of the buildings will be designed as per Bangladesh National Building Code (BNBC) and other codes of practices like ACI code.

The structures will be designed to sustain the effects of three broad categories of load namely:
Dead Loads
Live loads
Lateral loads

Dead loads comprise the weight of the structure itself and any permanent fixture, partitions, finishes etc. Live loads arise from the occupancy (intensity and location) in the structure. Lateral loads consist mainly of wind pressure and earthquake loads.

The Bangladesh National Building Code (BNBC-1993) includes the Seismic Zoning Map and various provisions for design of buildings for earthquake loads. Basic wind speed Map and design provisions for wind loads are also given in the proposed code. Assessment of wind and earthquake loading will be based on the proposed BNBC.

For structural analysis of the building frame a three dimensional (3D) static analysis will be performed by using of STAAD.ProV8i and/or ETABS and other software. A manual calculation will be also performed in different critical positions for cross checking. In reinforced concrete design computations, the ultimate strength design (USD) method will be followed.

Foundation design of the structure will be performed by analyzing the supplied subsoil investigation report. The foundation design consultants will analyze the subsoil report and select the suitable foundation type in respect of structural safety and economic considerations.

All working drawings will be prepared in appropriate working scale. The scale of the drawings and sizes of the drawing sheets will be decided such that they are suitable for convenient reading and easy reproduction. Specific schedules, notes and instructions will be furnished in the drawings, wherever necessary, to supplement information and to offer easy reference.

The structural design consultant shall submit requisite sets of working drawings and design computation to the client.

12/06/2013

2.0 ASSESSMENT AND VULNERABILITY STUDY OF EXISTING BUILDINGS

Assessment and vulnerability study is an important tool for knowing the real status of the old buildings. The Assessment will highlight and investigate all the risk areas, critical areas and whether the building needs Immediate attention. It should also cover the structural analysis of the existing frame and pinpoint the weak structural areas for

Static loads
Wind loads and
Earthquake loads.

If the building has changed the user, from residential to commercial or industrial, this should bring out the impact of such a change.

12/06/2013

2.1. Steps to be followed for Assessment and Vulnerability Study

2.1.1 It is very important that we must have Architectural and Structural drawings of the building; it will be helpful if we have detailed structural calculations including assumptions for the structural design. The assumptions can also include the allowable live loads; whether the building is designed for residential, commercial, light industry or heavy industry and whether any future provision for adding new floors is considered? What type of Earthquake loads is considered?

2.1.2 If the Architectural Plans and Structural plans are not available, the same will be prepared by measuring the size of the building and locating the position of the columns, beams and size of all such structural elements. Other structural parameters will be collected by Non Destructive Test (NDT), Core Cutting, and Chipping etc.

2.1.3 A detailed inspection of the building can reveal the following :

Any settlements in the foundations.
Visual cracks in columns, beams and slabs.
Cracks in shear walls and partition walls
Concrete disintegration and exposed steel reinforcements.
Deterioration in concrete.
Extent of corrosion in reinforcement.
Status of Balconies – sagging, deflection cracks etc.
Status of Architectural features viz. fins, canopies etc.
Leakages and dampness in walls resulting into cracks and corrosion.
Changes carried out affecting structure. Toilet blocks - Added or changes made? Change of user – from Residential to Commercial to Industrial? Change of Partition Walls?

2.1.4 It is important that various tests will be carried out in the old buildings. This will give an idea about the extent of corrosion, distress and loss of strength in concrete & steel.

Tests may include:

Concrete Core Cutting and Compression testing for columns, beams and slabs for Strength Assessment of concrete.
Rebound Hammer Test
Carbonation test for carbonation depth measurement for Steel.
Rebar tensile test by cutting rebar of the building etc.

2.1.5 Computation to find out the Structural Capacity – With the available or tested result data, structural assessment will be performed by the following
Japanese Standard for Seismic Evaluation of Existing Reinforced Concrete Buildings, 2001
ASCE Standard – Seismic Evaluation of Existing Buildings (ASCE/SEI 31-03),
(ASCE-American Society of Civil Engineers)
Other Conventional Methods

2.1.6 If the building is structurally safe, Retrofitting Design is not necessary otherwise, Retrofitting design will be performed by best fitted technology and economic considerations.

12/06/2013

2.2. Tests to be performed for Assessment and Vulnerability Study:

Non Destructive Tests:

The following NDT tests are required to be carried out on structural elements. However,
it is important that the testing scheme is prepared based on preliminary survey of the
Building or Structure:

Core tests to determine the estimated equivalent in situ compressive strength and to establish correlation between Rebound hammer test and in situ strength of concrete.
Rebound Hammer test to estimate the in situ compressive strength of cover concrete.
Carbonation test to assess the depth of carbonated concrete.
Cover test to assess the cover provided to RCC structural members.

12/06/2013

2.2.1. Core Test:
Steps to the Core Tests:
The reinforcement will be detected at planned location with the help of Rebar Locator to avoid cutting of reinforcement.
The Core cutting equipment will be fixed at the planned location and core is extracted.
The Cores are transported to the laboratory and visual observations of cores are recorded for interpretation purpose. Reinforcement bars, if encountered, are cut off.
The Cores are removed from water cut to the required L/D ratio of 2, wherever possible, exactly perpendicular to the longitudinal axis.
Both the ends are prepared by grinding up to the tolerance limit as specified for flatness and parallelism.
A thin layer of plaster of Paris is applied to ends to ensure proper contact.
And the cores will be ready for compression testing.

12/06/2013

2.2.2. Rebound Hammer Test:
Steps to carry out Rebound Hammer Test:
The plaster is removed at test locations.
For testing, smooth, clean, dry surface without any defect like honeycombing cracks and hollow sound is selected.
The area of approx. 300 mm x 300 mm is rubbed with carborandum stone to remove loosely adhering scales, or remains of plaster mortar, if any.
In this area 12 points at approximate 30 mm apart are selected in grids.
By holding the rebound hammer at right angles to surface of the concrete member 12 readings are taken at selected points.
Of these readings, abnormally high and abnormally low results are eliminated and average of the balance readings is worked out.
Taking into consideration the factors influencing hardness of the concrete surface like moisture condition of the surface, carbonation, test location within the member, direction of test etc. corrected rebound number is worked out.
The compressive strength of concrete against each rebound number is obtained from graph prepared on correlation established between rebound numbers at core test locations and equivalent cube strength values.
The statistical analysis is carried out for this set of values of compressive strengths obtained by above method.

12/06/2013

2.2.3. Cover Meter test Procedure:
The instrument used is Rebar Locator, which is able to perform following functions:
To locate the bar accurately.
To assess the clear cover to the bar.
To calculate bar diameter of the selected bar.
The instrument works on magnetic principle & has limitations of spacing between bars to identify the bars individually.
The limitation of rebar locator instrument to identify bars, its diameter is that depth of rebar from concrete surface should be less than to 70 mm depth and spacing of bars should be more than 150 mm.

12/06/2013

2.2.4. Carbonation Test Procedure:
The powder of concrete is obtained by drilling inside into concrete at selected location. Then the collected powder is made moist and then phenolphthalein indicator is dropped on it to check any colour change. If the colour changes to pink, indicates that concrete is not affected by carbonation & if no colour change is observed, indicates concrete is affected by Carbonation. We can also apply phenolphthalein indicator on the collected core to see the colour changes.

12/06/2013

3.0 RETROFITTING DESIGN OF EXISTING BUILDINGS:

Retrofitting design is essential only when the building has short coming to withstand with current dead and live load, wind load and seismic load.
Strength and/or Ductility of the building will be increased with the best fitted procedure of retrofitting. The guidelines should be followed from
Japanese Guidelines for Seismic Retrofitting of Existing Reinforced Concrete Buildings, 2001
ASCE Standard - Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06), (ASCE-American Society of Civil Engineers)

Following retrofitting should be used to increase the Strength and/or ductility of Buildings as required
RC Shear Wall
RC Wing Wall
RC Jacketing of Columns
Steel frame Bracing
Carbon Fiber Wrapping
Structural Slit

12/06/2013

3.1. Reinforced Concrete Shear Walls:
It is a strengthening method to fill a bare frame or a frame having window opening with shear wall. It mainly increases the strength of building. However, it is necessary to consider deeply the structural characteristics of whole buildings in the retrofit design, since the strength and restoring force characteristics of infilled shear wall will be changed due to flexural yielding of boundary frame or uplift of foundation.

12/06/2013

3.2. Reinforced Concrete Wing Walls:
This strengthening method is to install small wall panels which may not be considered shear walls with boundary columns. The objective of this strengthening method is to increase seismic performance of existing buildings by changing the existing independent columns to columns with wing wall for upgrading their strength. It is also possible to install wing walls to carry axial load of a column and to eliminate a problem of second-class prime elements, whose failure leads to building collapse.

However, there is a case that the seismic capacity of building is determined by the performance of existing beams, even though seismic performance of column is upgraded by installing wing walls. Thus, it shall be counted in the design.
Especially, buildings with short distance in beams shall be carefully designed to eliminate shear failure in beams due to beam shortening after installing walls adjacent to columns. It is possible by installing wing walls to upgrade the second-class prime elements, whose failure leads to building collapse. For example, installing wing wall in the direction of lateral load concerned to increase its strength is an effective retrofitting method when they are extremely brittle columns. To enhance the axial load carrying capacity of a column, wing walls are often provided in the direction perpendicular to the lateral load concerned.