The Structural System Chosen Construction Essay

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The United Arab Emirates government has been investing in managing the dramatic rise of the population levels in its cities. The UAE's population has grown exponentially to 8.26 million in mid-2010, a growth of 64.5% in four years, which needs effective and consistent strategies for the accommodations, especially in the Emirate of Abu Dhabi. The residential villas in Abu Dhabi are considered one of the most attractive solutions used to mitigate the demand on accommodations and the excessive rental rates in the down town areas. Due to availability of lands in urban areas, residential villas would definitely help in granting the UAEs population with a unique life style as villas will be the represented mark on the governments care.

This project presents one case of designing a residential villa. The role of the structural engineer is to produce a safe and economic structural design that suits the architectural requirements.

1.1 Project description

This project will demonstrate the analysis and design of a residential villa located in Abu Dhabi, the villa consists of 2 stories + roof and it provides 4 master bedrooms, maid's room, laundry room and a sitting room.

This villa will reflect the modern life style as it is perfect dwelling for a family aiming to live peacefully in a private yet luxurious manner.

The villa covers around 600 m2 area and a height of 11m.

The main law followed in this project is seeking safe outcomes in the design of this structure without neglecting the importance of constructing it easily and economically.

Figure 1 Building Geometry (ETABS)

1.3 The Design Objectives

The main objective of the project is to provide a safe and economical design. Accomplishing the design objectives required two main tasks. First, the structural design must satisfy the safety requirements and reach the standards of the code used. Then, several design alternatives shall be produced and prioritized according to the cost estimate analysis. To minimize the overall cost of the villa in parallel with achieving the market acceptability and the overhead profitability; the lowest structural system in cost shall be selected as the optimum design alternative.

1.4 Intended Outcomes and Deliverables

The construction material used for this project is reinforced concrete that shall be cast on site and the structure complies with ACI-318 design code. To submit this project, the following requirements must be met:

1. Details of the structural system chosen.

2. Structure modeling, analysis and design for each component (slab, beam, column, pile, Raft) using the software package ETABS and SAFE and showing the hand calculations for selected members that will show our understanding of what we are doing.

3. Complete doing the drawings for the project. It should be kept in mind that the contractor should be able to build the structure by referring to the drawings only.

4. A breakdown for the project cost and a detailed project schedule form the feasibility study to the completion of the work.

5. A summary of the contribution of each member of the team in the project.

1.5 Report organization

This report is composed of three main topics

1. Preliminary design

This chapter will present the outline of the project such the materials used, the codes followed , definition of the structural system and the loading criteria.

2. Design

The design of the structural system will be negotiated in this chapter as it demonstrates the reinforcement, stresses and dimensions of the structural system manually and program wise.

3. Cost estimation

This chapter is very important because the economic outcome will reflect the efficiency of the engineer in accomplishing such a project with an economical price yet sustainable and safe.

 

Chapter II

2. Preliminary Design

The engineering design is a chain of multiple processes and procedures; if one of them is not achieved, certainly, the design outcomes will not be accomplished. According to that, the analysis phase is considered the most important stage, in which it provides the structural engineer with the required data as well as gives an idea and expectation about the design alternatives. For an example, in the analysis phase; if it is found that the deflection is relatively high in the edges of the floor slab, this would probably help the designer to think about applicable solutions such as increasing slab thickness or adding spandrel beams .so in other words the analysis phase is considered as laying the main outline of the project.

The analysis of a villa has several tasks and requirements to meet the client's architectural point of view and matching this desire with our structural design to produce a safe yet beautiful outcome.

To keep up with this outcome we have to consider some important disciplines to go with such as the materials and the structural system which should match the cost and the quality expected.

Some Engineering Software is typically used to facilitate the analysis of the structural system to obtain accurate and reliable results. Therefore, it is believed that the better the analysis, the better the design.

2.1 Codes

To start first we have to determine the structural codes that will lead us in the design phase. In Abu Dhabi, the American Codes of Building have been enforced on all municipalities and private affairs therefore; two main codes were used;

-ACI 318M-08 American Concrete Institute (2008)

-ASCE 7-10 American Society of Civil Engineers (2010).

2.2 Construction materials

Reinforced Concrete (RC) has been chosen for this project rather than other materials for many reasons such as safety, cost, material availability and construction scheduling. Concrete performs well during both natural and manmade disasters. In addition due to its wide availability, concrete prices remain very steady despite the fluctuation and substantial increases in other building material prices. Therefore, this construction material has become the most common material used in UAE.

Therefore we have decided to use reinforced concrete for this project with these properties:

2.3 Concrete

These properties of concrete are applied for all the structural elements designed further on.

- fc' (compressive strength) 40 MPa

-E (modulus of elasticity) 29,725.5 MPa

-Wt of reinforced concrete 24KN/m3

Figure 2((http://www.engineeringcivil.com/modulus-of-elasticity-of-concrete.html))

2.4 Reinforcing Steel

Rebar FY (flexure and tension) 420MPa

Es 200000MPa

2.5 Software:

For the current project, the following software is utilized for the structural analysis and design:

 

2.6 ETABS 9.7.1

CSI ETABS is considered one of the most powerful extended 3D analysis and design of building systems as it is one of the structural software's that is commonly used in the structural fields .

ETABS was very helpful in:

1- Modeling the structure and identifying the structural and supporting system.

2- Defining the area sections of the villa (slabs, columns, beams)

3- Calculating the wind loads as per (ASCE 7-05) code which has been already defined in ETABS.

4- Projecting the deformed shape resulting from the lateral loads

5- Defining actual story drift

6- Designing columns.

7- Subjecting member stresses / forces

8- The ability to complete the design process in other programs simply by exporting the model with its distances and loadings to the standard software's in ETABS.

2.7 SAFE 12.3.1

CSI Safe is an integrated program which is considered as the completion of the ETABS analysis and design procedures especially in slabs.

Safe was very helpful in:

1 - Analysis of slabs and rafts as it can calculate deflection and punching shear values.

2 - Facilitates our decision about the reinforcement alternatives for the slab.

3 - Generates professional drawings using a built in program called CSI Detailer.

4 - Safe can help in modeling our actual piles through using spring support.

5 - Projecting the slab/Raft stresses and forces.

6 - Designing the raft by importing the model with the total factored loads of the structure from ETABS which helps in the punching shear checking and allowable settlements.

2.8 Loading criteria

Structural loads or actions are forces, deformations, or accelerations applied to a structure or its components.

Estimating the loads that may be applied to the structure during its life time is a very important task to overcome structural failures. No loads that may reasonably be expected to occur may be overlooked. That's why as per ASCE 7-10 the following loads were determined.

Loads on any structure will be composed of:

1. Gravity loads

2. Lateral loads

2.8.1 Gravity loads

Gravity load are signified to the loads acting downward or vertically gravity wise such as dead loads and live loads.

2.8.2Dead load

Dead loads are loads of constant magnitude that remain in one position. They include the weight of the structure under consideration as well as any fixtures permanently attached to it. For a reinforced concrete building some dead loads are considered to be frames, walls, floors etc...

2.8.3 Super imposed dead load

It is considered the service loads or finish load applied by the finishing work such partition like blocks or gypsum board or the flooring like tiles and ceramics.

It can be also helpful during construction by implementing the load of the slab above because in the stages of construction if we will raise the slab on top we will use jacks and supports to support the top slab while concreting so technically in the construction phases the bottom slab will carry the slab on top.

2.8.4 Live load

Live loads are loads that can change in magnitude and direction. To clarify we can say that people and furniture are considered live loads.

The table below shows the loads subjected to our villa.

Table 1 Design loads

Level Super Dead

(KN/m2) Live Load

(KN/m2)

partitions

Finish floor

Screed + insulation

Story 1 3.6 2 - 2

Story 2 3.6 2 - 2

Roof - 2 2 2

2.8.5 Lateral loads

It is typically considered to be those which act parallel to the ground plane and may occur at many angles other than perfectly horizontal. It's generally considered to act transversely to the primary structural system such as wind load and seismic loads.

2.8.6 Wind load

Wind load is the force on a structure arising from the impact of wind on it. Preventing wind damage involves strengthening areas where things could come apart. The walls, roof and foundation must be strong, and the attachments between them must be strong and secure.

This force can severely damage your structure with three resulting forces:

1. Uplift load - Wind flow pressures that create a strong lifting effect, much like the effect on airplane wings. Wind flow under a roof pushes upward; wind flow over a roof pulls upward.

2.

3. Shear load - Horizontal wind pressure that could cause racking of walls, making a house tilt.

4. Lateral load - Horizontal pushing and pulling pressure on walls that could make a house slide off the foundation or overturn.

Due to this high affect of wind loads on structures we should take it into consideration during the analysis of the villa and apply it to our design.

2.8.6.1 ETABS Wind Analysis

In this project we used ETABS to complete the analysis of wind load and its effect on our structure using ASCE 7-05 code which was already identified in ETABS.

The analysis results will be demonstrated later on, but we should understand first the codes parameters and prerequisites that comply with our case.

2.8.6.1 Theoretical background

Basic Wind Speed, (V) (ASCE 7-05, 6.5.10)

The basic wind speed is defined according to the ASCE 7-05 standards as "the three-second gust speed at 33 feet (10m) above the ground in exposure B". Based on meteorological studies the wind speed varies from location to another. The basic wind speed in Abu Dhabi is specified by the Abu Dhabi municipality as 100 mile/per hour (40 meter per second). (ASCE 7-05, 6.5.10)

Importance Factor, (I)

The importance factor is defined in the ASCE 7-05 standards as "a factor that accounts for the degree of the hazard to human life and damage property". In our case the occupancy category of the villa is (II), and the corresponding importance factor is 1.0.

Exposure Category

The exposure category presents the location of the building with respect to its surrounding condition. In ASCE 7-05 standard there are three types of exposure categories that describe different exposure conditions which are:

1. Exposure B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Use of this exposure shall be limited to those areas for which terrain representative of Exposure B surrounds the structure in all directions for a distance of at least 2,630 ft [800 m] or ten times the height of the structure, whichever is greater.

2. Exposure C: Open terrain with scattered obstructions having heights generally less than 30 ft [9.1 m]. This category includes flat, open country, grasslands and shorelines in hurricane prone regions.

3. Exposure D: Flat, unobstructed shorelines exposed to wind flowing over open water (excluding shorelines in hurricane prone regions) for a distance of at least 1 mile [1.61km]. Shorelines in Exposure D include inland waterways, lakes and non-hurricane coastal areas. Exposure D extends inland a distance of 660 ft [200 m] or ten times the height of the structure, whichever is greater. Smooth mud flats, salt flats and other similar terrain shall be considered as Exposure D.

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