Structural Theory and Design
Design Assignment 1

 

1 Introduction

The aim is to design a structure capable of supporting a mezzanine floor of dimensions 30 by 8 meters. This will be done by taking into account the strengths and weights of all materials used as well as the specifications needed to comply with the design.

 

2 Design Layout

 

3 Design Calculations –
Sufficient Ultimate Capacity and Deflection

 

3.1 Cross Section 1 – Concrete Slab
As shown below in the diagram the first layer involves the slab and the force that this encounters through gravity.

The following table describes the factors important for testing if the concrete has a sufficient ultimate capacity. The dimensions of the slab are outlines in the cross sectional diagram. The concrete is grade 32. The widths between the beams supporting the slab is 0.5m and therefore creates a section that can deflect of 0.125m high X 3m long X 0.5m wide. This area has been considered in the following calculation.

 

ln order for the reinforcement to be effective it must have a Ku value of less than 0.4. This means that the concrete is lightly reinforced and that the steel reinforcement will yield before the concrete does to ensure it has maximum effectiveness. ln order to reinforce our concrete slab we used D500S1102 Steel mesh applied 30mm above the bottom of the slab.

 

3.2 Cross section 2 – Beam series 1
The first series of beams is placed at 0.5m intervals underneath the slab. They are consistent over the full length of the slab. Due to the beams underneath they cover a span of 3m and are simply supported. The reason for these beams is that it provides a more stable fixture for the slab. This then provides important support for issues such as slab deflection and creep that may occur over an extended period of time.

 

A cross section of the beam is located below with the relevant calculations of the extreme fiber stresses displayed below. The following table shows the beam in terms of the ultimate capacity. This proves that the section is sufficient for the load imposed on the beam.

 

3.3 Cross section 3 – Overhanging Beams
The Beams that support the running beams are spaced at 3m intervals across the 30m and run the full 8m length to the glass façade. The beam is made up of a simply supported section and a cantilever section. The forces acting on the beam are from above as it experiences the reactions from the above beams that are supporting the concrete slab. These point loads are consistent over a symmetrical length creating point loads at 0.5m intervals.

 

To measure the reaction on the supports of this beam are calculated through the use of moments and the balance of forces. The distribution of the reaction forces can be seen below with the diagram. The calculation of reaction forces used the view that this distributed load can be consisted a point load in the middle of the beams. This is shown in the diagrams below.

 

 

3.3.1 Sectional Properties

 

3.3.2 Bending Moment Diagram
Once the reactions have been found it is possible to use the central difference method. This created the following bending moment.

 

3.4 Cross section 4 – Column Beams
The column beams are present between each column. They are 6m in length and support the above beams as previously defined. The point lads are contributed from above creating 22km force onto the column beams. The tables below test the ultimate strength as compared with the maximum moment create as well as the deflection of the beam.

 

3.5 Cross Section 5 – Columns
The columns are also made up of I sections. These sections have a total height of 4m above the ground and are subject to the axial force provided by the above members. The shapes and dimensions of the columns dictate their properties. The cross section of each column can be seen below, including the sectional properties.

 

 

3.6 Truss Bracing
Truss bracing is used in order to reinforce the columns and beams from falling in a sideward manner. The bracing type used was knee bracing allowing a strong brace between the columns and the beams to exist. This knee bracing is observed with the 3 dimensional diagrams below. The reason that knee bracing was used is that it can easily be attached through welding rather than plating as the angles may become difficult. The knee bracing will provide adequate protection from horizontal forces that may arise.

 

4 Overall Design Issues

 

4.1 Joins
4.1.1 Welds
For the chosen the top runner beams (layer 1 beams) should be connected using welds. These welds will provide the strongest bond between the beams whilst not affecting the structural integrity of the beam. The limitations with this choice are the time and cost. For the professional development of the structures welds would take a longer period of time. The diagram beside shows the region (red areas) that would be used as the weld area.

 

4.1.2 Plates and welds
The other join that would be used in the design is the plates and welds, This could column sections to support the beam from the underneath. This would provide an added strength to the support of the beams and hence limiting any other movements. The plates can be seen as the red section and the welds shown by the orange lines in the diagram below.

 

4.1.3 Plates and bolts
To connect the bottom beam layer to the columns then plates could be used. This would involve welding and bolting a plate to the flange of the column. This would create an eccentricity that has been calculated above. The design can be seen below with the plate and the bolts highlighted on the column.

 

 

4.2 Design Limitations and Improvements
On further investigations there are areas of the presented design that could have changed and been improved depending on the beam types used.

 

4.2.1 Other Design Options

4.2.1.1 Beam and Column Design
There could have been larger beams used in the design. This would mean that the runner beams could be removed and then have stronger column beams. Some of the columns could also be removed as they currently carry only a small amount of their load capacity. These changes would save materials and improve the design.

 

4.2.1.2 Slab Design
For the above beam changes to be made then there would have to be a larger slab used. This would then limit the deflection that would be experienced between the beams.