In order to determine the effect of wind loading on the various types of steel sections two Staad- Pro analysis models were modeled and subjected to equal wind loading of 36m/s wind speed and
23
location is a small town in Rift Valley(BS 6399: 1997 Part2: code of practice for wind loads ). The two models are namely:24 meter Three leg Tubular Guyed Mast.
24 meter Three leg Tubular self supporting lattice Tower.
The structural behaviors of these towers were then assessed from the Staad- Pro analysis output. Aspects looked at include: Deflection at tower top, nodal displacements individual member loads and Failure ratios. Rigidity is one of the requirements in tower design for telecommunication use. This is because wave transmission is from tower to tower and deflection at the tower top is undesirable as this would interfere with the signal. (Refer to the Staad- Pro analysis output attached).
24
Figure 7: Staad Pro structural simulation model for a 3 leg tubular self supporting tower3.1.2 An overview of the Staad- Pro Analysis Software and modeling
STAAD- Pro is a general purpose program for performing the analysis and design of a wide variety of types of structures. The basic three activities which were carried out to achieve that goal i.e. a) Model generation b) The calculations to obtain the analytical results c) Result verification are all facilitated by tools contained in the program’s graphical environment. The Staad manual contains four sample tutorials which guide the user through those 3 activities. Staad- Pro is an integrated 3D analysis, design and draughting package. The system is used by over 3500 companies and is supported worldwide by offices in the USA, UK, France, Germany,
25
Norway and India. Staad- Pro is applicable to almost every type of building structure, from simple portal frames and multi storey buildings to more complex structures such as bridges, offshore structures. Staad- Pro includes 2D, 3D elastic, P- Delta and dynamic analysis. There are extensive modeling features and structures can be analyzed using beams and finite elements. The user can choose from the graphical input or generator which includes a library of structure, edit a simple ASCII file or use AUTOCAD to create the structure geometry then import it into Staad- Pro.Staad- Pro can automatically optimize the weight of the structure, group members together and give a steel take off for estimating purposes. The design can be carried out using any type of section, including angles, channels, plate girders etc. The program includes 10 standard steel section tables including British and European sections and the user can also define personalized sections for analysis and design.
The design codes implemented in Staad- Pro include British steel BS5950 the latest issue of Euro code 3, the French, German, Norwegian, Swedish, Australian, Canadian, and American steel and concrete codes. Other European design codes are in development.
The package is multilingual. The user can change between French, English and German. Other languages are under development. As well as general building and structural applications Staad- Pro has found extensive use in the offshore industry and in the bridge design. Staad- Pro offers Bridge design to BS5400 for steel and composite bridges. The offshore packages include modules for wave loading fatigue and transportation analysis. Design options include the American code, API punching shear checks and the Norwegian NPD or NS3472 steel codes.
Staad- Pro 2004 in which this project is done is the next generation of the STAAD product line, the most powerful structural engineering software in the world. With over 150,000 installations, 15,000 clients, design codes for 30 countries and NRC/NUPIC certification, Staad- Pro analysis software has a feature that allows the user to check the degree to which a given member in the structure is put to use. This degree is given in a range between 0 and 1. A value of 1 shows that the member is put to full use i.e. 100% utilization, values greater than this show that the member has been over utilized and will therefore fail. Therefore it is safe to keep the values between 0.5 and 0.9. At this range members will be utilized by between 50% and 90%.
3.1.3 Outline of the procedure for calculating the wind loads on Lattice Towers and Guyed Masts
Lattice towers and Guyed Masts of square and equilateral sections constitute special cases for which it may be convenient to use overall force coefficient in the calculation of wind load. The
26
wind load should, for convenience be calculated for the condition when the wind blows against one face.The wind loaf F acting in the direction of the wind should be taken as
Where:
Ae effective area of the face
q Is the dynamic pressure of the wind and, Cf is the overall force coefficient
For square lattice towers the maximum load occurs when the wind blows onto a corner. It may be taken as the wind load for the face on wind. For triangular lattice towers the wind load may be assumed to be constant for any inclination of the wind to face.
3.1.4 Solidity Ratio calculation
The solidity ratio ø is equal to the effective area of the frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction.
When single frames are composed of circular section members it is possible that the larger members will be in the supercritical flow regime (i.e. DVs > 6m
2
/s) and the smaller members will not (i.e. DVs <6m
2
/s), there may also be some details fabricated from flat sided sections. DVs is
the product of the dynamic wind speed and the diameter of the section. In this situation the wind force acting on the frame should be calculated using an effective force coefficient equal to:
Cf= Z. cf (super) + (1-Z) A (circ. Sub). Cf (sub) = (1-Z) A (flat). Cf (flat)
A (sub) A (sub)
Where:
Cf (super) is the force coefficient for single frames comprised of the subcritical circular sections.
Cf (sub) is the force coefficient for single frames comprised of the subcritical circular sections.
Cf (flat) is the force coefficient for single frames comprised of flat sided members.
A (circ.Sub) is the effective area of the circular subcritical members: A (sub) =A (circ.sub) + A (flat)
Z = Area of the frame in a supercritical flow Ae
27
The wind load on the structures was calculated for:1. The structure as a whole 2. Individual structural members
3. Individual cladding units and their fixtures. The assessment of wind load was made as follows
The basic wind speed
The basic wind speed V appropriate to the region where the structure is to be erected was determined. The basic wind speed was multiplied by S1, S2, and S3 to give the design speed Vs
for the part under consideration. The basic wind speed V is the 3- second gust wind speed estimated to be exceeded on the average once in 50 years.
S1- Topography factor
S2- Ground roughness, building size and height above ground factor
S3- a statistical factor
Vs = VsS1S2S3
The design wind speed was converted to dynamic pressure q using the relationship q= kV2
The value of k= 0.613
The value of q was then multiplied by an appropriate pressure coefficient Cf and area of the part
under consideration to give the pressure P exerted at any point on the surface. (Please refer to the wind analysis section of this report).
Wind loads obtained as a result of these computations were loaded onto the Staad- Pro models for analysis.