STRUCTURAL ENGINEERING REPORT
CIVIL AVIATION AUTHORITY OF ZIMBABWEDESIGN REPORT
VICTORIA FALLS AIRPORT CONTROL TOWER
1.0 STRUCTURAL ENGINEERS DESIGN REPORT
TABLE OF CONTENTS
Design Considerations 3
The control tower at Victoria falls is a glass enclosed elevated structure for the visual observation and control of the air and ground traffic. Due to the length of the new runway which is 4km and the need for adequate eye height and visibility to runway threshold 12 and runway threshold 30, the tower is 28.15m high above the ground level.
Foundation adopted for the tower is a raft foundation because of the low bearing capacity of the existing soils. Kalahari sands have a low bearing pressure and are classified as collapsible soils. The foundation has to be excavated to 4m below natural ground level and control compacted. Acceptable quality material has to be imported and compacted in specified layers to achieve a bearing capacity of 150kPa. The main concrete raft for the tower has a 12530mmx12530mm square footprint as opposed to a circular footprint. The original circular footprint was under designed and would have led to the structural instability of the Control Tower. The concrete raft is 1000mm thick. The main building is founded on similar soil raft at shallow depth utilizing a system of pad footings and ground beams.
The control tower is a concrete structure which is circular in shape with 300mm thick outer walls. It has got a lift shaft inside and the thickness of the lift shaft is 250mm thick. Design wind speed of 100km/hr was used and a statistical analysis was performed based on the recorded wind speeds of the last 50 years. Consideration and technical analysis was carried out to check whether damping of the tower was necessary. Aerodynamic modifications were made by adding stiffening beams at 5m or less intervals on the tower. This was an important consideration which had been overlooked by the Chinese designers. The cabin level structure has a higher mass than other floors. This gives some aerodynamic stability to the tower. The control tower frame is braced at all levels up to cab level.
The main building structure consists of beams and columns.
3.0 Design Parameters
It was important to verify whether the tower as designed was consistent with the following codes. In this regard a detailed analysis and calculations based on the following codes were used in the design of the control tower and the main building: BS8110: Structural Use of Concrete BS6399: Loadings for Buildings BS5950: Structural Use of Steelwork In Building BS5628: Use of Masonry FAA Order 6480.7D (2004): Airport Traffic Control Tower and Terminal Radar Approach Control Facility Design Guidelines.
The recommended typical loading conditions, as indicated in SANS 10160 General Procedures and Loadings to be Adopted in the Design of Building are indicated in the following table: UsageAllowanceComment
Lounge roomLive load2.5kN/mAllow for light partitions:1.0kN/m
Conference RoomLive load4.0kN/m
Storage areas for furnitureLive load5.0 kN/m2These areas to be specifically indicated on architectural layouts
Corridors Live load4.0 kN/m2
Ablution areasLive load3.0 kN/m2
Accessible flat roofsLive load2,0 kN/m2Screed: thickness:40mm
Note: As a general rule, an allowance of 0,5 kN/m2 has been made for ceilings and services.
The material properties for the principal structural elements are set out below:
4.1.1Reinforced ConcreteThe following values were assumed for design purposes:Design Strength Foundations30 MPaColumns30 MPaStrongroom walls30 MPaAll other structural elements25 MPaYoungs Modulus 26 kN/mm @ 28 daysAmbient Relative Humidity45%Coefft. Of Thermal Expansion10x10-6/C4.1.2ReinforcementHigh Tensile Reinforcement 450 MPaMild Steel Reinforcement 250 MPa
4.1.3SteelworkStructural Hollow Sections, Grade300WARolled Sections, Grade350WCold Formed Purlins225MPa
4.1.4MasonryLoad-bearing (if applicable)14MPa (min)Face brickwork 14MPa (min)Partition walls and internal walls10 MPa (min)
The design and structural layouts done by the Chinese designers were not corresponding to architectural drawings. This can be proved by main building layouts. The curved beams on the main building have different radius as compared to the radius of the control tower. The radius of curved beam on main building is R3.2m whilst the radius on the control tower is R2.85m. This will cause the two structures not to tie up properly and this will also affect the reinforcement of the beams and the slab as well.
Materials to be used on the Control tower and main building, was also important to be considered in the design as this was also overlooked by Chinese designers. The Chinese designers did not specify the type of steel to be used, they just mentioned the size of steel only. This resulted in the changed of type and diameter of steel used in the design. For the main building the Chinese designers used slab thickness of 120mm and the main steel reinforcement used was 10mm diameter bars. But after considering the permissible superimposed loadings values the slab thickness was changed to 200mm thick, with main reinforcement of Y12mm diameter bars.
The Chinese designers did not provide the drawings for the staircases for both structures i.e Control tower and main building. Chinese designers did not also provide the bending schedules for Control tower and main building.
Generally the design which was done by the Chinese designers was not economic, this can be proved by the sizes of beams used on the roof beam layout, some beams were 600mm deep while others were 500mm deep whereby 400mm deep beams were sufficient after considering permissible superimposed loading values.
1NCUBE BURROW CONSULTING ENGINEERS:VICTORIA FALLS CONTROL TOWER