MODEL BRIDGE DESIGN

INSTRUCTION

PROJECT (DAB20202)

Introduction The aim of this project is to encourage students to use their structural analysis knowledge and creativity to solve practical problem in building small model bridge and encourage effective communication between group members. The students are required to test their materials to find the tensile, compressive strength and modulus Young using Universal Testing Machine or other appropriate methods. Some calculations might be needed to obtain the suitable member size and deflection using hand calculation and check using suitable computer programs. The number of students in a group is not more than five members. Materials The models shall be made from commercially available rectangular balsa stock and glue. Element Dimensions The length of each structural element of the model must not exceed 150 mm. The cross-section of each element must not be greater than 5 mm by 5 mm for square and 5 mm diameter for circular cross sections. Model Specifications

- Clear span 800 mm (Distance between centre of supports approx 850-900 mm) - Maximum overall width 100 mm - Maximum overall height 300 mm - Contact base area at each end must not be greater than 100 x 100 mm - The bridge model shall be simply supported on the support base of the rig for

load testing - The model must be able to support a minimum load of 5 kg placed at the mid

span loading area on the deck. - The total mass of the bridge plus glue must not exceed 200.0 g

- The bridge shall be "free standing".

- No fastening mechanism except mechanical interlock of the balsa pieces or commercial glue is permitted.

- Hanger or flat surface on the deck must be provided at the centre of the bridge for placing loads which consists of 100 mm diameter iron weights.

Load shall be added until either an audible cracking sound together with visual evidence indicates the failure of some structural member or glue joint of the bridge, or until a suitable reference point on the roadway at the centre of the span has been lowered by more than 2.0 cm.

Assessment The following criteria shall be used to determine the best design.

- Ratio of final collapse load to self-weight (The higher the better) - Concept and ease of construction - Presentation and workmanship - Aesthetic and overall appearance of the model.

Report The report should consists of :

(a) Consideration of three(3) bridge geometries design. (i) Use Lindpro software to determine the deflection at the centre span of the

three bridge geometry under 10 kg (100N) load. Usually the lowest deflection signifies stiffer structure and are more suitable to be chosen as a model.(Make sure you use the E value of balsa in the software.

The E value of balsa can be obtained as follows:

Clear span 800 mm

PLAN

100 mm

Support base fixed to the position of hanger

testing rig (max. 100x100mm)

Bridge model simply

ELEVATION supported on the

Weight testing rig

Testing rig Testing rig

support base support base

E value of balsa can be obtained using simple bending test of the balsa wood specimen say 300 mm in length. Using the bending test apparatus in the Light Structure Lab., put incremental loading at the centre of the specimen and note the deflection using the dial gauge. Since deflection at the centre of the beam due to point load, = PL3/48EI Plot a graph of (cm) y axis against P (N)x axis. You should get a linear graph passing through the origin (0,0). Calculate the gradient, n of the graph, where n = L3/48EI L cm, I = bd3/12. b and d is the balsa specimen in cm. Calculate the value of E (N/cm2) and use this value in Linpro (ii) In your Linpro analysis of the 3 bridge geometries indicate the member

that has the largest (i) tension force (ii) compression.

a) In your opinion can the balsa member withstand the largest compression force?

b) Can the balsa members joints withstand the largest tension force? c) Explain how you can reduce the largest force in tension and compression. d) Explain how you can prevent the largest compression member from buckling failure.

(b) Considerations of why the final design is chosen? The final design is chosen base on the following reasons: (i) The least deflection at mid span (ii) The least largest compression force (iii) The least largest tension force (iv) The least weight (less members and less joints) (v) Must have lateral truss to counter lateral movement Explain what reason or reasons you choose the final design. (c) Draw the final model showing plan and elevations.(autocad drawing preferably) (d) Actual failure report including pictures.

Indicate where the failure occurs in your model after test. Explain the type of failure whether it is (i) tension failure (ii) compression failure (iii) joint failure (iv)sway failure (v) combinations of failure.

(e) Recommendations (How to improve the bridge load carrying capacity)

How can you improve your bridge to take more load?

(f) A CD: A short video of how the model bridge is made and how the bridge performs during loading. Oral presentation by each group member should be seen in the video.

Datelines Model will be tested on the 12th weeks of the semester. Final report should be submitted by the 14th weeks of the semester. The report should be no more than 15 pages.