Saturday, February 28, 2009

On TecQuipment Structures Teaching Apparatus

The Department of Civil Engineering (De La Salle University-Manila) will introduce a laboratory component in the undergraduate courses in the Theory of Structures and the TecQuipment’s (TQ) structures teaching apparatus will be used. Among the TQ apparatus purchased were for Frames (Deflections and Reactions), Beams Deflections, Pin-jointed Frameworks, Shear center, Unsymmetrical Bending, Beam Shear and Bending Moments. We are presently being familiarized on the use of the equipment by a local representatvie of TQ. There were some problems initially during the demo but the TecQuipment Ltd (UK) Engineering Manager, Andrew Darby BEng (Hons) immediately responded to the problems encountered. I can see a great potential for the TQ Structures Teaching Apparatus in enhancing the understanding of structural analysis concepts. I am looking forward to using these equipment in our laboratory class in structural analysis. As a matter of fact, I plan to write a teaching and learning manual using these equipment.

Thursday, February 19, 2009

Two Popsicle Stick Bridges from DLSU

I would like to feature two popsicle stick bridges submitted by my undergraduate students at the 5th DLSU-CES Bridge Building Competition. Bridge No. 09 by Morgan Say and Jet Tugado is a Warren-type truss bridge. It actually ranked no. 4 in both the Strength and Design category. The bridge has the properties, L = 626 mm, W = 1073 g (quite heavy). It's deflection at P = 10 kg was only 3.2 mm but at failure, it posted a very large deflection D = 19.20 mm. Again the arrangement of the diagonal members may have contributed to the large deflection. The diagonal members in this bridge are under compression. If only the weight and the deflection were reduced, it may have joined the top 3. Actually it is ranked no. 1 based on the P/W ratio only.
The second bridge (Bridge 16) by Jefron Gaw, Allan Mariano and Lex Alviar was withdrawn from the competition because its depth D exceeds the limit, making it impossible to test the bridge using two point loads. The bridge has an arch shape. The bridge is quite heavy W=1425 g. It is a well designed bridge and it may have performed well in the competition, if it only passed the specifications. The bridge was actually tested upto failure using the UTM and observe the failure mode - a bending failure at midspan.

I would like to commend the participants for creating the nice popsicle stick bridges. You are all winners!

Saturday, February 14, 2009

GRASP Analysis of the Top 3 Popsicle Stick Bridges

In the strength category of the 5th DLSU-CES Bridge Building Competition, the bridges with weight W were subjected to two-point loads using a UTM (Read the blog on bridge testing). The values of P and D at failure were noted to get the Strength rating S=P/(D*W). The bridge with the largest S wins the competition.

The value of S depends on the stiffness (P/D) of the bridge and the weight W. The competition measures how efficient the materials were used to obtain a structure with large ratio of stiffness to weight.

Using the GRASP software, the top 3 winning bridges (B13, B03 and B06) in the strength category were analyzed . Observe the very interesting deflected shapes of the models. The figure also shows the members with axial tensile forces (labeled T). Why did the top bridges perform better than the others? What are the factors that contributed to the large stiffness (P/D) of the bridges? One major factor is the material property of popsicle sticks - the tensile stress capacity is larger than the compressive stress capacity in popsicle sticks. It would be easier to break the popsicle stick due to compression than to tear it due to tension. Hence, if you want to efficiently use the strength of popsicle sticks, design your bridge such that tensile forces not compressive forces are developed in most of the members. In the top 2 bridges - B13 and B03 - relatively large tensile forces were induced in the diagonal members compared to the compressive members.

Another factor is buckling failure in the compression members. If you want to increase the capacity of the members against buckling, then provide braces. The top 2 bridges have horizontal braces at the top and bottom preventing lateral buckling of the members. Bridge B06 which is 3rd in the competition has relatively larger compressive forces in the top chord and diagonal web members. Observe the buckling failure of the top chord. If only horizontal braces were installed at the top, the bridge may have developed a larger capacity and less deflection before failure.

Another factor which increased the stiffness of the bridges, particularly the top 2 bridges is the use of a deep box girder. This results to a relatively light-weight bridge but effective against bending. Sticking together popsicle sticks forming stiff griders like in the bridges shown result to very heavy and not very efficient bridges. They may carry a larger load (P) before failure but the ratio with weight may be smaller because of the large bridge weight.
Modeling your bridges and analyzing them using a software like GRASP before the actual construction will guide you on how to improve the bridge designs. You can redesign the arrangement of the truss members, increase the depth or know the location of braces to prevent buckling failure.

Tuesday, February 10, 2009

Best Popsicle-Stick Bridge Designs

In the recently concluded 5th DLSU-CES Bridge Building Competition last 7 Feb 2009, the bridges made from popsicle sticks competed for the Best Bridge Design based on the following criteria:

  • Creativity and Innovativeness in the design and form: 30%
  • Application of bridge design principles: 30%
  • Practicality and implementability: 20%
  • Neat and well-polished bridge: 20%

Four judges examined and evaluated the bridges. After about an hour of evluation, the scores were tabulated. It was a tight race for the winner. The winning bridge from Don Honorio Ventura College of Arts and Trades (DHVCAT) won by a hairline against the bridge entry from the Technological Institute of the Philippines (TIP), Manila. The 3rd placer is the bridge from the Technological Institute of the Philippines (TIP), Quezon City. Building popsicle-stick bridges using glue takes a lot of planning, patience and ingenuity. Cutting the popsicle sticks to fit the form of the bridge, gluing the sticks and polishing involves a lot of time. In general, the popsicle-stick bridges submitted were impressive. Many of the bridges were unique and may serve as models for future bridges. The students have demonstrated their skills and understanding about bridge construction.

Saturday, February 7, 2009

DLSU CES 5th Bridge Building Contest

The 5th DLSU Bridge Building Contest was held on Feb 7, 2009. Seventeen bridges made from popsicle stick bridges were submitted from various engineering schools - Don Honorio Ventura College of Arts and Trades (DHVCAT), Technological Institute of the Philippines (Manila and QC), FEATI University, Far Eastern University (FEU) , Pamantasan ng Lungsod ng Maynila (PLM), University of the East (UE-Manila) and De La Salle University (DLSU-Manila). The bridge must span a distance of 560 mm and must have a width not more than 140 mm, height not more than 200 mm, depth not more than 100 mm, weight not more than 1.50 kg. There were two categories in the competition: Bridge Design and Bridge Strength. In the Bridge Design category, the criteria used were creativity, innovativeness, application of bridge design principles, practicality, implementability and neatness. The winner for this categroy is one of the entries from DHVCAT. In the Bridge Strength category, the bridges were tested using the UTM and the load P and deflection D at failure and the bridge weight W were used to get the Strength Rating (see blog on bridge testing). The winners in this category were 1st: PLM, 2nd: TIP-Manila and 3rd: DLSU-Manila.

View more photos
Photobucket Album