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.