Sunday, November 30, 2008

Structural Engineering for Kids 3 - The 3 Little Pigs and the Big Bad Wolf - Lessons on Wind Loads

Remember the story of the Three Little Pigs and the Big Bad Wolf? This is a story where kids will learn the importance of designing structures against external forces such as wind due to typhoons and hurricanes. The big bad wolf wants to eat the pigs. And to do that he must be able to destroy each pig’s house by blowing on the house. The big bad wolf represents external wind forces that a house must resist. So to protect themselves, the pigs have to build their own houses. Pig 1 built a house of hay. Pig 2 built a house of twigs, while Pig 3 built a house of stone. And you all know the story’s ending, the big bad wolf’s strong blowing power destroyed the houses of hay and twigs, but not the house of stone. Structural engineers must design structures against wind loads. Codes like the National Structural Code of the Philippines (NSCP) specifies wind zones with corresponding basic wind speeds in determining the design wind loads. For example, Metro Manila belongs to Zone II with a basic wind speed of 200 kph, while the Northeastern parts of Luzon and Visayas under Zone I have a speed of 250 kph. According to the Outdoor Advertiser's Association after the Typhoon Milenyo in 2006 struck Metro Manila, they designed their billboards for 120 kph (PDI Oct. 1, 2006) and this is less than 200 kph. The toppled billboards due to Typhoon Milenyo showed us what will happen to structures not designed, constructed and maintained properly against wind forces. Photo courtesy of Engr. Willy Lopez (DPWH): Fallen billboard due to Typhoon Milenyo 2007

Wednesday, November 26, 2008

Popsicle Stick Bridge Testing

Popsicle stick bridge building competitions usually choose the winner based on the strength/weight ratio. Usually loads are applied at the mid span of the bridge by placing incremental weights. The load at failure (P) is then determined and divided by the bridge weight (W) to get the bridge performance.
In the DLSU CES Bridge Building Competition, a Universal Testing Machine (UTM) is used to test the strength and displacement of popsicle stick bridges. Since the load will be applied on the bridge deck, an innovative method was devised so that it can be applied to truss bridges. Two 14 mm steel bars spaced at specified distance are placed on the bridge deck. A hollow steel cage is then placed on the two steel bars. The UTM is then applied on the steel cage which then transfers the load to the two steel bars and then to the bridge deck. A displacement transducer is attached to the UTM to measure the displacement (D) in mm. The load is applied and the value of P (kg) is obtained. The value of P can be predefined or when the load dial of the UTM stops which indicates bridge failure. The bridge rating, S = 1000(P/D)/W is then determined. The S rating represents the bridge stiffness (kg/m) per unit weight (kg). For example, the bridge shown in the video below was tested and it’s P = 30 kg, D = 25 mm and W = 1.50 kg. Hence, it’s rating , S = 1000(30/25)/1.50 = 800 (kg/m)/kg. By using the S rating, strength, displacement and weight become parameters in ranking the performance of the popsicle stick bridges.

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Video Clip: Popsicle Bridge Testing using a UTM at DLSU CE Lab

Friday, November 21, 2008

Structural Engineering for Kids 2 - Shaping Up Humpty Dumpty

You may be wondering why Humpty Dumpty fell from the wall if he has legs and hands. Well this is because of his shape which is very unstable. When he sat on the wall, he is not anymore supported by his legs and a little push will make him roll and thus he fell from the wall. If Humpty Dumpty is an inverted triangle, he will be more unstable. He will stand on the tip of the triangle and he has to balance himself otherwise he'll fall. But a rectangular or upright triangle- shaped Humpty Dumpty will be difficult to be overturned even if he is not supported by his two legs since the bottom side is flat for both shapes and the center of mass is near the bottom.

Shapes play a major role in the behavior of structures. Observed that because of the stable upright triangular shape, the Pyramids of Egypt and the Eiffel Tower have remained structural wonders. In earthquake design of structures, rectangular and upright triangular shaped buildings and towers are more preferred because of their regular shape. To build tall skyscrapers like the Taipei 101, the floor area has to be reduced as it goes up. An inverted triangle is the worst type of configuration for a building because it will behave like an inverted pendulum where the inertia forces will be concentrated at the top and there is very little resistance to lateral movements. The overturning moment will be too large such that it can easily be toppled down. Irregularly shaped structures, of course, can still be built provided they are properly modelled, analyzed and designed.

Tuesday, November 18, 2008

Structural Engineering for Kids 1- Humpty Dumpty & Structural Failures

Remember the nursery rhymes and songs your parents have read and sung for you just to put you to sleep? You or your parents may have not realized that when you were babies, you were unconsciously introduced to basic concepts of Structural Engineering whenever these nursery songs are played.
Humpty Dumpty sat on a wall
Humpty Dumpty had a great fall
All the king’s horses and
All the king’s men
Couldn’t put Humpty Dumpty
Together again
This nursery rhyme is a favorite of my two kids. It is a lesson on structural engineering failures and instability. The egg-shaped Humpty Dumpty is an example of an unstable structure. A little push will make it roll and fall. Similarly, buildings or bridges will be damaged or will collapse if they are not properly designed and built against expected forces. Proper design of foundations, detailed analysis of connections, and correct design of structural members are necessary ingredients against structural failures. Minor damages such as simple cracks may be acceptable but structures must not collapse, such that they are beyond repair endangering people's lives.

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Inspired by the Humpty Dumpty music, I created a short video clip about the damaged buildings during the 1990 Luzon Earthquake. Humpty Dumpty here represents Luzon and its damaged structures. Perhaps, nursery rhymes and children songs may be a good way of introducing Disaster Awareness and Preparedness to kids.

The idea of this blog was inspired from Henry Petroski’s book, “To Engineer is Human.” Petroski argues that “the ideas of engineering are in our bones and part of our human nature and experience.” This is a good book specially for non-technical people who wants to understand concepts of engineering design.

Friday, November 14, 2008

Understanding Eathquakes& Disasters:Photo-Video

Computers and multimedia equipment have become popular tools in public presentations. In most conferences or seminars, a multimedia projector has become a common equipment. Recognizing this fact, the use of multimedia may be used for effective public awareness and education. However, there is a lack of multimedia software or presentations that are readily available for public use. It is for this reason, that I pursued the creation of photo-video presentations to be used in schools and public lectures. The photo-video presentations consists of photos or images integrated with text and music by a video editing software. The techniques and tools needed in producing personal multimedia presentations are discussed in a paper in my website: http://mysite.dlsu.edu.ph/faculty/oretaa.
Understanding Earthquakes and Disasters: Photo-Video Presentations” is a project funded by the DLSU-Manila University Research Coordination Office (URCO). It consists of eight short presentations (watch the video clip below to know the 8 photo-video presentations) which can be played using a DVD player or a computer with Windows Media Player or similar software. The main focus of the photo-video presentations is the impact of earthquake hazards – ground shaking, surface rupture, liquefaction, tsunami, landslides – to the community and infrastructures. By presenting the effects of earthquakes, the public will understand their vulnerability to the different types of seismic hazards. And by knowing ones’ vulnerability, appropriate actions can be done to mitigate the adverse impacts of the hazards and avoid a disaster. Contact me if you want a copy of the presentation.
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Introducing the 8 Titles in "Undertanding Earthquakes & Disasters: Photo-Video Presentations"

Friday, November 7, 2008

Building Popsicle Stick Bridges

I built my first popsicle stick bridge about 12 years ago when I was introduced to the bridge building contest organized by the students of the DLSU Civil Engineering Society (CES). My bridge was a through truss bridge. A through truss bridge derives its strength from the arrangement and strength of the truss elements. The load is transferred from the bridge deck to the truss elements and then to the supports. There are other types of bridge forms that can be created. A deck or girder bridge resists loads through bending – hence you must design the bridge deck such that it will have a large moment of inertia.The arch bridge, on the other hand, transfers the load to the abutments or supports through compression.

Bridge building is a fun and challenging activity. By applying basic strength of materials and bridge design principles, a little creativity and a lot of patience, you can create your own masterpiece similar to the CES bridges shown. So why not test your skills on popsicle stick bridge building by joining the 5th DLSU CES Bridge Building Competition? The competition is open to all civil engineering students in the Philippines. The best bridge design wins P5,000. The bridge with the largest “stiffness/weight ratio” wins P10,000. Check-out the rules in the ads shown in this blog site.
There are more postings related to popsicle stick bridge building and bridges in this blog. Go to this link.
Get some building tips at
Check-out http://pghbridges.com/basics.htm for various forms.
Photos of CES Bridges by Joenel Galupino: http://misfortunes.multiply.com/photos/album/60/MISFORTUNE_55

Wednesday, November 5, 2008

Introducing Damper Baby

Reducing building vibrations due to wind and earthquakes in tall buildings is one problem that must be addressed by structural engineers. There are various ways of doing this. One method is by installing passive and active dampers. When I visited Taipei 101 in 2007, I was introduced to “Damper Baby,” – the name given to one of the dampers installed at the currently world’s tallest building. Taipei 101 has a total height from the ground to the spire of 509 m. It has 101 stories above ground and five underground. In designing the tower, engineers had to consider the fact that " it will stand about 650 feet from a major fault line, and that it will face winds of 100 mph." Deflections resulting from extreme wind loads and earthquakes will be minimized by "three separate tuned-mass dampers: a primary, low-frequency 635,000kg damping sphere almost 6m in diameter formed from 41 layers of 12cm thick steel is suspended between the 92nd and the 88th floor to counter overall tower sway, while two smaller, higher-frequency dampers 7 tons in weight are installed inside the 20 m tall mast to counter mast vibrations. " The damper can reduce building vibrations by as much as 40%. Another technology of reducing vibrations is by the use of active control systems. Taipei 101 has also active control systems which eliminate vibrations. The video below is an exhibit at Taipei 101 showing the effect of active control systems in buildings.

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Saturday, November 1, 2008

Riding the World’s Fastest Elevator at Taipei 101


I reached level 89 of Taipei 101 last June 2007 using the world's fastest elevator with a top speed of 1010 m/min (63 km/h or 37.5 mph) The ride from B1 to the observation deck at level 89 was comfortable and took only 39 seconds. The elevators installed by Toshiba contain pressure control systems which adjust the atmosphere inside to prevent ear-popping. Taipei 101 is currently the official world’s tallest building, but come 2009, Burj Dubai in UAE with 141 floors will claim this title once its construction is finished. Observatory elevators with speeds of 18m/s (40 mph) will be installed at Burj Dubai. With fast elevators like this, it will take less than 2.5 min to go up the conceptualized 2.4 km tall Dubai “Vertical City” Tower! Read my blog on "Reaching for the Sky". Reference: http://www.allaboutskyscrapers.com/sp.taipei_101.htm and http://www.burjdubai.com/