Building to survive a Philippine earthquake. The recent Bohol Island earthquake (magnitude 7.1) once again reminds us that the Philippines is located in the “ring of fire” Pacific earthquake zone. When we built our own house in the Philippines, we learned some of the building construction principles and measures that can make a life of death difference or possibly the difference of surviving an earthquake with an intact home rather than a pile of rubble.
We have friends who built houses in areas affected by the 2013 earthquake, on both Bohol Island and in also in Cebu City. Our friends were definitely frightened by the quake and the many aftershocks. The good news is that there was not widespread damage to well constructed homes of reinforced concrete and hollow block. This may partly be due to the fact that the developed areas are fairly far from the quake epicenter. The Bohol earthquake, although it was hundreds of miles away from our home on Panay Island, gave our house a good shake.
Our part of Panay Island had experienced a magnitude 8.2 earthquake in 1948. A magnitude 7.1 earthquake releases the energy equivalent of 680,000 tons of TNT. An 8.2 earthquake releases the equivalent of 21,000,000 tons of TNT. This is truly sobering. Yet we know that many pre-1948 buildings in Iloilo City are still standing and in good condition.
Above is the Iloilo Hall of Justice, a P120 million built in 1992 which was abandoned due to structural failure after the 5.5 magnitude earthquake of February 6, 2012.
In 1948 Panay Island was much less developed. Doubtless all or almost all the hollow block buildings on the island were built after 1948. The 1948 quake damaged or destroyed dozens of buildings. The huge and beautiful church in Oton was destroyed. The Alimodian church was heavily damaged. Bell towers were toppled, including that of the Jaro Cathedral. Of course these buildings were of very heavy un-reinforced masonry construction. Hundreds of pre-War reinforced concrete buildings have survived quite well. None of these were built with hollow blocks. Engineers were well aware of Philippine earthquake dangers and built very strong public buildings. The strength of these massive Commonwealth-era government buildings were one reason that the Japanese forces were so hard to dislodge during the Battle for Manila.
There have been some small, but still damage-causing quakes since 1948. A much bigger quake could happen at any time. Tigbauan, where we live, is in the most dangerous Mercalli earthquake zone.
We asked the engineers to design the house with earthquake survival in mind. They did so. The size and quantity of reinforcing bar was more typical of that used in a two story house. The columns were substantial. We took additional steps which we thought might help. We used 6″ rather than 4″ hollow block for the exterior walls. As the blocks were filled, the larger block had a much more significant concrete core . We tried to police the project to be sure proper reinforcing bar was used, in some cases tearing down and rebuilding when problems were found. We used a very strong 1-2-3 (concrete, sand, gravel) mix. We installed reinforced concrete lintel and tie beams. We thought we were doing what was necessary to build an earthquake resistant house.
Recently we’ve been reading up on earthquake resistant building design. We learned that the really damaging earthquake motion is lateral shaking, not just vertical shaking, that lateral forces are the major destroyer of buildings. Structures built with concrete columns have great compressive strength but may not withstand lateral forces very well.
Another concept we encountered was that buildings should have “ductility” — that is that they should be able to flex without breaking. We deduced that a concrete column and hollow block building is about as un-ductile a building as you can get. A wood or bamboo building is likely much better at accommodating lateral forces without damage.
Another concept is preservation of building integrity — that if different portions of the building are not well connected, the building will break apart. This integrity is accomplished by tying the building together with stiff exterior and interior “shear walls”. We’re assuming that this is especially necessary in the non-ductile hollow block house. You’ll be told that the weak block in Philippine houses is not a problem because the structural strength is in the beams and columns, but weak block can’t produce a strong shear wall.
One of the best resources for the layman that I’ve found is the book, “Peace of Mind in Earthquake Country” by Peter Yanev, Chronicle Books, San Francisco, 1974.* Yanev analyses the strengths and weaknesses of the various forms of residential construction. Since almost all construction in the Philippines (except for high rise or bamboo buildings) is hollow block, here’s what he says about hollow block buildings.
“When the hollows of concrete blocks are properly reinforced with vertical and horizontal steel rods and then carefully grouted with poured concrete, the walls of concrete-block buildings form solid and continuous sheer-wall units. Thus, reinforced concrete-block buildings can exhibit great strength and resistance under the stress of earthquake forces. Attention to detailing and workmanship is critical however. Because the walls are an assemblage of separate block units joined by poured concrete and mortar, they can share many of the weak points of brick construction if the mortar is faulty or poorly executed, if the poured concrete grouting does not completely fill the cavities, if the steel reinforcing is inadequate, or if the connections with the diaphragms of the building are weak or insufficient.”
Yanev then goes on to list twelve specific construction standards needed to ensure a strong block building. Here I’ll mention a few areas of concern based on our experience with trying to build a strong block home.
Miserably weak 4″ hollow block are standard for building homes in the Philippines. These 4″ block just don’t have a large enough cavity for the concrete grouting mentioned above. Further, workers don’t seem to understand the need to carefully pack the cavities. I had to fight against workers using old, re-wetted mortar instead of concrete. We did use decent quality 6″ block for our exterior walls. See /our-house-project-cement-blocks/. The big cavities in the 6″ block welcomed good filling. The 4″ block we used in building our interior walls was crumbly, poor quality stuff and we have no confidence in their strength. If we had used 6″ block throughout, we’d be more confident in the strength of the building. One reason they are not used is that so much concrete is needed to fill them — the very reason they should be used! In our view, the actual additional cost of using and filling 6″ block is minimal as compared with the benefit.
We mostly stuck with local standards for vertical and horizontal reinforcement bars in the block walls; 12mm vertical bars every 60cm and 10mm horizontal bars every 60cm. When my workers inexplicably changed to using 10mm vertical bars, I had them tear down and rebuild the affected wall areas. The bars are supposed to be continuous. As a practical matter, the bars were spliced. Lifting block over three meter vertical rebar is not practical. My workers did not know correct splicing . Often the splices were far short of a 40X bar diameter standard spice — 48cm for 12mm, 40cm for 10mm. See /our-philippine-house-project-rebar-splicing/ for more discussion.
The rebar protruding out of the left side of this column will be spliced to the horizontal rebar in the hollow block walls. The protruding rebar is too short for an effective column-to -wall tie.
Here’s an example of a house corner where the two walls are barely tied together at the corners. This is a house where heavy 16mm rebar is being used, but in our view, the strength of the house is unnecessary compromised. Better corner ties would not have increased the cost at all.
Reinforcement around door and window openings. Yanev says that inadequate reinforcement around door and window openings is a major cause of failure. In an attempt to keep cool, we have lots of exceptionally big 5′ x 8′ windows. We did put in a continuous lintel beam on the exterior wall. Then two more blocks up we poured a roof beam using 16mm rebar. Door openings inside have lintels over each door. The 16mm top tie beams were also used on the interior partition walls. If we had it to do over again, the roof tie beam and the lintels would have been replaced by a well-reinforced 32″ tie beam, combining two courses of block, the lintel beam over the windows and the roof beam above.
If you look closely at the photo above, you can see the lintel beam above the windows and the roof beam in the plywood forms. They are separated by two rows of 6″ block. If the block had been eliminated and a single 32″ band of reinforced concrete constituted the lintel and roof beams, the house would have been much stronger at very little additional cost. This would have been an especially wise move given that the very large window openings do compromise the building’s strength.
Weight up high on the building is another hazard. With lateral shaking, a clay or cement tile or a cement roof can act as a pendulum, magnifying the shaking motions. The situation will probably be worse with a two story building. Tile roofs also require a heavier structural framework to support them, adding to the problem. The low hip roof (quatro aguas) found on many Philippine bungalows is ideal. There is little mass up high and a sleek roof profile to avoid catching the force of typhoon winds.
So, we started out hopeful that we’d build a house with excellent earthquake survival qualities. The most we can say now is that our house is more rugged than most, but falls short because we accepted the concrete column and beam with hollow block infill model of construction which prevails in the Philippines. From an engineering perspective, this may not be an especially good choice for earthquake country. The reality is that in many cases poor materials and untrained workers compound the problems inherent in the design. Readers concerned about seismic survivability may do well to explore reinforced concrete and steel frame designs. Also, bear in mind that the Philippine hollow block residence is economical, can be built of locally available materials and is very well adapted to surviving the much more common hazards of life here, typhoons and flooding. Big earthquakes may come every 100 years. Typhoons arrive like clockwork every year! Also bear in mind that these views are those of a homeowner, not an engineering professional.
*This book is available used from many online book sellers for a low price. If you’re ordering from the U.S., U.K. or Europe for shipment to the Philippines, I’ve found that prices and shipping rates from ABE (http://www.abebooks.com/docs/HelpCentral/Search/index.shtml#11) are generally better than Amazon.
We want to thank reader Ceazar Nieva for suggesting another publication, The Seismic Performance of Reinforced Concrete Frame Buildings with Masonry Infill Walls - A tutorial developed by a committee of the World Housing Encyclopedia, a project of the Earthquake Engineering Research Institute and the International Association for Earthquake Engineering. It has a wealth of information and is available online for free as a PDF download at
I urge all who are considering building a concrete house in the Philippines to download and study this publication and to insist that their architect or engineer do the same. Please let us know if you have a problem with this link.
Updated Oct 23, 2013