Earthquake Philippines – design right

Earthquakes
When we hired the engineer to design our house we were aware that the Philippines was included in the “ring of fire” earthquake zone and that our part of Panay Island had experienced a magnitude 8.2 earthquake in 1948.  In 1948 the 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.  The huge and beautiful church in Oton was destroyed.  The Alimodian church was heavily damaged.  Bell towers were toppled, including in the Jaro Cathedral.  Of course these buildings were of very heavy unreinforced masonry construction.  Many pre-War reinforced concrete buildings seemed to have survived quite well.  American engineers were well aware of the 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 big quake could happen at any time.  Tigbauan is in the most dangerous Mercalli earthquake zone.  (inset map?) 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 rebar 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 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 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.
One of the best resources 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.  We did use decent quality 6″ block for our exterior walls.  See _________. 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 I 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.  These are smaller bar than Yanev recommends.  He recommends 5/8″ bar, which is about the same as 16mm.  We can’t imagine using 16mm bar for block reinforcement.  The use of 16mm bar in 4″ hollow block is so far from Philippine reality that its makes one wonder about the general adequacy of Philippine practice.  When he speaks of good block construction he is describing something far from the Philippine reality — perhaps solidly filled 8″ block with 16mm rebar reinforcement and multiple tie beams. 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 ________ for more discussion.  PHOTOS
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 combined into well-reinforced 32″ tie beam, combining two courses of block, the lintel beam over the windows and the  roof beam above.  PHOTOS.
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 is not 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!
*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.

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.

maribojoc_bohol_damaged_house

Collapsed two story house, Maribojoc, Bohol

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.

Villanueva Building, Iloilo City, built in 1925

Villanueva Building, Iloilo City, built in 1925

Iloilo_hall_of_justice (1 of 1)

Hall of Justice in Iloilo. Built 1992 and abandoned due to earthquake damage.

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.

finance_bldg_manila

Finance Building, Manila, 1945 – withstood huge assault

Oton Church – destroyed in 1948 Panay Earthquake

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.

Philippine Hazard Map

Philippine Hazard Map

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.

An almost perfect column

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.

Poorly tied corners

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.

corner

Reinforcing bar should wrap around corners

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.

The roof beam is complete

The roof beam is complete

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

http://www.world-housing.net/wp-content/uploads/2011/05/RCFrame_Tutorial_English_Murty.pdf 

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

Comments (23)

  1. great i really read all those article …its a really great help and i let my husband read this because we are planning to built a house in caraga region in surigao and he is insisting a 4″ hollowblock …. and the map its really big help thank you so much for this…

    • We used both 4″ and 6″ hollow block on our house. What is your husbands concern about using 4″ hollow block?

      • My own observation is that workers don’t see filling the cores of the block as the wall goes up as being that important. I found them using mortar rather than concrete sometimes. That’s convenient for them because to use mortar between the block and concrete to fill the cores they have to maintain two mixes. It’s much easier just to use the mortar for both. The size of the hollow core on four inch block is really small. I question how much good the filling actually does. The hollow cores on 6″ block are quite a bit bigger and can hold much more concrete. My theory is that six inch block creates a stiffer, better shear wall and therefore able to resist earthquake shaking better. Is my theory correct? Not sure.

        Bob

  2. Dose everybody takes this much care in Philippines while constructing house. Is this the general practice ….

    • I see a lot of variability in construction quality. Some is terrific, some is downright scary.

  3. I am giving away my design for earthquake resistant masonry construction to any that need it or can use it. Fell free to use it if you can and share please share it with any others who could benefit from it.

    • Thank you for this most intriguing alternative. I hope our readers will give it careful consideration.

    • Have you consider liquefaction hazard in your construction since i observed from other picture you posted that your lot is a rice field before or flood prone

      • Unfortunately, we learned more about earthquake hazards after we built rather than before. We did make it clear to the engineer that we wanted our house to be earthquake resistant. The plans called for more rebar. Back when we were considering a two-story design, they proposed using 25mm rebar for the columns — unheard of locally. In other ways, as I have pointed out, there were design failings. It’s a long list. Regarding liquification, I have seen a hazard map of Panay. The clay soil areas were not marked as high hazard areas. The higher hazard areas seemed to be sedimentary coastal and riverine zones and of course recently filled areas.

  4. john, a week ago ,my son inform me that they will study earthquake, philippine setting. Iam wondering how can i help him, because he is a 2nd year college student taking up architecture. i decided to get info through internet, until i found out this article. when i read, i feel that i m a engineer or an architect or simply a carpenter.. because ive learned the proper building of structures in an earthquake prone area. tnx

  5. Hi Bob,

    Excellent website/blog. My wonderful Filipina wife and I have now read everything you have written and with much interest. Thank you.

    Perhaps of interest, we’ve decided to build a 32′ Ecoshell house on our land on Palawan. This seems maybe an ideal answer to housing in the Philippines. Would you, or your many many readers, have any opinions regarding this type of house construction?

    References:
    http://www.monolithic.com/topics/domes
    http://www.monolithic.com/topics/ecoshells
    http://www.monolithic.com/topics/blogs-design

    • Hi Stephen,

      It’s been my experience that ANY monolithic pours are stronger than conventional methods. This would also be true if someone wanted replace block construction with a continuously poured wall.

      This is what I would do if I were to build in an earthquake prone area like the Philippines.
      The objective is getting out alive.

  6. It is worth considering using steel beams (with holes in them for keying in mortar/concrete) as lintels above the doors and windows (particularly larger windows) as these are areas of particular weakness in earthquakes. Steel beams would be able to withstand the forces of an earthquake better than reinforced concrete (which is more brittle) and would not add greatly to the overall cost of a house.

    • Peter,

      We ended up making our lintel beams into a continuous tie beam. I think that’s a better approach because it’s all tied together, including at the columns and corners rather than having individual lintels at each door or window. I just wish that we had thought of merging the roof and lintel-level ties beams into a single solid, reinforced concrete tie beam with strong ties at the corners.

      Bob

  7. Pingback: Our Philippine house project: shopping for cement blocks at goILOILO.com

  8. A couple of years ago, there was a group of architects and engineers who where studying the earthquakes in San Francisco. They made a scale down models of brick buildings. One model was conventionally placed rigidly under a few inches of dirt, the other was set on a glider. When they created a shaking motion like of an earthquick, the model that was on a glider sustained very minor damaged, while the conventionally erected one collapsed completely

  9. Earthquake.

    Hi Bob.

    I have been studying structure damages in the 1990 7,7 earthquake that hit Luxon.
    Many photo shows damage on pillars near the foot, over ground level or below.
    Concrete structures are simply chrushed like in a large grinder.
    My conclusion is, that the lateral movements isnt well enough take care of in many building.
    OK in the top of pillars wtih reinforcement that tie beam and pillars together and make them stron there.
    But i the bottom there are no lateral reinforcement, its like a havy structure, standing on long weak legs, somewhere the legs are going to brake, and most certainly in the ground level region as they are unsupported here.
    Some photos show that the load is much higher that expected, they pillars are simply wrongly calcualted, but in general, they might have taken the lateral damagind force if there have been better lateral support.
    Diagonal support of some kind might have helped this.
    What ever its steal beam’s, or in site molded and reinforced walls, placed strategic to support lengt and width of the building, and concrete slabs to take rotational lateral movements, that can come from movements hitting not angular to support walls, that can again create the building to move in rotations.

    Building technic, and theory about this issue is very interesting.

    Do you know which rules the Phil engineers use to calculate the reinforcement in your building.
    American standard, British standard, EU norms, or Any deveriations of this as a phil standard. ???

    With regards
    John

    PS.:
    I have developed a calculation system in EXCALL that can deal with EU standards for concrete reinforced structures. So that is why im interested. :)

    • John,

      Thanks for your excellent comment and sorry for the late answer! I don’t know where the Philippines gets it’s structural engineering codes but in most cases Philippine codes are generally based on U.S. ones. Many of the multi-story buildings which failed had a very open structure on the lower floor or floors, evidently to accommodate open lobbies, parking, atriums and so forth, so they had not much to constitute shear walls.

      I continue to wonder how effective weak (lousy block, lousy filling of hollow core) 4″ hollow block walls are as shear walls. The idea seems to be that the walls are not important in resisting earthquakes. That seems wrong to me!

      Bob

  10. Great article about the construction for earthquake area. Because of the earthquakes and typhoons that is one of the reasons that I want to build a Monithlic Dome. I posted a website on one of my comments before. These Domes are great for the Philippines and I will build one, when I do not know.

    From the sounds of it you should have a great house that for the most part will servive a earthquake. Even the best built houses have a weakness, the question is will that be the weakness that brings the house down. Hope you hard work and attion to detail pay off, it should.

Comments are closed.