Our Philippine house project: the structural heart of the house – concrete columns and beams

Our Philippine house project: the structural heart of the house – concrete columns and beams


Our Philippine house building project: reinforced concrete columns and beams, the heart of the house.  What we did right in building our Philippine house and suggestions on how to do a better job.  Learn from our experiences!  Just keep in mind that we are not engineers or design professionals.    We urge you to have your building engineered by an engineer.  We share our personal experiences and insights.  They may help you ask the right questions as your house is designed an built.

Roof beams are nothing new…

 

Parthenon

Hollow block walls in Philippine residential construction don’t have much load carrying capacity or the shear strength need to withstand lateral shaking.  That’s why its so important that the reinforced concrete columns and the  roof and lintel beams are properly specified and constructed.  They may look fine until the shaking begins.  During the day that we written this we’ve had a number of tremors.  Poor design or construction can have disastrous consequences.

Hyatt Terraces Hotel, Baguio

Hollow block hotel, 1964 Valdez earthquake, Alaska

This post will show how we constructed our columns and beams and will discuss some ways in which we could have done a better job — if we had known then what we know now. Take advantage of what we’ve learned.  While Bob was generally knowledgeable about construction in the US, he had little experience with concrete and reinforcing bar (rebar). Most of the Philippines, Panay Island included, experiences severe earthquakes.  An 8.2 magnitude quake struck our area in 1948, causing extensive damage.  We told our engineers we wanted   home designed to survive  earthquakes.  Here are some of the issues we faced.

  • Our crew, which was hardworking, intelligent and experienced, really knew nothing about structural engineering.  What they knew was learned haphazardly from years of work building residences and other modest structures.  They had not worked on higher-end commercial projects where, presumably, they would have worked under the supervision of an engineer and learned basic engineering practices for using concrete and rebar properly.
  • We assumed that the plans developed by our engineers would ensure, if followed, that our project would result in a soundly engineered house.   In some aspects, the design of the beams and columns, quantity and size of rebar, this goal was met.   Most people who looked at the way we built the house thought it was ridiculously overbuilt.  They may have been right in terms of concrete mixes, column and beam sizes, and quantity of reinforcing bar.  We used good materials but, unknowingly, did not always follow good engineering practices.  Why?  In many critical areas, the plans were woefully deficient.  They did not  provide details that were needed to properly tie together rebar joints for maximum strength.  There were no rebar splicing details. There were few details about such critical issues such as  adequately tying corners and walls together so that the building can act as a unit to resist earthquake stresses.  Given that there were few instructions in the plans, the workers followed their own instincts and prior experiences. While Bob was not knowledgeable about such matters, he was on the job every day and became uneasy about some of the work.  He called in the engineer and the inadequate splicing of the roof tie beam were corrected.

Engineer and crew meet to talk over rebar splicing problems. See engineer’s chalk marks on wall giving splicing instructions.

However some other rebar placement and splicing work is seen to be inadequate.  One conclusion which can be drawn is that the owner can’t depend on workers or engineers to ensure that one’s Life home is properly built.  The owner must educate himself, in advance, about building principles.  One book I highly recommend is “Peace of Mind in Earthquake Country” by Peter Yanev.  I only wish I bought and read it before building our house rather than after!  You can buy the book inexpensively from amazon.com.  There should be an ad for it to the right.  If you use that link, MyPhilippineLife.com will get a small commission.  Since the book can be obtained for three or four dollars, don’t worry too much much about our profiteering!

  • What was the engineer’s response to our complaints about the missing engineering details?  Basically that the workers should know how to do these things.  Obviously, they did not.  If they did, why we would we need an engineer?
  • In some respects, the plans could have incorporated more advanced engineering practices, especially given the mandate to make the building earthquake resistant.  For example, our house has unusually large windows.  These can very substantially reduce the shear strength of the building.  It would have been better if we had strengthened the wall areas surrounding the window openings. We have bond or tie beams above the windows windows (lintel beams) and at the top of the wall — the roof beam.  A bond beam below the windows would have also strengthened the structure. We had a few downright engineering missteps.  One of these was leaving out a beam needed to support the roof rafters over the porch.  The solution involved demolishing some work already done and installing a pretty dodgy beam and column add-on.  Overall, our engineering experience was disappointing to us.  A recommendation that we use nine meter long rebar rather than six meter would have resulted in less splices and a stronger building.

We are going to assume that someone is going to check and double-check the column layout — that columns are in the right places, that the building is square, that the columns are plumb.  Our foremen and crew were pretty good about that.  Now we’ll look at how we did things in a series of photos showing what we did and pointing out some pitfalls that you can avoid.  We are going to be honest about our shortcomings in the hope that others can benefit.  We are covering concrete, concrete mixes and concrete vibration in another post and hollow blocks in yet another.

Digging the column footers

The workers insisted on using mahogany rather than coconut lumber for batter boards.  They were right.  We were able to reuse the mahogany many times.  Coco lumber is much poorer quality. The crew did a good  job laying out the house using simple tools — fish line, a water level and my 25m tape.  The only problem was that they assumed that the “front” of the house faced the road.  It actually faces the mountains.  They had to reverse some of the layout.  What a start!  Anyway, they did a good job.  Again, it’s critical that the layout work be near perfect.  Small problems with the layout can cause big problems later.

Constructing the rebar column cage

 

Column rebar going up

Column rebar cage and footer.

The rebar mat supporting the columns is constructed grid of sixteen pieces of 16mm rebar wired together. The mat and the footer are 1.2 meters (about 4′) square.  The footer excavation is 1.2 meters deep.  The column rebar cage is of 12mm rebar, the stirrups 10mm.  As you can see in the photo, the bottoms of the vertical rebar are bent and wired to the mat.

Thus, for our one story house, a six meter rebar extends without a splice from four feet underground to a few inches above the roof beam in which it is embedded.     As you can see in the photo, this particular column is at a corner of the house, is L-shaped, and consists of seven 12mm rebar. There is nothing to apologize for here.  It should be an usually sturdy column resting on a substantial footer, possibly suitable for a two-story residence.

The photo above also gives a good overview of how the footers were done.   Our workers put a shallow layer of large gravel in the bottom of the 1.2 meter deep footer excavation, perhaps 4” of it. (No gravel was indicated in the plans.  We’re not sure if this was necessary or desirable.)  Then the rebar mat goes in the bottom and the column cages are put in and wired to the mat with 16 gauge galvanized tie wire.  Then the column footer is poured.  The plans call for the footer to be 1.2 meters square by 25 cm (10″) depth.  Hint.   Make sure your workers support the mat above the bottom of the footer so that the column rebar is embedded in and bears on the concrete footer, not on the dirt underneath.  My workers had to be instructed to do this.  To raise the mat you can use stones to lift up the mat so that the concrete will flow under the mat.   Also make sure that the footer extends the full depth into undisturbed native soil.  We had done some filling of the area where the house was to be built,  so our excavations had to be deeper — 1.2 meters plus whatever depth of fill there is.

Column forms were of mahogany 2×2 and 1/2″ marine plywood

Once the concrete footer has set, the plywood forms are secured around the column rebar assembly.  Use good materials for the forms.  During the project these materials will be used and used and reused.  Coco lumber and thin plywood will not hold up. Of course it’s essential to check and double check that the columns are exactly in the right place and that the forms are plumb. The columns, which total about fifteen feet high, are formed up and poured in sections.  Make sure the rebar is centered in the forms.  As you’ll see, it’s very difficult to fill every void in a column form.  The the usual Filipino method is to use quite soupy concrete which flows easily, but of course the column will be weaker.  Our L-shaped corner columns have advantages.  They make strong corners which end up being the same thickness as a six inch hollow block.  That means there is no exposed column to work around during finishing. You’ll have a simple, square corner.  The downside is that the column form is filled with lots of steel which the cement may have a hard time flowing around.  Apply the rule of thumb that the aggregate should be no larger than 75% of the smallest gap the concrete need to flow through. See http://www.concretenetwork.com/aggregate/gradations.html. Any big stones are going to catch in the rebar and prevent a proper flow.  Keeping the column concrete thick enough to be strong and thin enough to fill the form is tricky.

 

We used a gasoline powered concrete vibrator to try to ensure there were no voids in concrete as we filled the forms.  The vibrator has a long flexible shaft with a vibrator on the end. The use of vibrators is common in non-residential concrete work.  More on the vibrator HERE and HERE. We pried the forms off our first column and this is what we found.  We were all pretty discouraged.

Demolition of column

 

Another column problem

I was in Iloilo buying supplies.  When I got back I saw a suspicious looking patch on a another column.  The tried to hide this behind some mortar.  This is what it looked like after a removed the mortar.

A good column, but other problems

This L-shaped corner column looks to be  just about perfect.  The rebar protruding from the column will be spliced into the horizontal rebar in the hollow block walls. The protruding rebar on the right is far too short to make a strong splice.  When the earthquake shaking starts, the building corners are critical to holding the building together.  It’s hard to imagine these tiny splices doing the job.

Corner and tee column plans

Compare this to my rough drawing showing stronger rebar placement in corners.  The outside rebar should be bent around the corner and extend as far as possible into each wall.  Of course these long rebar tails are a bother on the construction site.  Also, consider using longer rebar in beams than the standard 6M (20′) lengths.  That means less splices and more strength.

The ends of the inside rebar should be bent down into the block cavity.

Poorly tied corner

I did not have a good photo showing how the corners were tied in our house but the above photo of another house under construction illustrates the problem very well.  This house uses lots of rebar — no expense is being spared but note how little attention is being paid to tying the corners together.  It is these corners which will come under tremendous stress in a seismic event.  We’d love to hear an explanation from an engineer as to why things are done this way.  It’s probably practicality.  These complex rebar cages are put together on the ground and raised into position, just as if they were wooden beams.  Long protruding ties would be a nuisance.  The end result is that much of the potential strength of the building is compromised.

 

The columns and the walls go up simultaneously.  Here you can see the vertical rebar sticking up from the hollow block walls.  Although we had big windows, no extra rebar or reinforcement was specified.  If you have big window openings, we suggest you add two additional vertical and horizonal rebar in the hollow block at each side and above and below the window opening. The wall footers and walls will be covered in more detail in another post.  Note that the bamboo staging is secured with old-fashioned Manila hemp rope.  According to the crew this rope will not stretch and grips the bamboo much better than synthetic rope. And where does Manila hemp come from?

Abaca (Manila hemp) being unloaded at Port of Iloilo in 2010.

 

Another problem

In the photo above you can see the hollow block wall almost touching the rebar cage for a column, leaving little to no room for column concrete.    Since our columns are almost exactly the same depth as the 6″ hollow block we used on the exterior walls, forms for these columns were simplified.  Although Bob was not especially knowledgeable about concrete columns, it was pretty obvious to him that there was a problem in not having the rebar well embedded in concrete. Bob brought the problem to the engineer and she agreed.  These hollow blocks had to be cut back  to allow room for the full column. Frankly, the engineer somewhat grudgingly admitted the problem.   That the experienced crew could not see the problem and that the engineer was not especially concerned encapsulates some of the problems facing the foreigner (or Filipino) trying to build a quality house in the Philippines.

Rebar corrosion

Rebar corrodes.  Corroding rebar can break apart columns and beams.  If any rebar is exposed to moisture the corrosion is much faster.  In commercial projects, hot dip galvanized or epoxy coated rebar may be used to slow down corrosion.

Coated rebar.  Galvanized is better but more expensive.

The next step was the lintel beam

Forms for lintel beam

Window and door openings weaken walls. Lintel beams carry the weight of the wall above door and window openings and can also help tie the building together.  Lintel beams typically are only over the door and window beams, but since we have so many and so big openings, we decided to make the lintel beam into a continuous tie beam.  Our lintel beams were 15cm x 20cm (6″ x 7″) and used two 12mm rebar with 10mm stirrups. Usually lintels are not so strongly built  as other beams as they are bridging small spans.  It might have been better if we had used four 12mm rebar.  More on that later.

Lintel rebar

This photo shows some nice long splices in the lintel rebar, but the splices are one atop the other.  They should be staggered.  See below.  In the red circle, a very short splice in the vertical hollow block rebar.

Lintel beam – forms off

It looks like there’s a curve in this beam but it’s aspherical distortion in my zoom camera lens.  A few notes.  The rebar stubs extending downward from the window opening will be used to weld in the steel casement windows.  The ends of the column rebar are the very ends of six meter (20 foot)  rebar the other end of which is securely anchored in the footers 1.2 meters (4′)  underground.

Above the lintel beam are two courses of hollow block.  On top of those goes the roof beam — the main structural beam in the building.  This is where we missed a key opportunity to strengthen the building at very little additional cost.   We could have, and should have  made a single, strong 80cm x 15cm reinforced concrete beam from the top of the windows to the top of the walls, replacing the lintel beams, the two courses of hollow block and the roof  beam.  Combined with rebar wrapping in the corners, this would have greatly improved the shear strength of the entire structure and would, in our amateur opinion,  have been a good engineering response to the many large window and door openings.   Of course no such improvement was in the plans, suggested by the engineers or thought of by myself or the workers until it was too late.  So we hope that our readers may be able to raise this issue with their engineers and perhaps end up with a better, stronger house.

So, the next steps were to lay the top two courses of block and form-up for the roof beam.

Splicing error in roof beam rebar

This photo shows the 16mm rebar framework for the concrete roof beam.  The plans called for 15cm x 25cm (6″ x 10″) room beam with 16mm rebar.  We decided to build a 15cm x 30 cm (6″ x 20″) beam

A visit by our engineer confirmed our suspicions of a  problem with the arrangement of the rebar in the beam. Rebar comes in six meter lengths.  As shown in the photo above, the workers spiced all the rebar in the center of the span.  The engineer directed that splices be staggered with no splices at mid-span in the bottom rebar and no splices at the support columns in the top of the beam. Everything above was taken down and redone.  Our plans lacked a rebar splicing plan.  This has caused endless required corrections and wasted time and money. Our foreman and workers just don’t know the engineering principles. We asked the engineer to prepare a splicing plan so that the workers (and owners) will be sure that things are done properly.  We never got one. We suggest that you insist that your architect or engineer include a complete splicing plan.

Here’s a few rebar splicing guidelines we learned.  They are only rules of thumb.

  • The splices for reinforcing bars in the top of the beam should be between columns.
  • The splices for reinforcing bars in the bottom of the beam should be approximately over the support columns.
  • The reinforcing bar splice overlap should be a minimum of 40X the diameter of the rebar.  For example the splice on a 12mm rebar should be a minimum of 48cm.  For a 16mm bar, the splice should be at least 64cm.  Longer splices are better.

House_40CM_splice

This photo (above) shows 16mm rebar spliced with a 40cm splice. The minimum overlap should be 64cm. This rebar cage had to be disassembled and redone.

Rebar cage at intersection of roof beams and column.

Rebar cage at intersection of roof beams and column.

Our engineers left a porch support column and a beam out of the roof plans.  Our workers spotted the problem. The engineers came up with an ad hoc solution — adding slender (15x15cm) columns so as to not spoil the appearance of the porch.  Both we and the crew thought the columns were too small.  It this photo you can see the tangle of rebar coming together at this tiny column.  It’s hard too see exactly where there is room for concrete.  We used small aggregate (the rule of thumb is that aggregate should be one-fifth the size of the smallest rebar opening) and gently vibrate the concrete.  Poor planning by the engineers.

To put in an additional porch column, we had to demolish part of an already completed wall and wall footer and pour a column footer under the existing wall footer.

A forest of bamboo scaffolding

A forest of bamboo scaffolding.  Roof beam.

Bamboo scaffolding in Kowloon, Hong Kong, 2005

Bamboo staging in Hong Kong

The roof beam is complete

Pouring the roof beam. Concrete vibrator in used.

The crew worked feverishly to finish pouring the roof beam.  This is the last structural concrete work.

Beams and columns done

Despite our missteps, it does look pretty strong doesn’t it?  It takes at least two weeks for the beam to cure, then the steel roof trusses can go on.  At this point we  laid off some of the laborers as there was not so much heavy work left to do.  The welders kept busy with the roof trusses and windows, the masons with interior walls.  Our next coverage is WALLS, ROOF and WINDOWS.

READ THIS BEFORE YOU BUILD.  Thanks to reader Naldy Bulan for recommending an excellent, free UN publication on building in the Philippines:  Handbook on Good Building Design and Construction in the Philippines at http://www.unisdr.org/files/10329_GoodBuildingHandbookPhilippines.pdf

Read all our Philippine House building project pages at /building-our-philippine-house-index/