Making sure rebar splices are done correctly will help make your Philippine house stronger at virtually no additional cost. Said another way, improper splicing results in a weaker structure and saves no money. The Philippines is one of the world’s most highly seismically active regions. Improper splicing will make the structure more vulnerable to earthquake damage, even from a smaller tremors. The adequacy of rebar splicing is one of the biggest disappointments in building our Philippine house building project. The workers, although experienced and intelligent, did not know anything about the engineering principles behind rebar splicing. The plans provided no guidance. The engineers contended that the workers should know about splicing. They did not. Bob knew nothing about rebar splicing, but in especially egregious cases, sensed that something was wrong. The engineer was brought back to show the crew how to spice, but in some instances only after it was too late to make changes.
Once again, I advise buying the book “Peace of Mind in Earthquake Country” and reading it before you talk to architects and before you build. You really only need to read the few pages on concrete block buildings. The book is available very cheaply as a used book from Amazon or ABE.
Philippine buildings generally consist of steel reinforced concrete columns or posts supporting steel reinforced concrete beams with the spaces filled in with fairly weak concrete block forming the walls. The reinforcing steel is referred to as “deformed rebar”. Ours is a one-story house, so a single standard six meter long rebar (about 20 feet) can extend from the footer (1.2 M deep) to the topmost roof beam without splicing. That’s a good thing. Our columns used 12mm diameter rebar. All columns and beams also include rings of reinforcing steel called “stirrups”. The stirrups are held in position using “tie wire”. Together the rebar and stirrups make up the rebar cage which will go into a plywood form, the form will be filled with wet concrete to make a column or beam.
The beams are a different matter. Since the room beam length for our house is 13.5M and 15M, the 6M beam rebar must be spliced. How and where is splicing is done affects the strength of the beams and of the house. Our good, experienced workers could neatly make the rebar splices but really had no idea of how much overlap there should be at the splices or where the splices should be. Splicing was done incorrectly and had to be redone, in one case three times. This wasted time and money and caused considerable frustration.
Since our workers had worked on dozens of houses this leaves one wondering about the errors embedded in them. On commercial buildings, there will likely be a supervising engineer, but on residences — who knows? In the case of our house, if Bob had not been there, the critical roof beams would have been already poured and the mistakes, hidden.
The above photo shows the completed rebar cage for our east roof tie beam. The rebar is 16mm diameter. At the top and bottom are two sets of rebar spliced together at the center of the beam. This is wrong in two ways. The top splice is OK as it is over the center of the span. The bottom splice is wrong because it should be over a column. Both splices have far too little overlap at 30 or 40cm. The minimum is 64cm for 16mm diameter rebar.
Here are the engineering rules-of-thumb we garnered regarding splicing:
- Splice beam top bars at locations between support columns.
- Splice bottom beam bars at or near support columns.
- The splice overlap should be a minimum of 40X the diameter of the rebar. So, for 16mm rebar the splice should be a minimum of 64CM, for 12mm rebar, 48cm.
So, the above splicing violates every one of these rules. The splice overlap was 30CM, less than one-half of what it should have been. All bars were spliced in one location. This rebar cage had to be taken down and completely redone. Without supervision this beam would have been poured and the mistakes hidden forever, or until the first earthquake.
The rebar splicing errors which went uncorrected were mainly splicing the horizontal rebar in the hollow block walls and tie beams to the corner columns. These splices, properly done, can create a strong band of reinforcement around the building.
In this photo of a column going up, you can see the stubs of horizontal rebar protruding out through the forms. They are spaced so that they will align with the horizontal 10mm rebar in every third course of hollow block. The splice should be at least 40cm and they all should not be in the same place. It’s my view that these corner ties are not very good. Compare to this diagram showing rebar wrapped around the corner and extending well into the wall, perhaps as far as to the first door or window opening. This type of reinforcement will bind the structure together much more strongly.
Here is another suggestion. Rebar generally comes in six meter lengths, about 20′. Whenever you can eliminate splices by doing so, use nine meter lengths. This means fewer splices and a stronger building.
Here’s a good splice in a lintel beam. It has good overlap and the splices are staggered. On the left you can see the rebar cage for one of our quite complicated tee-shaped columns.
Here’s a pretty good article on rebar, and rebar cage fabrications: http://www.wikihow.com/Tie-Rebar
Another thing to consider is whether to paint rebar or pay extra for coated rebar. Certainly, any exposed rebar should be kept painted.
While our attention to these construction details may seem over-obsessive (they certainly seemed so to our construction crew!), the Haiti disaster and now the Chile earthquakes show that proper engineering and construction can make the difference between life and death or at least between being able to live in your home after an earthquake as opposed to being homeless and starting all over again. And, in this case doing it right costs the same as doing it very wrong.
Read all about our Philippine House building Project at /building-our-philippine-house-index/