Roof beam

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Roof beam

Postby thomas.fontaine » Wed Mar 05, 2008 3:55 pm

While on most of the detailed drawings I have seen so far vertical posts at roof level are connected together with concrete beams, 0.4 x 0.2 or whatever, on some of the house in construction in thailand I have noticed that posts are sometime connected using steel beam, I-shape, U-shape or whatever.

Steel beam seem an attractive solution as it is labor cost effective but I was wondering about the pros and cons of using steel beam at roof level.

Any post on that?
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Re: Roof beam

Postby thomas.fontaine » Thu Mar 06, 2008 8:09 am

To avoid any confusion, I am talking about steel beam conecting the posts, not the steel structure that support the roof.

You can see those steel beams on dozer's house. Even though steel price is high, you don't have to put all the form work needed by concrete beam + no rebar + no concrete.
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Postby Nawty » Thu Mar 06, 2008 9:51 am

H, here is a photo of a house I am currently building.

The I beam was built into the design for show on the exterior, it is not seen on the interior.

I love the way it has turned out, it looks very slick now as it is all painted same smokey grey colour as the walls and sofit and has a great effect I think.

Originally my design was to have the I beams cut at an angle of around 45 degrees on the ends, but i changed it to 90 degrees and think this looks much better suited to the house style.
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Postby thomas.fontaine » Thu Mar 06, 2008 12:12 pm

So, basically, you choose steel beam for architectural reason. Is there any structural advantage to have steel beam instead of concrete beam?
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Postby Nawty » Thu Mar 06, 2008 3:36 pm

Steel is stronger, it is also liter, so the height of your house and then the roof will be a lot less.

I have some photos somewhere, I will try to find and post of a house out in BangNa we went to look at. It was constructed entirely of steel frame and went up very quickly.

Dont however be fooled into thinking that the cost saving in labour and some materials will be saved in the long run here in Los.

What you save with stell on labour, you will no doubt lose elsewhere in some other ridiculousy slow section of construction for whatever piss poor reason they can muster up.

Sure in the west with schedules that can be adhered to, but not here. It is very hit and miss as to if you will save in the long run with it. So make the decision on other foctors as well.
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Postby thomas.fontaine » Thu Mar 06, 2008 4:31 pm

OK, I understand your point. But still it's always better to design something cost effective, even if you end up with surprise along the way...If your design is 'costly' and complex from the start, the hit will surely be even bigger.
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Postby Nawty » Thu Mar 06, 2008 6:45 pm

Here tis.
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Postby Nawty » Thu Mar 06, 2008 6:56 pm

ooops
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Postby thomas.fontaine » Thu Mar 06, 2008 7:42 pm

That's interesting.

How do they lift all the steel beams in position, do they have a small crane for some period of time or do they lift them the old egyptian way?
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Postby Nawty » Fri Mar 07, 2008 9:07 am

For my house, they use the egyption method....have a pic of that somewhere also, lumping this huge beam around in thongs and barefeet, rolling it accross the concrete beams of the roof.

In this pic they would have used cranes as this was an estate with 40 or so houses on it, all built same.
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Postby thomas.fontaine » Mon Mar 10, 2008 6:47 pm

So, about this concrete beam story at roof level...

Anybody with either good technical or purely architectural reason to choose for concrete beam instead of steel beam to connect the posts?
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Postby jazzman » Tue Mar 11, 2008 11:42 am

It depends entirely on whether you are planning a single or a two storey house.

From the pics it would appear that Nawty has used what are often called an RSJ - Rolled Steel Joist, which is a heavy I - beam also used for lintels. It was probably the most reasonable solution in the rather special design of his house.

The steel that is used in Thailand for ring beams and purlins in single sorey houses, is usually box section, 2 x 4 x 3.2 which is available in three grades ranging from 500 - 1,500 baht per standard 6 m length.

Some engineers choose to used two C-section girders welded face-to-face to create a very strong 4 x 4. if you are building a single storey house, there is no reason whatsoever to have a concrete ring beam, which is very much more cost and labour intensive. If you are contemplating a span of over 7 metres or so somewhere - such as to aviod a central pillar in the middle of a large living room - you could install a concrete beam just over that part, but doubling up on the steel would still be easier and cheaper and just as effective.

Some building contractors cut corners and ignore the engineers' specifications. Instead, they use C-section beam for the ring beam, and after the tiles have been laid, the weight squashes the open ends of the beam together causing the roof to sag. Not a pretty sight - and extremely expensive to remedy.

If your house is an unusual design, your best bet is to consult an engineer or talk to a well informed agent of the CPAC company, or just get the CPAC roof installation handbook which explains what to do and what to use without baffling the customer with science.
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Postby thomas.fontaine » Tue Mar 11, 2008 1:20 pm

jazzman wrote:It depends entirely on whether you are planning a single or a two storey house.


OK, thanks Jazzman, very comprehensive answer, as asual. Could you explain why you mentioned that "it depends on whether you are building a single or 2 storey house", since I don't see it clearly in your answer?
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Postby jazzman » Wed Mar 12, 2008 11:00 am

A concrete R-beam would be totally superfluous and very costly for a single storey house. It is designed for heavy load bearing such as upper concrete floors. In a standard construction, it is still going to be the columns that support the roof, and at roof level you would have steel girders to support the rafters.

Steel C-, and box section girders as described above are perfectly adequate for a single storey house - unless there are some unusually wide spans to be conquered; in which case, that span could be achieved by a concrete beam.

As mentioned before: if in doubt, consult your engineer, architect, or consultant, particularly if you will not be present during the construction. Either way, you will need to ensure that no substandard workmanship or corner cutting takes place and that the engineer's instructions are adhered to. The roof space is an area where the contractors think the client will not look.
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Postby thomas.fontaine » Wed Mar 12, 2008 12:17 pm

Your position conforts my initial thought: unless instructed by the structural engineer, concrete beams at roof level are superflous.

Regarding engineers in thailand, I might be completely wrong but I have the vague feeling that they will do whatever you ask them to do or not to do. Therefore, if you don't specifically mention that you believe concrete beam are superflous, they may come back with a design that includes concrete beam, regardless of the cost. This is of course not right if the engineer is also the one that is going to built your house, as in this case he will surely try to optimise the design to lower the cost down.

Therefore, my humble advise to other CTH members, if I may, is: OK to ask an engineer to design your house, but better to bring with you some knowledge of what is costly and what is not.
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