Long Beam Span

rodsika

Are there any structural engineers here or know the stuff. I'd like to know this.

Can structural engineers design long span beam for example 12 meters reinforced concrete beam that is as good as standard 6 meter beam in seismic performance or are 6 meter beams always better compared to 12 meters beam even if the latter were designed for ultimate seismic performance?

Studiot

For vibration analysis you have to consider the whole structure (including the foundations), not just the individual components separately since the connections play a great part in the result.

So it depends what your 6m and 12m beams are connected to and how they are connected.

rodsika

I mentioned "seismic analysis", you wrote "vibration analysis". Unless they are identical? In what ways?

Studiot

Vibration is just the more general term as there are other sources of excitation besides seismic activity.

For example the beams of motorway bridges are subject to continual vibration and are quite a bit longer than 12m.

That is less important what is important is the connection conditions of your beams.

Another important factor is the natural frequency of vibration.

rodsika

In a building with 12 meters frontage. How much generally is 12 meter beam span more expensive compared to two 6 meter span in the overall cost of the construction, materials? Is it like 20-30% more expensive?

Studiot

Two 6m span beams would require a centre support that is not present with a 12m one so would add to the cost.

You have been asking about reinforced concrete. This is usually priced at a rate per cu metre placed so the cost would likely be the same for the beams either way. I don't see that any heavier duty craneage would be required 12m is not such a significant increase in size.

rodsika

Supposed you would be given a choice by the architect/engineer whether you want a 6 meter or 12 meters RC beam span in your house or office building? What would you choose? I'm still thinking of two 6 meters RC beam span because there are 3 connections instead of 2. In construction they don't do it perfect or sigma 5 accuracy. Hence with the 3 connections in the 6 meter span.. at least there are 3 connections instead of 2... in this latter case of 2 connections, if there is some subtle weakness in one of the beam-column joint.. it could fail. In your country. Do you commonly see 12 meter RC beam span houses or office? Isn't 6 meter the standard?

Studiot

Most building component sizes are decided by architectural considerations not structural ones. If a longer span beam is specified by the architect then the structural engineer will design a suitably stronger one.

Beams of 6m span are probably so lightly loaded that they are only just above what we call 'minimum steel'. That is the steel required to control cracking.

It is unusual to use RC beams in the UK in any case, steel is more normal.
Suspended floors are normally RC slabs or prestressed.

rodsika

I think you forgot that the rebars of longer beam span would be quite larger and there will be much more rebars in 12 meter RC beam span versus two 6 meter RC beam span so the cost of the former would be higher.

Studiot

No I didn't forget that.

I said that under standard building contracts reinforced concrete is priced at a rate per cu metre placed. That is the UK standards method of measurement.

Civil engineering contracts are priced differently so the reinforcement, concrete and formwork are all measured separately.

It is actually unlikely that there would be a greater number of reinforcing bars in the longer beam. Just that they would be larger diameter.

rodsika

Consider a lot size of 12 meters frontage and 15 meters depth. Supposed they put 4 beams side to side and one girder at middle front to back. This means the front and back have columns at the center. Only two RC beams of 12 meters (without columns) are put at middle of lot with beam depth of 0.8 meters. Would they make the girder at middle (front to back) also with girder depths like 0.8 meters to support the whole 15 meters front to back lot size.. or would the girder be much smaller just to support the slabs? What is usually the case?

Note I'm not constructing any of this, of course but just want to understand the design of certain office near mine. Thanks.

Studiot

Do you mean something like this?

So the upper floor is divided into 4 slabs 6m x 6.5m.

Attached Thumbnails

 

rodsika

Yes. But with the difference there are two 12-meter beams in the middle between the front and back making up 4 side to side beams (not 3 as your drawing shows).

Also the girder is not steel i-beam but RC beam and it has same level as the side to side RC beam.. meaning it is embedded like in normal column-beam joint so the definition of girder as something supporting smaller beams don't hold. I wonder if you also defined a girder as something just to support slabs and not exactly the beams (wherein the 12 meter RC beam can stand on their own).

Studiot

OK I think I understand.

There are 3 columns at the front, 3 at the back, and 4 down each side, making 10 in all. avoiding counting corners double). They are presumably evenly spaced. They are connected by a system of rc beams at the top. This network of beams divides the support for the upper floor into 3 strips 4.3m x 12m. When complete this makes an rc frame.

I would guess that the upper floor might be then constructed by lifting precast beams or floor units to span 4.3m onto the frame and screeding over the top. That would avoid slab formwork and 4 - 5 m precast beams are not that large and readily handleable by a small crane or even a digger arm. If the floor units can be cast off site or on site whilst the rc frame is being constructed there will not be some weeks waiting time for the floor concrete to cure.

rodsika

Thanks. In my place. We don't use precast slabs. But simply pour fresh concrete into the floor support. Anyway. The building will be two storey. They plan to use metal hip roof on the roof of the second storey. Now do you know how heavy a hip roof is compared to using pure concrete slabs or roofdeck? Like is a hip roof with full truss support 1/2 heavier or 1/4 heavier than pure concrete roofdeck or same weight given the same building size?

Studiot

Rough guess

75 to 100mm concrete slab 240 kg per sq metre = 180*12*13 to 240*156 = 25tonne to 35 tonne

metal sheet roof 6 - 10 kg /sqm plus 4 trusses at 750kg each = 3 to 5 tonnes

rodsika

Thanks. I read today that irregular size lots can experience torsion as a result of unequal lateral movement. Pls see attached picture. Supposed the lot we talked about has the left side 14.57 meters and the right side 17.29 meters.. that is the right side is 2.72 meters longer. How much torsions would be produce for earthquake or seismic movement that goes up and down, side to side and diagonal? Or can the size be considered rectangular approximations and torsions negligible? If not. Can you design columns and beams that would make the left and right side equal and preventing torsions?

Attached Thumbnails