Roof Trusses with Huge RC column supports 2

[mes7a]

There are existing huge RC columns above the roof (0.5 x 0.5 meter size). Instead of another floor. We will just put trusses to support a lightweight plastic or metal roof. There are 4 bolts of about 14mm at the corner of each column (see pictyure picture). If baseplate would be attached to it with the I-beams welded to it. What would be the problem if there is very strong wind or hurricane. Would the bolts just break? I read that in lightweight roof trusses and I-beams.. the support needs to be flexible so all the forces won't be concentrated on the 4 bolts and the support would just bend. What connections would I need to do?

 

[BAretired]

Where did you read that? What recommendations did the writer give to achieve flexibility?

14mm bolts seem a bit small, but in general, bolts properly anchored in concrete should perform adequately if they have sufficient shear capacity to resist factored wind load.

BA

 

[JAE]

Can you ask your supervisor or someone with experience? These are pretty basic questions.



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[KootK]

Would the bolts just break?

Maybe. There's no inherent reason that this system shouldn't work but the connection requires proper engineering considering base plate, anchor bolt, and concrete failure modes.

the support needs to be flexible so all the forces won't be concentrated on the 4 bolts and the support would just bend. What connections would I need to do?

I don't understand this concern. Can you elaborate or provide a sketch of the failure mode that you're envisioning? I would think that you'd want/i] the forces running through your anchor bolts. There's really no other load path into the supporting structure.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

In seismic movement, the entire metal roof would rock back and forth and the bolts may be overwhelmed in shear and just break down. Whereas if the column is flexible, it would be the column that would bend as it absorb the energy and oscillate like a pendulum and not the connections be stressed. A rigid support/column would have all energy focused on that bolt connections I read this back then. So you guys also use huge rc columns to support roof trusses?

 

[KootK]

I think that a better approach would be to simply design the joint for the forces that you expect there rather than trying to trick the connection into not seeing any load. Unless you're proposing a rigid podium approach for the seismic design of the penthouse, the part of the structure intended to dissipate seismic energy will be the lateral system for the main building (shear walls, moment frames, etc.).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I see no reason why concrete columns couldn't support your trusses. If you want some serious feedback, I'd recommend posting a few sketches of the system that you have in mind.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

I see no reason why concrete columns couldn't support your trusses. If you want some serious feedback, I'd recommend posting a few sketches of the system that you have in mind.

Kootk. If the columns are distance 6 meters apart and square bars that is sized 250mm x 150mm x 8mm thick is put around the perimeter between the columns by welding it to 10mm metal plates (see red in picture) expansion bolted at the sides of the columns. The square bar would support 1 meter high RC parapet wall above the garble roof perimeter (note the rafters are not trusses but just square bars of certain size enough for it). If the 9 columns move during seismic activity, would the square bar be pull apart from the metal plates? What is the action that you usually do in this case? Do you need a bigger steel beam to connect the columns (again size 0.5x0.5 mtr and 3 meters high) together? Thank you.

 

[BAretired]

I seems to me I've seen that sketch before.

BA

 

[BAretired]

-Yes, I have seen the sketch before on thread507-393506.

I suggest that you delete the above thread as you are confusing the issue by having two threads on the same topic.

You really should get your terminology straight. The proposed beams are not square bars, they are rectangular hollow steel sections. Some would call them rectangular tube sections.

It is not clear why you would have a one meter high RC parapet wall above the gable (not garble) roof perimeter. Is that an architectural requirement?

Placing the perimeter beams on the side of the existing concrete columns seems to be an unusual practice. It is possible, but it would likely be better to seat the beams on top of the columns as you had suggested in the other thread.

Do you have any architectural cross sections through the building? This would seem to be the usual place to start.


BA

 

[KootK]

Thanks for the sketch OP. As others have said, the only way to really know is to analyze it and find out what happens. In my personal estimation, I believe that the columns will be just fine without stiff beams connecting them. They are large columns and they should work as flag poled cantilevers for seismic and wind. If you're really concerned,perhaps you could just install some light cross bracing between columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

-Yes, I have seen the sketch before on thread507-393506: Your style to put roof beams or rafters.

I suggest that you delete the above thread as you are confusing the issue by having two threads on the same topic.

You really should get your terminology straight. The proposed beams are not square bars, they are rectangular hollow steel sections. Some would call them rectangular tube sections.

It is not clear why you would have a one meter high RC parapet wall above the gable (not garble) roof perimeter. Is that an architectural requirement?

Placing the perimeter beams on the side of the existing concrete columns seems to be an unusual practice. It is possible, but it would likely be better to seat the beams on top of the columns as you had suggested in the other thread.

Do you have any architectural cross sections through the building? This would seem to be the usual place to start.

The purpose of the one meter high RC parapet is firewall requirement. The apex of the garble should be below the top portion of the firewall. See picture:

Now reason the perimeter beam is welded to the sides of the column is because the top of the column will support the rafter. And the rectangular perimeter beam has to have the longest side vertical so as to plant rebars or dowels drilled into the beams and into existing walls below to support the one meter RC firewall. Please see the following connection. Please suggest if you have idea how to put the perimeter beam above the columns.

In case you can't understand the strange connection details. On the left side, the column sides were expansion bolted with metal plates (2 sides of columns). Then the rectangular hollow sections (SB1) were welded to it. The reason it is not welded on center but on sides is because the wall has to be put on the edge of property line (to avoid wasting inner space). The RT1 is the rafter connecting to the center columns (3 total from the earlier picture of 9 columns 3 rows). So what other connections do you know where the perimeter beams can be put on top of the column yet the top can still support the rafter and perimeter beam with long side vertical (again in order to plant dowels drilled in holes in the perimeter beam doweled towards the existing wall below)? Thank you.

 

[BAretired]

If the RC parapet is a required firewall, you can't support it on an exposed steel beam. Can you simply extend the lower wall up to the top of parapet? Then run the steel beam inside the wall on top of the columns.

The rafters would likely be spaced at 1500 mm on center, is that about right? The rafters could bear on top of the perimeter beam and extend to the inside face of the parapet wall where they could laterally brace the wall if required. The perimeter beam could be a WF instead of an HSS to simplify connections.

It is a little difficult to say what should be done without knowing a bit more about the project. The parapet wall needs to be reinforced to resist wind moments. Perhaps it could be masonry instead of solid concrete, but it will need reinforcement to be continued from the wall below if that is possible.

I'm not sure why you have a sloping roof. It appears to create a drainage problem.

BA

 

[mes7a]

If the RC parapet is a required firewall, you can't support it on an exposed steel beam. Can you simply extend the lower wall up to the top of parapet? Then run the steel beam inside the wall on top of the columns.

The firewall is only 6 inches in thickness. Same size as the steel beam.

The rafters would likely be spaced at 1500 mm on center, is that about right? The rafters could bear on top of the perimeter beam and extend to the inside face of the parapet wall where they could laterally brace the wall if required. The perimeter beam could be a WF instead of an HSS to simplify connections.

No. The rafters are not spaced at 1500mm on center but 6000mm (6 meters) on center. Please see the following sketch of the roofing plan.

There are only 3 rafters (horizontal) that is supported by each row of columns (3 rows).

Is the rafters too small to cover this wide distance of 6 meters on center? The rafter as stated is composed of a rectangular hollow steel of size 250mm x 100mm x 8mm thickness with purlins across them as support for the lightweight thin metal roof (NOT heavy roof tiles). The rafter is rested on the middle columns that has shown distance of about 5.5 meters (oc). The reason the roof is sloped from center (garble) is to let rain flow to the side instead of getting stuck at the middle. Is the rafter sizes and distance sound in your experiences?

 

[BAretired]

I have not checked any beam sizes. I was just trying to understand your roof framing. The framing plan clarifies that.

My concern about supporting a firewall on a steel beam is that the steel beam is not fire rated which means the firewall is not fire rated.

It is not clear how wind on the parapet will be handled. Is it your intention to resist wind by torsion in the perimeter HSS?




BA

 

[mes7a]

I have not checked any beam sizes. I was just trying to understand your roof framing. The framing plan clarifies that.

My concern about supporting a firewall on a steel beam is that the steel beam is not fire rated which means the firewall is not fire rated.

It is not clear how wind on the parapet will be handled. Is it your intention to resist wind by torsion in the perimeter HSS?

If you will see the layout.. the rafters are barely connected to the perimeters HSS. The rafters are just connected/welded to metal plates in the columns. So to handle wind, the welder parts and metal plates and expansion bolts have to be stiff to resist wind. Why. There there be a torsion transfer material to take the wind energy?

About firewall. We just use hollow blocks with concrete inside to serve as fire wall.. which is really not enough. Eventually it can give way.. so no problem with the steel giving way too. We can't just add concrete to the parapet height because the wall below is 3 meters high already. So the beam perimeter HSS will serve as wall stiffers. Do you have any idea how to plant dowels/rebars into an I-beam.. is this done at all? The I-beams need to be drilled and dowels connected below and above it connecting the parapet wall to the wall underneath it. But it seems HSS can be drilled more easily than an I-beam.. How do you support a parapet wall using steel beam?

 

[BAretired]

Good question. How indeed.

BA

 

[mes7a]

Good question. How indeed.

How about in your constructions. When your build beams using steel (like I-beams). How do you put concrete wall over it.. unless you use steel wall too?

Also for rafters. Should they always be connected to something flexible to transfer the energy from the wind.. or can the rafters themselves be the flexible torsion sink of the wind (the rafters flexing)?

 

[BAretired]

I can't remember ever using steel beams to support concrete walls so I can't answer your question. If you need a parapet above the steel roof to serve as a continuation of the firewall, it would seem to make sense to simply continue the lower wall to the top of the parapet. The wall would need to be laterally braced at the roof level and I'm not sure you can achieve that with your lightweight plastic or metal roof.

Also for rafters. Should they always be connected to something flexible to transfer the energy from the wind.. or can the rafters themselves be the flexible torsion sink of the wind (the rafters flexing)?

Rafters may be connected to "something flexible" but there is no mandatory requirement to do so that I am aware of.. If the rafters are connected to the edge beam, they could be sized to take the wind moment from the edge beam but that cannot be done if you attach the edge beam to the face of the concrete column.

The idea of breaking the wall to accommodate an HSS is simply the wrong concept in my opinion. If the purpose of the edge beam is to provide lateral support to the wall, then put it inside the wall so that the wall can pass through to the top of parapet uninterrupted. The HSS would be carrying horizontal load from wind but the parapet would bear on the wall below and the steel beam would carry no gravity load.

BA

 

[mes7a]

Rafters may be connected to "something flexible" but there is no mandatory requirement to do so that I am aware of.. If the rafters are connected to the edge beam, they could be sized to take the wind moment from the edge beam but that cannot be done if you attach the edge beam to the face of the concrete column.

The idea of breaking the wall to accommodate an HSS is simply the wrong concept in my opinion. If the purpose of the edge beam is to provide lateral support to the wall, then put it inside the wall so that the wall can pass through to the top of parapet uninterrupted. The HSS would be carrying horizontal load from wind but the parapet would bear on the wall below and the steel beam would carry no gravity load.

We use hollow block walls infill with cement.. not concrete.. hollow blocks are not hard.. if you drop them from 6 feet.. they can break in 2. It's not pure RC wall. This is the reason we need to put a stiffer beam every 3 meters height of the wall. Why.. how high can you make your wall without any stiffer beam? Do you use pure RC wall or also hollow blocks infilled with cement? RC wall would be very heavy.. almost the same as a shear wall.

So now knowing hollow block walls have limit of 3 meters before putting another horizontal stiffer beam. Any idea what beams to use in the existing 3 meter high columns.. RC beams would be difficult because the whole columns are also poured with only 4 bolts coming out from corner as the first picture of this thread shows.

Do you think hollow block walls are qualified as firewalls? What do you use as firewall in your country? And do you know of pure metal parapet that can be made 1 meter high?

Metal roof are very common... do you usually use HSS or I-beams or trusses.. can trusses take the moment from the wind? What do you usually use as perimeter beam to support the rafters? What is the commonly use connections in your place and what objects do you use to transfer the wind load into? is it rafter or perimeter beams?

Thanks a lot.