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Roof Trusses with Huge RC column supports
2

Roof Trusses with Huge RC column supports

Roof Trusses with Huge RC column supports

(OP)

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?

RE: Roof Trusses with Huge RC column supports

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

RE: Roof Trusses with Huge RC column supports

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

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RE: Roof Trusses with Huge RC column supports

Quote (OP)

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.

Quote (OP)

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 [i]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.

RE: Roof Trusses with Huge RC column supports

(OP)

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?

RE: Roof Trusses with Huge RC column supports

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.

RE: Roof Trusses with Huge RC column supports

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.

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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.

RE: Roof Trusses with Huge RC column supports

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

BA

RE: Roof Trusses with Huge RC column supports

-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.

BA

RE: Roof Trusses with Huge RC column supports

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.

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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

RE: Roof Trusses with Huge RC column supports

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

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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.

Quote:

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?


RE: Roof Trusses with Huge RC column supports

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

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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?

RE: Roof Trusses with Huge RC column supports

Good question. How indeed.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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)?

RE: Roof Trusses with Huge RC column supports

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.

Quote:

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

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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.

RE: Roof Trusses with Huge RC column supports

Hollow block walls are definitely qualified as firewalls in Canada. Vertical cells are reinforced and filled with concrete as required. If you place a bond beam at the 3m level and tie into your roof diaphragm, you will be satisfying the 3m maximum unsupported height requirement. The parapet will simply act as a cantilever carrying wind load and the weight of parapet will be supported by the wall below.

If you maintain your present roof framing arrangement, you don't need an edge beam other than a typical purlin to carry a half panel of roof deck. It would be normal practice to bolt the edge purlin to the bond beam at about 1200mm o/c.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

Hollow block walls are definitely qualified as firewalls in Canada. Vertical cells are reinforced and filled with concrete as required. If you place a bond beam at the 3m level and tie into your roof diaphragm, you will be satisfying the 3m maximum unsupported height requirement. The parapet will simply act as a cantilever carrying wind load and the weight of parapet will be supported by the wall below.

The reason a bond beam at the 3m level can't be tied into the roof diagphragm is because there must be spacing between the bond beam and roof diaphragm to serve as rain gutter as the following architectural detail shows.



Quote:

If you maintain your present roof framing arrangement, you don't need an edge beam other than a typical purlin to carry a half panel of roof deck. It would be normal practice to bolt the edge purlin to the bond beam at about 1200mm o/c.

If you will see in the roof framing plan I posted prior which i'll reproduced below (unchanged):



You will see that the columns at the lower end are not aligned but are diagonal. This is why the edge beam has to be able to carry the rafters at least at the lower part of the picture. A purlin may not take the load.. unless you mean it can take the rafter load at the left and right side where the rafter frames into the edge beam (which you suggested a purlin is enough)?

Also our firewall is only 6 inches in thickness.. so how to insert a steel inside. unless you mean very small purlin.. because the original HSS perimeter beam is also 6" being as stated before 250x150mm x 6mm thick.

Anyway. My architect said the HSS rafter at size 250mm x 100mm x 8mm thick is expensive because it is 8mm thickness and special order and suggest what if trusses instead. In your place. Is the mentioned HSS size really special order and expensive.. would trusses be cheaper instead of the rafter in my framing plan? Again noting the actual metal roof won't be touching the perimeter beam because of the rain gutter so the metal roof (which you call roof diaphragm) is just supported entirely by the rafter and purlins crossing it perpendicularly (as the framing plan above shows).

Thank you a lot.

RE: Roof Trusses with Huge RC column supports


Quote:

The reason a bond beam at the 3m level can't be tied into the roof diagphragm is because there must be spacing between the bond beam and roof diaphragm to serve as rain gutter as the following architectural detail shows.

Maybe you could use the rain gutter as a structural element to provide lateral support to the wall. Discuss with architect.

You will need an edge beam because of the offset columns at Y3 and Y5. It may be worth looking at the possibility of spacing your rafters at 2000 o/c and spanning steel deck parallel to the ridge beam. That uses more rafters but saves the cost of purlins.

HSS 250 x 150 is a standard size in Canada. HSS 250 x 100 is not standard. It is usually better to stick to shapes which are listed in the steel handbook.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

Maybe you could use the rain gutter as a structural element to provide lateral support to the wall. Discuss with architect.

You will need an edge beam because of the offset columns at Y3 and Y5. It may be worth looking at the possibility of spacing your rafters at 2000 o/c and spanning steel deck parallel to the ridge beam. That uses more rafters but saves the cost of purlins.

HSS 250 x 150 is a standard size in Canada. HSS 250 x 100 is not standard. It is usually better to stick to shapes which are listed in the steel handbook.

Our architect said HSS with thick 8mm are more expensive than I-beam. Is it true in your place HSS are generally more expensive than I-beam for the same sections or strength?

My colleague and I will redesign everything as I-beam (rafters and perimeter beam). And we will put the perimeter beams on top of the columns.. what my colleague suggested is welding the dowels of the parapet reinforcement to the I-beam perimeter beam itself. What is your comment and experience about welding 10mm dowel/rebars to the I-beam vertical to serve as parapet hollow block inner reinforcment?

RE: Roof Trusses with Huge RC column supports

I will make a few comments, as this roof structure is like Australian practice in some respects.

1) Yes, hollow sections when used as beams are generally more expensive than wide flange sections.

2) I assume the concrete columns are cantilevered, and you are not using a roof diaphragm. If your roofing is the metal sheeting, its purpose is to shed water, not to serve as a diaphragm.

3) Using concrete block for a parapet atop a steel beam makes no sense. You could use a fascia truss, fabricated from relatively light members, and clad it with a compressed fibre cement product to achieve the fire rating you want. Support the fascia truss on upstand brackets off the rafters. The fascia beams on X1 and X3 are then superfluous, except where you have the offsets at Y3 and Y5.

4) As you mentioned seismic, projecting elements like masonry parapets are one of the main offenders in loss of life. If you must have a reinforced masonry parapet, suggest you provide a concrete beam for it to start on, or else extend the reinforced masonry wall from below, and brace it at the roof level.

Not solving all your connection problems, just giving a few suggestions as to the way we would do it in Australia...

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

I will make a few comments, as this roof structure is like Australian practice in some respects.

1) Yes, hollow sections when used as beams are generally more expensive than wide flange sections.

2) I assume the concrete columns are cantilevered, and you are not using a roof diaphragm. If your roofing is the metal sheeting, its purpose is to shed water, not to serve as a diaphragm.

3) Using concrete block for a parapet atop a steel beam makes no sense. You could use a fascia truss, fabricated from relatively light members, and clad it with a compressed fibre cement product to achieve the fire rating you want. Support the fascia truss on upstand brackets off the rafters. The fascia beams on X1 and X3 are then superfluous, except where you have the offsets at Y3 and Y5.

4) As you mentioned seismic, projecting elements like masonry parapets are one of the main offenders in loss of life. If you must have a reinforced masonry parapet, suggest you provide a concrete beam for it to start on, or else extend the reinforced masonry wall from below, and brace it at the roof level.

Not solving all your connection problems, just giving a few suggestions as to the way we would do it in Australia...

Yes, the concrete columns are extended from foundation without any splice and size reduction. Yes, the light roofing purpose is to shed water.. not serve as a diaphragm. You have a point that a firewall parapet standing on exposed I-beam doesn't make sense. I can imagine the I-beam melting and the firewall parapet falling inside, yikes! So we'll design either the fascia truss.. or extend the wall from below and brace it at roof level. Brilliant idea.. yours and BaRetired. But still we need one meter or so of perimeter steel beam to support the rafters at the offset at Y3 and Y5.. If not steel beam.. how do we support the rafters at the offset? Thanks.

RE: Roof Trusses with Huge RC column supports

At Y3, you could just make the fascia truss on grid X1 strong enough to pick up the rafter. Then, at Y5, you can again either extend the whole truss to the column, or provide a beam fabricated within the truss which would extend to the column. Either way, as this member would extend beyond the roof, you would have to enclose it with cladding. When columns don't line up, you have to be a bit inventive.

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

At Y3, you could just make the fascia truss on grid X1 strong enough to pick up the rafter. Then, at Y5, you can again either extend the whole truss to the column, or provide a beam fabricated within the truss which would extend to the column. Either way, as this member would extend beyond the roof, you would have to enclose it with cladding. When columns don't line up, you have to be a bit inventive.

Do you have example of any fascia truss that is very strong? That you see in books, references or pictures. This is better than lateral support because the latter would extend the roof sheeting higher to brace the wall (if brace is below roof sheets).. I can't imagine extending the brace outside or above the roof sheets because this would create many leaks later on with many flashings to waterproof the brace extending outside/above the roof sheets.

RE: Roof Trusses with Huge RC column supports

No, you don't brace the fascia truss. The chords, top and bottom, are typically channels, web horizontal, sometimes hot rolled, sometimes cold formed, depending on the situation. The chords span between the upstands which are cantilevered from the rafter ends.

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

No, you don't brace the fascia truss. The chords, top and bottom, are typically channels, web horizontal, sometimes hot rolled, sometimes cold formed, depending on the situation. The chords span between the upstands which are cantilevered from the rafter ends.

I meant I know either to use fascia truss or brace it. We would choose fascia truss instead of bracing it. Anyway. We will calculate the fascia trusses and whether it can be tiny enough to have sufficient concrete cover. If not sufficient. Maybe better not to have a firewall parapet at all than to have one that would topple over in the event of fire (from exposed steel beam).. lol.. A lot of thanks to all the people who helped here.

RE: Roof Trusses with Huge RC column supports

I don't know what you mean by "sufficient concrete cover" on a fascia truss. There is no concrete on a fascia truss.

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

I don't know what you mean by "sufficient concrete cover" on a fascia truss. There is no concrete on a fascia truss.

Oh. I don't know where to buy "compressed fibre cement" to clad it with as you stated.. we don't have it locally.. so I plan to enclose it with sufficient concrete cover to act as fire protection. Remember in beams and columns, the rebars are protective by concrete cover. Won't this be sufficient??

Because if not sufficient (by empirical computations). We may have to ditch the whole parapet firewall. Because it can be more dangerous if it falls over from melted beam considering what's below are just thin roof sheeting for water to glow over gutter.

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

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.

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.

BAretired. I've been reading your messages over and over again the past 2 days.. about 30 times. I thought when you mentioned to put the beam inside the wall. I thought you were referring to within the wall.. that was why I was asking the beam is same size as the wall (how could the beam be inside the wall). But I realized today you were referring "inside the wall" to the other side not outside. I think my last resort is to use your solution before ditching the whole firewall idea. Because I don't have the concrete fibre cement Hokie required to clad the fascio trusses. I'd like to ask the following which I don't quite get. You said " 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". Are you saying the edge beam would not be connected to the column? Or if it would be connected at top of column. It's the same as your "that cannot be done if you attach the edge beam to the face of the concrete column".. because if welded on top of column. The edge beam can't transfer the moment to the rafter.

If you were mentioning about edge beam hanging with the wall itself. Of course the rafters can't be rested on it. When you said "they could be sized to take the wind moment from the edge beam" (I read this over 35 times already). You meant the parapet would rotate ("wind moment")? transferring this to the beam.. but the only way for this to happen would be if the parapet rested on the beam. But if it is just beside the parapet.. how can it transfer wind moment (making the beam rotate)? You also mentioned to extend the rafter up the wall as lateral support (separate idea to all this). I'd just like to explore your idea of the rafter being sized to take the wind moment from the edge beam. Just please clarify what you meant by it (like whether the beam should be welded on top of column) before I'll share the idea to our team of 5 structural designers tomorrow who will compute the members sizes and dynamics of it all. This is our last resort. Thank you.

RE: Roof Trusses with Huge RC column supports

(OP)
to continue what I was saying BaRetired.. we must not just consider wind lateral loads (which you call wind moment) but seismic loads which can sway the walls back and forth. This is the main problem if you will make it high all the way to parapet. During strong seismic movement where the ground shakes, the wall would sway to left and right.. so consider this.. noting the hollow blocks are also just weaks. Even if you can extend rafter up to brace it. The bolts may give way when seismic shaking goes to the right pulling out the connections. This is the exact reason why we need maximum wall height of 3 meters and putting stiffer below and above to avoid the walls from toppling down in seismic. So please reconsider your ideas using seismic left and right shaking to see if the parapet is still feasible. If not we will just ditch it. You are our last hope towards it. Thank you.

RE: Roof Trusses with Huge RC column supports

I don't know why you need a firewall, but if you need a firewall, it has to be either solid concrete or masonry. Steel members covered with cladding will not give you a firewall, at least not by the definition in my code.

As for seismic events, there are design provisions in the code to cover them. I tend to agree that masonry is not an ideal material if serious seismic shaking is likely to occur. Fortunately for us, we do not have that problem in Alberta so I am not too familiar with seismic design.

If the architect is calling for a masonry firewall, then it has to be continuous from bottom to top. It cannot be interrupted by a steel beam. The wall must be laterally braced at the 3m elevation. The parapet must be cantilevered from the wall below or otherwise braced to the roof structure, possibly using diagonal braces from the top of parapet to the rafters.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

I don't know why you need a firewall, but if you need a firewall, it has to be either solid concrete or masonry. Steel members covered with cladding will not give you a firewall, at least not by the definition in my code.

As for seismic events, there are design provisions in the code to cover them. I tend to agree that masonry is not an ideal material if serious seismic shaking is likely to occur. Fortunately for us, we do not have that problem in Alberta so I am not too familiar with seismic design.

If the architect is calling for a masonry firewall, then it has to be continuous from bottom to top. It cannot be interrupted by a steel beam. The wall must be laterally braced at the 3m elevation. The parapet must be cantilevered from the wall below or otherwise braced to the roof structure, possibly using diagonal braces from the top of parapet to the rafters.

It is difficult to laterally brace the wall at the 3m elevation. You are only consider wind energy going into the wall or building. What about seismic energy that can pull it to fall on the neighbor. The brace are just attached to hollow block. the block can break especially after certain seismic cyclic loading. This is the reason we always confined them inside the columns and beams (bottom and top). So even without parapet, we will need to put a perimeter beam to hold or confine the 3 meter wall below.

Anyway. I think we'll just ditch the whole firewall thing.

RE: Roof Trusses with Huge RC column supports

It is not difficult to brace the wall at the 3m elevation. Use an edge beam flush with the inside face of wall designed to resist the lateral force from the wall caused by wind or seismic action. Drainage of the rain gutter must be done without unduly weakening the edge beam.

Ditching the whole firewall thing is not your decision to make. It is the Architect's decision. Why does he want a firewall? If the building is on or near a property line, a firewall may be a code requirement.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

It is not difficult to brace the wall at the 3m elevation. Use an edge beam flush with the inside face of wall designed to resist the lateral force from the wall caused by wind or seismic action. Drainage of the rain gutter must be done without unduly weakening the edge beam.

Ditching the whole firewall thing is not your decision to make. It is the Architect's decision. Why does he want a firewall? If the building is on or near a property line, a firewall may be a code requirement.

The building is on the property line with the neighbor also on property line (almost touching each other). I'm trying to convince the architect the parapet may be unstable. The hanging column thing is unique situations that other structural engineers in the office are running out of idea. Anyway. We'll analyze your bracing idea. When you said the following which I have read 50 times already "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."

1) you said "they could be sized"... which "they" are you referring to.. the rafters or edge beam?

2) What do you mean by wind moment? how can wind have moment unless you are talking about the lateral pressure caused by the wind? Can you call it a moment? Or are you talking about torsion? But this torsion is a problem we have thought of if the parapet were welded on top of the perimeter beam as we initially envisioned. Again.. pls clarify meaning of "wind moment".

3) Are you talking about putting 5 rafters instead of 3? Because with only 3 connecting at columns. I can't see how the edge beam can affect the rafters. But 3 rafters would be sufficient. If so.. would it able to take the wind moment from the edge beam.. by whatever wind moment you meant.

4) Are you talking of putting rafter on top of edge beam.. or edge beam on top of rafter?

RE: Roof Trusses with Huge RC column supports


The building is on the property line with the neighbor also on property line (almost touching each other). I'm trying to convince the architect the parapet may be unstable. The hanging column thing is unique situations that other structural engineers in the office are running out of idea. Anyway. We'll analyze your bracing idea. When you said the following which I have read 50 times already "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."

I was referring to the situation where you were bearing the parapet on top of the edge beam, a concept which I do not recommend for reasons stated earlier. In that case, lateral forces from wind or seismic activity could only be carried by torsion in the edge beam. The rafters could be sized to take that torsional moment if they were properly connected to the edge beam. The above comment was made in response to a question which you had asked, but it is not my recommendation, so please ignore it.

1) you said "they could be sized"... which "they" are you referring to.. the rafters or edge beam? The edge beams could be sized to take the torsion and the rafters could be sized to take the torsional moment from the edge beam.

2) What do you mean by wind moment? how can wind have moment unless you are talking about the lateral pressure caused by the wind? Can you call it a moment? Or are you talking about torsion? But this torsion is a problem we have thought of if the parapet were welded on top of the perimeter beam as we initially envisioned. Again.. pls clarify meaning of "wind moment".

Wind does not have moment. Wind moment refers to the moment in a member caused by wind. If the parapet is of height h and the pressure is w, the wind moment per unit length of edge beam is wh2/2. If the rafters are spaced at s, the bending moment in the rafter is swh2/2.

3) Are you talking about putting 5 rafters instead of 3? Because with only 3 connecting at columns. I can't see how the edge beam can affect the rafters. But 3 rafters would be sufficient. If so.. would it able to take the wind moment from the edge beam.. by whatever wind moment you meant.

I was thinking of spacing the rafters at 2000 o/c, i.e. at the third points of the 6m span. The rafters could bear on top of the edge beam and the edge beam would be providing lateral support to the firewall using short stubs to fill the gap required for the roof drain.

4) Are you talking of putting rafter on top of edge beam.. or edge beam on top of rafter? See (3) above.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

I was thinking of spacing the rafters at 2000 o/c, i.e. at the third points of the 6m span. The rafters could bear on top of the edge beam and the edge beam would be providing lateral support to the firewall using short stubs to fill the gap required for the roof drain.

One rafter is so expensive.. we would retain only 3 spaced at 6000 o/c instead of 9 at 2000 o/c. So if we will retain 3. Is what you describe still valid where we will use edge beam on the wall resting on top of column. Then the rafter either at level or on top of edge beam at column? Why should it be on top if we will just use the column (except at the offset at front)

I know about extending the rafter to the vertical side of the wall at the column part and even diagonally brace it at the rafter center.

Quote:

Use an edge beam flush with the inside face of wall designed to resist the lateral force from the wall caused by wind or seismic action. Drainage of the rain gutter must be done without unduly weakening the edge beam.

Must the edge beam be a wide flange or HSS (or what material) to flush with the inside face of wall. So I imagine the rebars/dowels inside the wall welding to part of the edge beam which is located on the surface.. then this would be welded to the 3 columns (I know the parapet would bear on existing wall and not on top of edge beam). Is connecting the edge beam on the 3 columns top enough. Meaning the rafter and edge beams won't have torsional effect or transfer on each other. The rafter only providing diagonal support on top of the wall... so how rafter and edge beam are connected doesn't matter. Thanks so much.

RE: Roof Trusses with Huge RC column supports

The rafter does not have to be on top of the edge beam. They could be flush if you prefer. The rafters do not have to be HSS. They could be light wide flange or I-beam sections. Spacing rafters closer together results in more rafters, but they could be smaller members because they carry less load. In addition, they eliminate purlins by using steel deck applied directly to the rafters. Steel deck provides a competent roof diaphragm and the close spacing of rafters provides a short span for the edge beam with respect to horizontal load from the wall.

The choice of member for the edge beam depends on the spacing of rafters. Under horizontal load from the wall, the span of the edge beam is the rafter spacing. For gravity load, the span is always the distance between columns.

The rain gutter requires a gap between the wall and the edge beam, so you will need some kind of stub projecting from the edge beam and embedded in a bond beam in the wall. I am assuming that structural steel will be erected first, then masonry.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

The rafter does not have to be on top of the edge beam. They could be flush if you prefer. The rafters do not have to be HSS. They could be light wide flange or I-beam sections. Spacing rafters closer together results in more rafters, but they could be smaller members because they carry less load. In addition, they eliminate purlins by using steel deck applied directly to the rafters. Steel deck provides a competent roof diaphragm and the close spacing of rafters provides a short span for the edge beam with respect to horizontal load from the wall.

By "flush" do you mean welding the rafter and edge beam horizontally?

Steel deck are so expensive. Aren't they?

Quote:

The choice of member for the edge beam depends on the spacing of rafters. Under horizontal load from the wall, the span of the edge beam is the rafter spacing. For gravity load, the span is always the distance between columns.

The rain gutter requires a gap between the wall and the edge beam, so you will need some kind of stub projecting from the edge beam and embedded in a bond beam in the wall. I am assuming that structural steel will be erected first, then masonry.

I had meeting with another structural engineer in charge of the roof project. He said the stubs between the wall and edge beam would be exposed and would rust.. unless you mean enclosing the stub with masonry. Also he said the stubs connecting to the edge beam won't be strong. What if the welding between it and edge beam fail.. then it's like not connected.. that's why he insists the parapet must be put on top of edge beam (I-beam) cladded with concrete for strong connection. The edge beam or I-beam would be sized to handle the torsion. He said if there is fire.. there is time to run before the steel melted in the edge beam. He said this connection is stronger than stubs connecting edge beam and wall. Also what is your idea of stubs.. is it just rebars protruding outside the concrete cover of the wall or bars? I have to convince him how to enclose all of them with concrete. I told him it's no longer a firewall if it's parapet on top of steel beam encased with bit of concrete. So I'll convince him to use your idea.. but the thought of the stubs weakening is the main problem now.

Quote:

The choice of member for the edge beam depends on the spacing of rafters. Under horizontal load from the wall, the span of the edge beam is the rafter spacing. For gravity load, the span is always the distance between columns.

You said that under horizontal load from the wall, the span of the edge beam is the rafter spacing. But let's say we don't want to add more rafters. Can't the entire horizontal load from the wall be taken up by the columns themselves distant 6 meters apart.. like just welding the edge beam to the columns and the connection itself resisting the entire horizontal load of the 6 meter wall?

RE: Roof Trusses with Huge RC column supports

By "flush" do you mean welding the rafter and edge beam horizontally?

Flush means rafter and edge beam have the same top elevation. (same bottom elevation too if they are the same depth).

Steel deck are so expensive. Aren't they?

22 gauge steel deck is used extensively for roofs in Alberta. I am not current with prices as I have been retired for seven years. Check it out with local suppliers.

I had meeting with another structural engineer in charge of the roof project. He said the stubs between the wall and edge beam would be exposed and would rust..exposed steel can be protected with epoxy paint unless you mean enclosing the stub with masonry I don't think that would be necessary. Also he said the stubs connecting to the edge beam won't be strong why not?..they could be short stocky HSS sections. What if the welding between it and edge beam fail.. what if the sky falls in? then it's like not connected.. that's why he insists the parapet must be put on top of edge beam (I-beam) cladded with concrete for strong connection bad concept as I have said before. The edge beam or I-beam would be sized to handle the torsion. I-beams are lousy in torsion and he should know that. He said if there is fire.. there is time to run before the steel melted in the edge beam. How much time? A firewall is usually rated at two hours or more. He said this connection is stronger than stubs connecting edge beam and wall. He is wrong. Also what is your idea of stubs.. is it just rebars protruding outside the concrete cover of the wall or bars? I have to convince him how to enclose all of them with concrete. I told him it's no longer a firewall if it's parapet on top of steel beam encased with bit of concrete. So I'll convince him to use your idea.. but the thought of the stubs weakening is the main problem now. The stubs could be HSS 75x75x6 welded to the edge beam with a plate anchored into the bond beam of the block wall.

You said that under horizontal load from the wall, the span of the edge beam is the rafter spacing. But let's say we don't want to add more rafters. Can't the entire horizontal load from the wall be taken up by the columns themselves distant 6 meters apart.. like just welding the edge beam to the columns and the connection itself resisting the entire horizontal load of the 6 meter wall? Yes, this can be done but your edge beam would have to be designed to span 6m with wall load applied horizontally unless the steel deck performs as a diaphragm.

As a final comment, let me say I do not wish to get into an extended argument with your engineer. If he feels strongly about his concept then I would suggest you ignore all that I have said in this thread and go with him. Forcing ones ideas on an unwilling recipient is likely to have disastrous results, particularly when he is in control of design. I have tried to tell you in general terms how I believe the roof should be constructed but there are many details which have to be carefully sorted out and I have no intention of becoming more intimately involved.

Goodbye and good luck.

BA

RE: Roof Trusses with Huge RC column supports

(OP)

Thanks so much BA and for emphasizing the torsions if the wall would be put on top of the I-beams. As for bracings. It's very sophisticated connections. The engineers are analyzing them as they haven't done it before in their life. They are just beginners anyway early 20s and just learning (you are mid 70s so experience is the great teacher I know).

If we are not confident with either (lacking the experiences). Then maybe we'll just put RC beams by chiseling the column.. or use special lightweight panel boards that I started a thread to ask about the experiences of other structure engineers.. Again thanks so much. You mustn't be retired :)

RE: Roof Trusses with Huge RC column supports

(OP)

Quote:

Wind does not have moment. Wind moment refers to the moment in a member caused by wind. If the parapet is of height h and the pressure is w, the wind moment per unit length of edge beam is wh^2/2. If the rafters are spaced at s, the bending moment in the rafter is swh^2/2.

Rereading your messages. I just realized/remembered wh^2/2 as the formula for the edge of a cantilever beam. And suddenly I realized the parapet can act like a cantilever beam during a hurricane where the wind is like the load. Since the original plan has no rafter to take the moment, then it is the welding at the plates that would take the moment spaced at 6 meters. My gosh.. this can almost break the welding and make the parapet fall inward. Thanks specially so much for this BA. Our options would be to brace the wall or if this thing is too complicated for the beginner team. We'd just design concrete beam tied to the column that would transfer the torsion to the beam.. or not confident about the rebars integrity after chiseling the oversize column.. we may just no longer build the parapet anymore. I thought a while ago of using very lightweight panel as parapet but in hurricane.. if w is large.. and the connections strong. it may shear the welding too. But at least the lightweight panel won't crush thru the roof.. but in hurricane.. it can act like projectile to the street. So it seems our choice to build a parapet would be either concrete beam or your bracing only. Without this thread. I couldn't have realized all the scenerios. Thanks a million BA! :)

RE: Roof Trusses with Huge RC column supports

(OP)
Hokie said:

Quote:

I don't know what you mean by "sufficient concrete cover" on a fascia truss. There is no concrete on a fascia truss.



Before BA told me I-beam is lousy on torsion. I told the designer to use I-beam as it's cheaper (after he said he couldn't think how to make your fascia truss enclosed with concrete). Instead what he produced was the above.. it has concrete around I-beam that even use stirrups. The longitudinal bars were drilled into the column.

Anyway. For such complicated beam. I'd rather just chisel the top of the column and use typical beam. The team doesn't have the knowledge to apply BA sophisticated bracing technique, although they will learn slowly from the concept. But I'm afraid chiseling it may weaken the bars and considering so much torsion is transferred to the column, the joint may break apart if the rebars give way.

My gosh. Just one adding one meter of parapet is so complicated, isn't it.

I told the architect it is so complicated. She said it is ok not to make the parapet. So the best thing is not to make it anymore... unless we can hire BA to come over as consultant for the project.. but he is retired.

Anyway thanks for all the tips and enlightening thoughts.

RE: Roof Trusses with Huge RC column supports

If the parapet is not required, then I agree that the best thing to do is to get rid of it.

BA

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