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]

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

 

[mes7a]

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?

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.

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?

 

[BAretired]

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

 

[mes7a]

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

 

[mes7a]

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!

 

[mes7a]

Hokie said:

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.

 

[BAretired]

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

BA