150 feet span roof 13

[dgkhan]

I have to design a circular roof 150 feet diameter. Shall I use two way steel truss. by the way what is space frame, is it two way truss? (the roof is flat so nothing parabolic)

 

[dgkhan]

Dik,
You are proposing a basic system. Anybody can design that. I wanted two way system to cut down the 96" deep truss which you proposed. I wanted 5 feet maximum depth of trusses. That's why I floated my question to get an idea. Any ideas of designing this roof to cut down depth is appreciated. and of course I will keep at least 2% slope.
and thanks for some discouraging remarks. All I can say that I can bet anyhting that there was a day when you also knew nothing. If you can learn, I can too and that's what I am trying. This forum is for this purpose.

 

[dik]

Hokie...
The roof can slope... with the main truss or transverse to it. It can also slope using the main truss as a ridge to the perimeter or as the main truss as a valley... You can also shape the main truss to achieve any number of ways... even to have a huge downspout in the middle of the circle (if you really wanted).

The framing has the advantage of the roof deck spanning one direction only. I arbitrarily picked 3" deck because it minimised the number of smaller trusses. The main truss can be deeper to improve the efficiency. The depth of the shorter span trusses can remain the same or can be varied to be more optimum.

Connections are simple and common to nearly all steel framing. No wierd and wonderful space frame connections. A space frame connection cost would be fatal unless you want to pay for the added elegance. It's boring, but cheap.

If the money is available, then radial trusses will do, but you have an interesting connection detail at the centre and the direction of deck span will have to change with each 'pie' shape, more difficult for lateral load analysis.

Columns can be located on the circumference as required with beams framing between them, picking up the steel deck and/or shorter span trusses.

Dik

 

[hokie66]

dik,
I wasn't suggesting that your scheme doesn't allow the roof to slope...was just pointing out that "flat roof" doesn't mean flat.

 

[hokie66]

One US company specializing in spaceframes.

http://www.deltastructures.com/

 

[dhengr]

Dgkhan:
You are right, this is a good place to come to learn, and for the most part, all are welcome. Furthermore, all of us did start in this engineering business with considerably less knowledge and understanding of the subject matter than we had after 20 years of experience. But, you must also admit that intelligent questions beget more meaningful answers. And, you don’t get a very high grade for your OP, IMHO. If you had combined your several posts in your OP, you would have better indicated some of your knowledge of the subject and elicited much better answers. If you ask a dumb, incomplete question, as you did in your OP, you can expect to get some hassle about you qualifications and some fairly simple reminders about things like flat roofs shouldn’t be flat, etc. If you know how to come here, try Googling ‘space frame’ or look in a few general Structural Engineering text books, so you come here well prepared with some basic understanding of your problem. You must admit that some of us will be a bit leery about engineering qualifications when an OP’er. says they are charged with designing a 150' dia. space frame, what should I do, and their first follow up question is ‘what’s a space frame?’

There should be no shame in asking an intelligent question, and if you give us an indication of your background and level of experience we will even adjust the definition of ‘intelligent question’ to fit your background. But, don’t pull our legs by pretending to be someone you are not, or by pretending to be doing a design you have no business tackling for lack of engineering knowledge or experience, and/or without some real serious local supervision. It’s a little like asking a butcher to do open heart surgery because he’s used a sharp knife before.

 

[SAIL3]

What DIk proposed is certainly a cost effective way to go.
May not meet the 5' max dept of a truss, though.
Using full length parallel trusses would spread the fdn loads out more evenly and by fiddling with the spacing of the trusses one might get a truss with max depth of 5' to work.
A space frame, in my opinion, could be quite costly. If cost is not an issue and esthetics is of more importance, then by all means, look into a space frame.

 

[dgkhan]

so far cost is not an issue. When we finish design than client might scream , than , we will go BACK to Dik's proposal. Right now I want imagineer rather than just engineer. BTW based on your input I will hire services of one of you.
Dik, I think you are from canada.Is that right. which city are you in?

 

[dik]

Truss depths were just a WAG... may be able to pare the main one down to 72" or so... You may have difficulty getting a space frame to work with 60" depth, but with the heavier members involved you may be able to reduce the thickness.

As noted earlier... the solution isn't elegant and if to be exposed, a space frame looks a lot prettier...

Dik

 

[JAE]

dhengr, good post.

 

[BAretired]

Even with a two way truss system, a 5' depth is probably not achievable for a flat roof.

A conical shape may be something to consider using radial members, a compression ring in the middle and a tension ring on the perimeter.

BA

 

[a2mfk]

dhengr- much nicer than what I wanted to say. But this guy rubs me the wrong way so I don't feel much like helping anymore. I don't design space frames anyhow. But I sure as hell know what one is. And I would learn what one is before I asked strangers to design my roof for me. (No offense to all you strangers.)

 

[dhengr]

I agree with BA on the roof depth and the conical shape might give you the depth you need and the roof slope you want too. But, I’ll offer a tack exactly opposite his: a tension ring or tall cylinder at the center, a compression ring around the outside, and radial tension cables or members btwn. the two, makes for a good roof system. Or it could be inflated too.

 

[BAretired]

Who am I to quarrel with a four star general?

BA

 

[slickdeals]

@dhengr, I believe the Green Point Stadium in Cape Town uses the same principle.

 

[a2mfk]

Last year in Atlanta, while watching my Packers march on to the Superbowl, I got a little bored when they were up by 28 and looked up at the roof. I am by no means a stadium engineer, so I started looking and studying and trying to figure out how that thing works with all the cables and what-not... I was pretty proud that I had it more or less figured out on my own, got home and googled it, and it was one of the first compression ring and tension structures of that type ever built. The construction loading and sequencing sounded like a huge challenge. Very neat and efficient structure when completed, but one of those things where you hope they did their calcs right!!

 

[dhengr]

A2mfk:
Where did you grow up? I grew up 20mi. north of Spooner and about 25mi. west of Hayward in the little berg of Minong. I think you are pulling my leg a bit about 5' snow drifts, at least as a common event, unless you grew up in Hurley. Although they will stand with a vert. face when you cut them with a snow shovel, so they couldn’t have induced much lateral loading. In fact, they would melt away from a heated building in a few days if you didn’t remove them. I do remember going out the back door a few times to get at a snow drift which had blocked the out swinging storm/screen door which we usually used.
Sorry for highjacking this thread for a moment.

 

[hokie66]

dhengr,
a2mfk hasn't grown up yet. He's a Seminole.

 

[dgkhan]

Just for records, I did search on Google at first place and did not find anyhting explaining enough how to design a space frame or anything more which I already knew. I know that it is a multi span system and I can model it in sap, etab or staad and see for results.
But I just wanted to know some technical guidelines if available.

 

[BAretired]

It seems to me that the design of this roof should take advantage of the circular planform either by using a dome shape or an inverted dome (if that is the correct description of dhengr's suggestion). The analysis should take into account unbalanced snow loads and wind loads, both of which add considerable complication to the design.

BA

 

[JAE]

dgkhan,
Here is the La Villita Assembly Hall in San Antonio, Texas (see attached photo).

This was a circular, cable roof assembly (not flat - a natural ponding concern!!) that was constructed of an outside compression ring and an inside tension ring, radial cables were pre-tensioned by pulling down on the interior tension ring, stretching the cables. Then concrete precast planks were placed on the cables. The tension ring was released, placing the concrete in compression, which held the concave shape you see in the photo.

This was "imagineering" at its best. But I tell you, if you are in the position to design something like this based on a Google search, please tell me where this building will be constructed (I like to avoid pitfalls).