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150 feet span roof
13

150 feet span roof

150 feet span roof

(OP)
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)

RE: 150 feet span roof

Do you need a space frame?  Can you use a clear span truss at the diametre and then frame into it with smaller trusses (7 or 8 different truss lengths) at say 10' centres (using 3" roof deck). The main truss panels can be spaced to match the smaller trusses.

Dik

RE: 150 feet span roof

I would consider radial trusses, all moment connected to a central steel cylinder of the same depth as the trusses.

Mike McCann
MMC Engineering

 

RE: 150 feet span roof

Agree with Mike.  Also...don't build a flat roof!  You can slope the trusses or specify tapered insulation on a flat deck, but DO NOT let them build a flat roof.  The ponding probability, deflection and progressive ponding collapse are all high...and a round roof section doesn't help things!

RE: 150 feet span roof

That's likely the pricey way to do it, and the way to do it if it's exposed and he needs a space frame.

Dik

RE: 150 feet span roof

...and, don't build it flat as Ron noted.

Dik

RE: 150 feet span roof

(OP)
what is a space frame

RE: 150 feet span roof

It's a frame in space - three dimensional - that would cover the whole area of the top of the dimension.  Spans all directions.  If you are looking for a light roof, this would be the way to go.

Mike McCann
MMC Engineering

 

RE: 150 feet span roof

To elaborate a bit, space frames are usually two way trusses as you suggested in your first post.  The top and bottom chords are offset, so the whole roof is formed by a maze of triangles.  There are a number of proprietary space frame manufacturers.  Most use circular hollow sections for all members.  The joint design is the tricky part, and again, there are quite a few proprietary solutions, the most elegant of which are also the most costly.

What do you or your architect intend as the roofing material?  The selection of a structural system would depend to a degree on how the roof slopes, the amount of secondary members required, etc.

RE: 150 feet span roof

Unistrut makes classical space frames.

Mike McCann
MMC Engineering

 

RE: 150 feet span roof

4
Do we want someone designing space frames spanning a 150ft circle if they don't know what one is?

I followed this whole post and don't really want a piece of that design. Maybe if you guys were all in my office.

RE: 150 feet span roof

a2mfk, you said what I was thinking, too.   

RE: 150 feet span roof

No, we don't want someone designing a spaceframe if he doesn't know what it is.  But he wouldn't be actually designing a proprietary spaceframe, so hopefully the vendors would know what they are doing.  Hopefully again, the OP would be able to incorporate a proprietary spaceframe into a building...if not, he probably can't design the roof at all.

RE: 150 feet span roof

Get some help - I agree with the above posts.

Have done things like this using three or four "main" trusses running parallel to a diameter.  Cuts down on the total "central" truss load.  Obviously they are different lengths.

RE: 150 feet span roof

Everyone thats an expert at designing space frames started with their first one. I think he is reaching out for help, but I agree OP should do more research before coming here. Pretty aggressive span too, would like to see final design

RE: 150 feet span roof

As pointed out, definitely not a flat roof.
Radial trusses from a central ring is generally an elegant solution. The only drawback with this concept is that it depends solely on the integrity of the central ring and it's conn to the radial trusses. If you are confident of quality fabrication/erection and effective maintenance, then it would be a viable option. Unfortunately, in these low-tech projects, one or all of these requiements may be lacking.
With that in mind, I believe I would tend to go with the parallel trusses as it results in a more redundant and robust structure and less sensitive to the potential shortcommings mentioned above.

RE: 150 feet span roof

dik- seems like that would work just fine. KISS. And outside the main girder the other joists could be pretty typical long-span joists.

RE: 150 feet span roof

It's probably the least costly way to do it... not very elegant.

Dik

RE: 150 feet span roof

When we talk about a "flat" roof, nobody really means flat...I hope.  Selection of the system should depend on the roofing system to be used.  Where I am, steel roofing would be used on about a 2 to 5 degree slope, supported on steel purlins.  Dik's structural system would work, but I would prefer more of a two-way load distribution.

RE: 150 feet span roof

(OP)
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.

RE: 150 feet span roof

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

RE: 150 feet span roof

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.

RE: 150 feet span roof

5
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.

RE: 150 feet span roof

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.  

RE: 150 feet span roof

(OP)
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?

RE: 150 feet span roof

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

RE: 150 feet span roof

dhengr, good post.

RE: 150 feet span roof

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

RE: 150 feet span roof

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.)

RE: 150 feet span roof

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.

RE: 150 feet span roof

Who am I to quarrel with a four star general?

BA

RE: 150 feet span roof

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

RE: 150 feet span roof

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!!

RE: 150 feet span roof

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.
 

RE: 150 feet span roof

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

RE: 150 feet span roof

(OP)
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.
 

RE: 150 feet span roof

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

RE: 150 feet span roof

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).

 

RE: 150 feet span roof

(OP)
JAE.
this roof has a downward slope so good candidate for ponding in my opinion. so I will avoid this approach. And do not worry, the design of this roof will be ok.If I do  not feel comfortable, I will hire someone.

RE: 150 feet span roof

(OP)
thanks zoparrat. It is the same concept other pointed out before with tension ring in center and radial trusses. Personally I think space truss will be more elegant. I only need to know how to design it. Do you have some reference available just like you posted for cycle wheel roof.

RE: 150 feet span roof

Why don't you visit the library at the nearest university and read up on the design of space trusses?  You cannot expect a crash course on a forum such as this.  Space trusses are designed using the same principles as any truss, namely statics but you first must decide on the shape of the space truss.

BA

RE: 150 feet span roof

BA....   What's a liberry?  I got my Ph.D. from Google and the internet, just fine, and I'm incensed that all you smart guys can't just give me the formula for space frames, then leave me alone, and quit telling me I should be giving this crazy idea more thought.  Do I get any extra credit for persistence, albeit a fairly naive insistence that even if I don't know what a space frame is, I will know how to design one, and I will know if-and-when I am uncomfortable designing an elegant space frame spanning 150'.  Next thing you know, you will be telling me to study a couple of the failures of these kinds of roofs, all done by engineers much smarter on the subject, than I am.  I'm going to Toys-R-Us tomorrow to see if they don't have a space frame or space invaders or spaced-out kit I can buy, I'll show you guys.

Dgkhan....  Wake up, this is serious business, not likely learned from an internet link or a twitter from a twit you don't even know or who may have no more experience than you do on the matter.  You have gotten some good advice and suggestions, now get a life, get some experience on this kind of structure under the guidance of someone who knows what he/she is doing, because you obviously don't.  Right now you and your thinking are dangerous, wherever you are.  

RE: 150 feet span roof

dgkhan,
If you are still considering designing a spaceframe on your own, I suggest you read up on the collapse of the Hartford Civic Center in 1978.  Better to know what not to do before you start learning what to do.  But for a 150' circular roof, a proprietary system would be my choice.  It's just a commodity item.  By the way, the Hartford Center is discussed in Levy and Salvadori's book "Why Buildings Fall Down".  You will find several of the other systems suggested above there as well.

RE: 150 feet span roof

(OP)
Thanks hokie66. As I said in my posts before that I need some exact book name / reference which no body could give. So I will contact the technical support of Bentley and CSI etc and also try to contact
my decade old teachers at Princeton, NJ if they are still there.
To all of you who have spend time in writing mocking comments, you can clearly read in my post that I will hire someone for help if I could not do it. Anybody interested and confident that she/he can do it multiply it's salary x 2 and I can hire.As a head of my company I also like to learn as mush as I can which created so much ado.

 

RE: 150 feet span roof

Take your own advice, if you are a real person and actually performing some type of engineering services, and hire someone to do this for you. You are way too wealthy and smart to bothered with a space frame.

And read this article, you may have a serious case of the Dunning-Kruger effect going on right now.

RE: 150 feet span roof

My apologies for being late to this online version of a flogging ;)

What is sad and scary is that the OP is probably some offshore engineer doing outsourced work for North American projects.  


dhengr – I just promoted you to a 5-star general and a2mfk to a 4-star.
 

RE: 150 feet span roof

@RacingAZ:
Not right. Just because some person is named "khan", you should not assume that he is an offshore engineer doing outsourced work.

RE: 150 feet span roof

@slick:
Not right. You're the one on the wrong here for assuming I did.
I'm not that stupid to base my hunch on an online handle.

I was curious that someone didn't know what a space frame is and yet is attempting to design a 150 ft. diameter roof. So I actually looked at some of his previous posts that led me to believe that he is one of those engineers.   
   

RE: 150 feet span roof

@RacingAZ: My apologies. I am a little sensitive when it comes to stereotyping. I am from India and lately, there has been a little too much immigrant bashing.

There are competent engineers in all parts of the world. I do understand what you are trying to imply by "sad and scary" when it comes to work being outsourced. A lot can get lost in translation. Let's hope its not one of those cases.

RE: 150 feet span roof

There should be no place on this forum for immigrant bashing.  But at the same time, the OP sort of set himself up for criticism by the way he phrased his question.  A person's ethnic background should have nothing to do with the responses he receives on this or any other forum.

BA

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