Light Framing Truss in Parts ? Lateral Load transfer mechanism? 1

[CFS339]

Dear All,

Is it a correct design approach? i.e.

There is a cold formed steel structure 30m x 33m. 30 meter truss is provided but the truss is supported at every 6 meters with a lattice columns. As bending moment should be avoid on the Cold formed steel columns so can I convert 30 Meters truss into 5 parts, each part individually supported on two columns and for vertical loads at as a simply supported truss ( Truss parts are connected just to columns at the base but not to each other). But while Analyzing it for Lateral loads , CAN we assume that these side by side parts of the truss are gonna transfer lateral load to the adjacent truss part.Please be noted that these truss parts are just kept side by side and only connected from the bottom track to the top of lattice columns. And act as a complete Truss assembly? What do u suggest is it a correct approach?

Regards

RWB

 

[hokie66]

Some sketches or drawings might help to explain your question, as I don't think anyone here will be able to decipher your description.

 

[CFS339]

Thanks Hokie!

Please do find the attached Image.Total Span of Truss is 30m But truss is divided into 5 (6meter each) simply supported parts. All intermediate columns are double each supporting the one truss truss part. The truss is connected to the bottom of the column only. Hope now u can imagine the scenario? I am asking whether it can be wise to analyize the frame as a whole to resist the lateral load coming from wind?

 

[CFS339]

Ridge Height of truss : 1.9 Meters
Eave Height: 0.4 meters

Height of Lattice Columns : 2.4 meters
Please do ask if u still need any sketch/point to understand the question.

 

[Once20036]

Based on the sketch you posted, it appears that you have a continuous top chord. This means that you`ll be developing positive moment at midspan and negative moment over the supports. If the tops of the columns are "pinned" you won't be transferring moment into the columns, but you`ll still have a continuous flexural member.
In order to create the simple span condition that you're describing, the top chord would need to be broken over each support, but that doesn't appear to be the case. Under lateral loads, I would agree that this continuous truss will transfer loads to each of the columns.

If the top chords are discontinuous over the supports, you`ll need to modify your model and put an axial release at the last portion of each top chord member. Whether or not this will transfer lateral loads to all of the columns gets more tricky. Is there a roof diaphragm that can transfer the loads? Can the lateral loads get out of the roof and down into the bottom chords? Are the bottom chords continuous over the supports?

 

[CFS339]

Dear Once,

Actually I have broken both Bottom and upper chord of the truss. Actually What I am thinking to do is to make Moments at the supports zero but at the same time I want to transfer Lateral Loads (Axial) from one part of the truss to the other i.e. for lateral load I want it to act as a single frame , may be by connecting Flanges of the top and bottom chords of truss parts by tension strap? Please do find the attached DXF file of the model for more details. This structure is representing the centerline model for the cold formed steel structure. Actually I want to make this Frame structure alone as a stable structure against Lateral and Gravity loads so that I should no provide Shear Walls.The ridge of the truss is quite high 1.8 meters so I have run the Joist beams in the longitudinal Direction to resist only lateral loads.
Need you comments and suggestions.
Regards

 

[CFS339]

or Frame will only take lateral loads if it is made Moments resisting frames i.e ( not simply supported connections between Truss and Columns?

 

[hokie66]

If it is not a moment frame, the columns have to take the lateral load as cantilevers. Is that what you want to try to accomplish? What takes the lateral load in the other direction?

 

[CFS339]

Dear Hokie,

What All I need to get a structure stable under the Lateral as well gravity loads in both direction. The building is of cold formed steel structure.Please have look at link (3D model what im designing) in my 3rd last post.And suggest me how to make a laterally stable structure? Since the height of the truss is quite big so I think I have to laterally support the truss in the other direction as well so I provided Joist beam of varying depths in the other direction (can be seen in model). In short need your suggestions/advice/tips to do this structure with cold formed steel.

Regards

 

[hokie66]

I can't open that file. In order to make your structure laterally stable in a global sense, you have three options. They are braced frame, moment frame, or cantilevered columns. Shear walls are a variation of the cantilevered column approach. Cold formed steel may not be the best option for an open structure of this size.

 

[CFS339]

thanks Hokie66 for you response.

Actually the file attached above is an AutoCAD file so you may have to right click on the file and click " open with" autocad. Hope it will open now.Than apply ("z"-"space bar"-"a"-"space bar") command to bring it on a model screen. Than convert it into 3D model from the autocad settings and then rotate it.Hope it will work now.Since It is opening with me.
The structure is not an open structure. The structure I have shown to you is MWFRS and it will have partition walls on all the four sides.According to you it means, in my sturture actually cantilever lattice columns are taking the wind loads which is very minor approximately 0.7 psf (13KN Point load).

 

[gobsmacked]

I haven't understood whether the lateral load you speak of is in the plane of the trusses (ie left to right, and vice versa) or do you mean at right angles to the plane of the truss (going into or out of the paper)? It seems you could have lateral loads in both directions. Therefore the frame drawing you need to provide is a plan of the whole thing, and elevations of wall bracing at the edges, or equivalent moment joints, showing how the lateral loads are eventually transferred from roof level to foundation level.

 

[hokie66]

CFS339,

I don't have AutoCAD, so can't open your file.

0.7 psf wind load is much too low for any location on Earth. 10 to 15 psf would be the norm in areas without high winds. With an enclosed structure, you must take the load on the external faces into the structure, probably by spanning from the ground to the roof structure, then by a system of bracing or diaphragm action within the roof structure, transferring these forces to the vertical resisting elements, and to the ground. And don't forget the wind loads on the roof.

Your descriptions and use of the nomenclature suggests to me that you need an experienced structural engineer as a mentor.

 

[CFS339]

Dear Hokie,

Sorry I was supposed to write 0.7 KPA ( 15 psf).And you are right I strongly need a Senior Structural Engineer mentor but unfortunately at moment that is not possible. I have to clarify my doubts somewhere online.
Let me try to explain it morely clearly with more snapshots.
1)

http://files.engineering.com/getfile.aspx?folder=6466210b-10d5-4c20-832e-2cdf1e5f6104&file=S3.jpg

This is how the whole MWFRS system looks like but this structures is cladded with LGS walls and roof panels so it is closed structure.
2) I have calculated the wind load according to ASCE 07-10 which comes out to be maximum 15 Psf (0.7 KPA) and multiple it with the tributary width (6 meters) of each the each lattice column of the outer side and applied it as a line load. But I have analyzed only one truss with maximum load. When the load is applied in transverse direction the lateral load will be resisted by these individual trussed frames. i.e

http://files.engineering.com/getfile.aspx?folder=3f45b141-ef03-404d-a3bd-4ad0d2c2c13a&file=S8.jpg

3) When the wind load is applied from longitudinal side this frame will resist.

http://files.engineering.com/getfile.aspx?folder=34e38907-11c6-487e-a547-7dfc56817707&file=S9.jpg

some of the more pics to understand the intersection of those truss and lattice beams resting upon column.

http://files.engineering.com/getfile.aspx?folder=0d5cd85e-ea1b-4d00-8c78-cf77fc24835a&file=S2.jpg

http://files.engineering.com/getfile.aspx?folder=535d7d32-89bf-4818-9cd6-d5114ad1cccc&file=S4.jpg

Must look this pic

http://files.engineering.com/getfile.aspx?folder=c678629c-8a50-412b-9439-6eaae5826d28&file=s6.jpg

http://files.engineering.com/getfile.aspx?folder=c8d919ea-e7ab-41d5-a307-2201f8bef11b&file=S7.jpg

The lateral forces at the roof level will be transferred to the lattice columns and then to ground.
I hope now i would be able to explain my scenario clearly and now need you comments/advice.

Regards

 

[CFS339]

Yes Gobsmacked,

the lateral load is comming from both side now have a look at the above post to have more clear understanding.

Regards

 

[hokie66]

OK, that gives me a much better picture of what you are doing. Back to your main question...without diagonal bracing in the vertical planes, or else some form of shear walls, you have to resist the lateral loading by moment action. This can conceivably be done by cantilevering the columns off the footings, but a much more practical and conventional approach is to make the connections in the superstructure moment resisting. Therefore, I would say your approach of connecting the trusses only at the bottom chord is not satisfactory. Why did you say that moment in the columns needs to be avoided?

 

[CFS339]

Dear Hokie,

First of all let me tell about the two restriction I have:

1) First of all we cannot transport such a huge truss from our factory to the site. It means we have to manufacture in the parts. Maximum 12 meters part.
2)Now It means we have two options either we connect truss parts exactly above the columns ( on site) or somewhere in the mid unsupported span. As I am not confident about our site erection team so if I dnt want to ask them connect those parts somewhere in between in the columns.
3) I want to avoid moment on the columns as cold formed steel cannot resist moments and local or distortional buckling can occur which means Member ( Stud) can fail even before reach yield stress.

So wht do u suggest in this case?

Thanks in advance

 

[CFS339]

Wht do suggest if i connect the truss parts with Tension strap on the flanges or on the web as shown in the attached pic rather than connecting them with screws i.e (center back to back studs with screws) ?

 

[hokie66]

I fail to understand why you say that the cold formed members of the columns cannot resist moments, when that is exactly what the trusses have to do.

As you are out of your depth with this design, I will not attempt any further assistance. Online forums cannot substitute for capable engineering.

 

[CFS339]

Dear Hokie,

I am very thankful for your really precious time. Please have a look at the PDF attached may be you understand what i am trying to say.
Since Cold formed steel is quite different from Hot rolled steel.And according to what I have read in research papers/ reports , the moment should not be transferred to cold formed steel studs (columns) because they are very thin (0.5mm to 1.5mm thick steel sheets) and always buckle before reaching their yield stress level.
Or if you still think I need to clarify my structural concepts I would be happy to read any suggested data (since at present, i dnt have access of any senior structural engineer)
Once again thanks for trying to help me.


Regards