Vierendeel HSS Truss Design 3

[asixth]

Hi guys

I am designing a vierendeel HSS truss fabricated from 5" x 3/16' square HSS sections (125 x 4 SHS). I am looking for a good reference that goes through the design of HSS joints which are to transfer moments.

Does anyone know of any good design examples in textbooks or codes (material written to any national design code will be fine). Likewise does anyone have there own procedure or tips for this form of design.

All help will be appreciated.

 

[WillisV]

HSS Moment connections are covered in AISC 2005 K.3.

 

[connectegr]

There are also some examples in the AISC Hollow Structural Sections - Connections Manual.

http://www.FerrellEngineering.com

 

[BAretired]

"Hollow Structural Section Connections and Trusses" by J.A. Packer and J.E. Henderson is a good reference.

I remember working for a fabricator a few years ago where the engineer of record required that all connections were to be designed for the full capacity of the members being connected. They required the fabricator's engineer (me) to stamp the drawings.

I pointed out that it was not possible to do that without cutting the chords and introducing welded plate stiffeners inside the chords, aligned with the walls of the web members.

Amazingly, that was what they opted to do. This made the trusses very expensive indeed. I thought then and still believe it was a very dumb idea, but we did it and everyone was happy...everyone except the fabricator, that is.

BA

 

[Ussuri]

The following link is to a Corus publication, which is based on the CIDECT approach adopted by the Eurocodes.

http://www.corusgroup.com/file_sour...tion/Tubes/CT16-SHS Welded Joints 9-08-05.pdf

 

[dik]

BAR...
I typically design members for approx 70-80% just to facilitate joint design...

Jeff Packer's book is excellent, IMHO...

Dik

 

[LOKSTR]

Asixth ,

There are bunch of design guides published by CIDECT.
In US and Canada, most of the publications are based around these guidelines.
Here is the link for that:

http://cidect.org/en/Publications/

 

[asixth]

Thanks for all the great references. I will need to visit my old university this weekend to see if I can find a copy of the Packer book.

I noticed the publication posted by Ussari is very similar to the Australian Steel Institutes publication on design of HSS connections for mitred knee joint connections. This seems to be the most applicable to my situation.

What I was concerned about is whether the flange forces from the vertical member that I have detailed in my sketch can be transferred from the vertical member to the horizontal flanges without any stiffening of the flanges. This would be similar to I-beam moment connection without web stiffener plates, it doesn't provide a direct load path.

The flange stresses for the vertical member from this moment must transferred to the web of the vertical so it can be transferred through the joint zone to flanges of the horizontals. Even a small moment transfer of 10kN-m (7.3kip-ft) generates a flange force of 65kN (14.8 kips) which is quite significant.

Does anyone have a typical detail they use for stiffening the joints for moment transfer?

 

[Ussuri]

Typical stiffening details would either be face reinforcement if chord face deformation was your failure mechanism or side wall reinforcement if it was side wall failure.

The type of failure mechanism is dependent on the geometry of the brace and the chord. For instance if your 150mm brace is the same section as the chord then the load is transferred through the side walls. If the brace is smaller than the chord then the chord face has to transfer the load.

There are some other publications by corus that accompany that. I'll see if I can locate them.

 

[Ussuri]

Worked examples using the Corus approach:

http://www.corusgroup.com/file_sour...bes/CT-SHS Joint Design Examples 15-02-06.pdf

Supplements to the original guide containing a few additional joint configurations:

http://www.corusgroup.com/file_sour...d joints Supplements - No 1 to 6 10-08-05.pdf

Info on welding for SHS joints:

http://www.corusgroup.com/file_source/StaticFiles/CT15-SHS Welding 09-05-05.pdf

Selection of different types of joints, with details.

http://www.corusgroup.com/file_sour...ts/Corus_Tubes/CT46-SHS Jointing 14-06-06.pdf

 

[apsix]

I use the ASI guide, Section 11; Y & T connections, but convert the moment to 2 sets of axial load, compression and tension.
I do this by obtaining the total stress (due to axial & bending) acting on one flange of the section and factoring it up to use as a pure axial design force for the whole section.

 

[asixth]

Thanks everyone for their help so far. I am not happy with the current design that I have. How would I go about strengthening a square hollow section T-connection to transfer moment?

I saw a post recently where plates were welded to the sides of the section to help transfer the stresses. thread507-269475 Would it be feasible to increase the section thickness for the horizontal members to help stiffen the connection.

 

[BAretired]

asixth,

Here are a couple of ways (see attached).



BA

 

[Ussuri]

Side wall plates would only help if its is side wall buckling you are trying to prevent. This is usually a concern when the walls of the SHS for both brace and chord are the same width. Otherwise you would be looking at something such as BAretired suggested.

My first step would usually be to increase the wall thickness of the chord and see if that helps. You can reinforce the joint but it adds to the cost of fabrication and can look unsightly. Extra weight of steel might be cheaper and easier.

 

[asixth]

Thanks BA

Is that taken from the packer book?

 

[Bobber1]

The tension chord splice, if there is one, will be fun too. I trust you don't have cyclical loadings.

 

[BAretired]

asixth,

From the Packer book, yes.

BA

 

[connectegr]

Panel points with gusset plates can provide additional assembly tolerances. Gussets can be easily design for local axial forces, moments, or couple forces. See attached

I also provided a splice detail. Using a cutting table a continuous backing bar can be cut from 1" plate. Depending on the prep the splice can have adjustment up to 7/16"

http://www.FerrellEngineering.com

 

[asixth]

Bobber1

There is no tension chord splice and the function of the vierendeel truss is to resist wind loads and gravity loads.

connectegr

Thanks for the detail. I will increase the wall thickness to avoid reinforcing the joint. A thicker section will increase material but this should be offset by the easier fabrication.

ussari

The Corus manual "Design of SHS Joints" and the accompanying design examples is a great reference. It steps through the design process with illustrations of the failure mechanisms which must be checked. I have basically extracted everything which is relevant to the design of equal width RHS T-joint connections subject to in-plane moments and axial loads and have programmed it into a spreadsheet. There is still a little bit of work which needs doing in the spreadsheet to convert from Eurocodes to Australian Codes but it is basically right. The design which I was proposing (125x4mm or 5x3/16") was cutting it a bit fine with chord sidewall buckling and bracing effective width so I am going to increase the wall thickness to 6mm (1/4").

Thanks to everyone who contributed.

 

[Ussuri]

Just to point out, the corus document is a limit state design. Your loadings need to be factored accordingly.