www.sefindia.org

STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

 Forum SubscriptionsSubscriptions DigestDigest Preferences   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups  RegisterRegister FAQSecurity Tips FAQDonate
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log in to websiteLog in to websiteLog in to websiteLog in to forum 
Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.

Analysis of trusses
Goto page 1, 2  Next
 
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion
View previous topic :: View next topic  
Author Message
vikram.jeet
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 2212

PostPosted: Mon Oct 29, 2012 12:06 pm    Post subject: Analysis of trusses Reply with quote

A] If loads are not acting at Joints

The truss members are subject to bending as well as axial forces combined and truss
will be costlier.

Analysis purpose
Manual - - transfer all loads to the joints based on their postion in panel.
Analyse the truss with nodal forces. Get axial forces in members .Add
the effect of bending assuming span of panel. Check the member for combined
bending and axial stresses.

B] If Joints are welded jointsi

Truss joints needs to be PIN jointed in ideal scenario.
In practice Riveted/bolted/welded joints are provided. Riveted and bolted joints are  
near to pin condition. Welded joints are rigid and thus some bending due to joint
rigidity cause about 10% higher requirements in welded trusses.but deflection  
of welded trusses is least.

On staad please try the analysis on two models - - - one with command STAAD TRUSS
and other model with command STAAD SPACE and see the difference.

best regds

vikramjeet



Dear sir, Trusses are supposed to take loads at their joints and joints are assumed to be pin connection. But, if loads are not acting at joints and joints have been done using weld connections then how it should be idealized for analysis purpose. Regards, Vijay kumar.
-- ญญ

Posted via Email
Back to top
View user's profile Send private message
Manoharbs_eq
General Sponsor
General Sponsor


Joined: 17 Jul 2012
Posts: 422

PostPosted: Tue Oct 30, 2012 4:04 am    Post subject: Analysis of trusses Reply with quote

Dear vikram,
IF we analyze truss  in staad with space then member demand will be high we need bigger and heavier sections.
It is always better to design truss members in truss module.




Rgds
Manohar







On Mon, Oct 29, 2012 at 7:42 PM, vikram.jeet <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           A] If loads are not acting at Joints

The truss members are subject to bending as well as axial forces combined and truss
will be costlier.

Analysis purpose
Manual - - transfer all loads to the joints based on their postion in panel.
Analyse the truss with nodal forces. Get axial forces in members .Add
the effect of bending assuming span of panel. Check the member for combined
bending and axial stresses.

B] If Joints are welded jointsi

Truss joints needs to be PIN jointed in ideal scenario.
In practice Riveted/bolted/welded joints are provided. Riveted and bolted joints are
near to pin condition. Welded joints are rigid and thus some bending due to joint
rigidity cause about 10% higher requirements in welded trusses.but deflection
of welded trusses is least.

On staad please try the analysis on two models - - - one with command STAAD TRUSS
and other model with command STAAD SPACE and see the difference.

best regds

vikramjeet



Dear sir, Trusses are supposed to take loads at their joints and joints are assumed to be pin connection. But, if loads are not acting at joints and joints have been done using weld connections then how it should be idealized for analysis purpose. Regards, Vijay kumar.
-- ญญ
     



     



Posted via Email
Back to top
View user's profile Send private message
mtamil
General Sponsor
General Sponsor


Joined: 07 Apr 2011
Posts: 187

PostPosted: Tue Oct 30, 2012 5:00 am    Post subject: Reply with quote

Pinned connections seldom exist in truss structures, unless special/true pinned connections are provided.

Now-a-days trusses are fabricated using welds and bolted truss are rare in India.

Welding truss members with gussets may not be assumed as pinned connections and the case is same in connections with 2 or more bolts.

The top & bot. chords of truss girder and Rafter & tie members of roof truss are generally fabricated using rolled sections which are continuous over the vertical/diagonals.

SP 38 also considered truss joints as rigid.

I think, truss joints are to be considered as rigid and due small moments shall be considered in the design.

Regards
Tamilarasan
Back to top
View user's profile Send private message
vikram.jeet
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 2212

PostPosted: Tue Oct 30, 2012 6:37 am    Post subject: Analysis of trusses Reply with quote

Yes - - - - As stated in earlier post ,Truss with rigid (welded) joints can be  
designed on STAAD SPACE model It will require about 10-15% higher
requirements due to joint rigidity.

Bolted and riveted trusses may have some play in holes and provide  
comparatively flexible joints and can be designed on STAAD TRUSS model

Deflection will be lesser in welded truss model- - but natural.

best regds

vikramjeet



Dear vikram,IF we analyze truss in staad with space then member demand will be high we need bigger and heavier sections.It is always better to design truss members in truss module.RgdsManohar
-- ญญ

Posted via Email
Back to top
View user's profile Send private message
sarfaraj.husain
...
...


Joined: 26 Jan 2003
Posts: 90

PostPosted: Tue Oct 30, 2012 9:52 am    Post subject: Analysis of trusses Reply with quote

@ Mr. Manohar

if we consider practical problems .......at site sometime it is very difficult to connect purlins at panel points ...in that case purlin connects to rafter.....so bending occurs in top chords....in that case defining as "truss" in staad may not be true.....( unless one can assure to site for proper connection)...........
but bottom chords & verticals can be defined as truss in space frame..........


sarfraj





From: "vikram.jeet" <forum@sefindia.org>
To: general@sefindia.org,  
Date: 10/30/12 03:02 PM
Subject: [SEFI] Re: Analysis of trusses



Yes - - - - As stated in earlier post ,Truss with rigid (welded) joints can be
designed on STAAD SPACE model It will require about 10-15% higher
requirements due to joint rigidity.

Bolted and riveted trusses may have some play in holes and provide
comparatively flexible joints and can be designed on STAAD TRUSS model

Deflection will be lesser in welded truss model- - but natural.

best regds

vikramjeet



Dear vikram,IF we analyze truss in staad with space then member demand will be high we need bigger and heavier sections.It is always better to design truss members in truss module.RgdsManohar
-- ญญ





-- ญญ

Posted via Email
Back to top
View user's profile Send private message
es_jayakumar
General Sponsor
General Sponsor


Joined: 24 Nov 2011
Posts: 1190
Location: Cochin

PostPosted: Tue Oct 30, 2012 2:43 pm    Post subject: Reply with quote

Dear members,

A case study on the effect of the rigidity of a truss joint on its member forces is attempted and attached. The effect of slightly off-joint loads is also discussed.

Regards,

E S Jayakumar



Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.
Truss.doc
 Description:

Download
 Filename:  Truss.doc
 Filesize:  162 KB
 Downloaded:  823 Time(s)

Back to top
View user's profile Send private message
VPandya
General Sponsor
General Sponsor


Joined: 09 Nov 2009
Posts: 757
Location: Ahmedabad

PostPosted: Wed Oct 31, 2012 4:40 am    Post subject: Model TOP CHORD as Beam Elements with Mz relesed. Reply with quote

Dear Er. sarfaraj.husain,

I assume that this is a PLANE  FRAME (TRUSS) Model problem. Not a SPACE FRAME (3D) Model.


I agree with you. Have TOP CHORD of Truss Model as Beam Elements with End Moments Mz released. Now Roof Purlin can come any where along the span of a Top Chord member and can support a concentrated load causing Positive Moment along Span and Vertical Shear at Joints. Top Chord connection with Truss Vertical or Diagonals will have to design not only for Axial Force but also for Shear Force.


All other members can be TRUSS Elements with connection designed for Axial Force only.

Some Engineers (Like me) prefer to Model Top Chord as a Continuous Beam spanning over Truss Verticals or Diagonals.

Note: All connections are simple Bolted joints. No Welded joints required.


Regards.

Vasudeo Pandya P.E. ; S.E.
Structural Engineer.


Last edited by VPandya on Thu Nov 01, 2012 4:29 pm; edited 1 time in total
Back to top
View user's profile Send private message
es_jayakumar
General Sponsor
General Sponsor


Joined: 24 Nov 2011
Posts: 1190
Location: Cochin

PostPosted: Wed Oct 31, 2012 1:59 pm    Post subject: Reply with quote

The previously discussed truss model with off-joint loads (purlins located on the rafter member), with principal rafter as a continuous beam and all other members with pin jointed ends is created by releasing the Mz of concerned member ends (as discussed by Shri. Pandya). This is analysed as plane frame. The concerned STAAD file is attached.
Mostly, the bottom tie members will also be continuous, as for principal rater. In that case, its joints need not be released. Readers may revoke the release of its ends and see if there is any marked difference in member forces.

E S Jayakumar



Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.
Truss off-joint load.rar
 Description:

Download
 Filename:  Truss off-joint load.rar
 Filesize:  573 Bytes
 Downloaded:  1427 Time(s)

Back to top
View user's profile Send private message
ravi_vairagi
SEFI Regulars
SEFI Regulars


Joined: 19 Nov 2011
Posts: 35

PostPosted: Thu Nov 01, 2012 7:06 am    Post subject: Reply with quote

Dear Jay kumar,

Thanks for the attached staad file.
now my question is

1.  why you provided two different support.(one is pinned and other is fixed but rel...).
2. why you released redudent member at both end and other member are on single end.

If you give above answer it help me lot in switch yard structure Design.

Warm Regards
Ravi Vairagi
Back to top
View user's profile Send private message
es_jayakumar
General Sponsor
General Sponsor


Joined: 24 Nov 2011
Posts: 1190
Location: Cochin

PostPosted: Thu Nov 01, 2012 12:50 pm    Post subject: Reply with quote

Dear Ravi,

1. I tried to model the truss with one support hinged and the other on roller, which simplification enables us carry out manual analysis, by making the structure determinate. I practice, both the supports will be either hinged or partially fixed (nobody will generally bother to provide a roller to a truss support, for a simple truss). While analysing using a software, you are at liberty to treat both the supports hinged (which is more ideal), as it does not call for extra effort for analysis, unlike manual analysis, which is tedious for indeterminate structures. In fact, you can enjoy a plethora of support conditions in STAAD. I am not much familiar to other design software packages.

2. I posted a STAAD model of a truss with principal rafter loaded with point loads from purlins, somewhere in between the joints. I wanted the principal rafter to behave as a continuous beam, with ends hinged, the way they really behave, when we provide a single structural steel section or one with more than one member welded together. This is in lines with Shri. Vasudeo Pandya's posting.  We cannot analyse the truss with the top chord as a continuous beam and remaining members as a truss (ie, with pinned joints at both the ends) in STAAD( please correct me, if I am wrong). Hence, I chose to release the Mz of such members, in order to induce the pinned end condition in them.
It is more appropriate to anayse the entire truss as a plane frame, when off-joint loads are applied. I showed the truss model in question purely for illustration  purpose. Please ask for further clarification, if any.

Best wishes,

E S Jayakumar
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion All times are GMT
Goto page 1, 2  Next
Page 1 of 2

 

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


© 2003, 2008 SEFINDIA, Indian Domain Registration
Publishing or acceptance of an advertisement is neither a guarantee nor endorsement of the advertiser's product or service. advertisement policy