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purlins on rafters directly & Unsupported length

 
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er_moinuddeen
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Joined: 21 Mar 2019
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PostPosted: Sun Jun 09, 2019 4:23 pm    Post subject: purlins on rafters directly & Unsupported length Reply with quote

dear all,
              plz guide me with regards to the subject
1) can we directly support the purlins only on Rafter(rcc/steel)without the steel truss, for a purlin span of 12 Meter.
2) what is the correct method of calculating the unsupported length when there are members(bracings) in one direction(x) and no members(bracings) in other directions(y) as for the frame in our project whose pics are attached, because according to authors n IS code its the distance between floor top n bottom of the shallowest member



unsupported length of col in the direction of no beams(bracngs).jpg
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unsupported length of col in the direction of no beams(bracngs).jpg



purlins of 12m span directly resting on rafter (without truss).jpg
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purlins of 12m span directly resting on rafter (without truss).jpg


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mandeep_singh_kohli
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Joined: 29 May 2019
Posts: 11

PostPosted: Tue Jun 11, 2019 12:11 pm    Post subject: Re: purlins on rafters directly & Unsupported length Reply with quote

Hi,

Regarding your question 1.) yes you can support purlins directly on the rafter w/o truss. You will just have to design the purlins for the full length of 12 meters with pin supported at its two ends on the rafter.

2.) Here you need to understand what special thing does a brace introduce into the system because of which we use it to break the un braced length.Also understand that the way we diesgn structures is 2-D, although the model is 3-D but we design our 3-D structres as a combinations of different 2-D  orthogonal frames having 2 separate lateral load resisting systems in each orthogonal direction.

So if you have bracing in X direction but not in Y direction the efective length will be the maximum un braced length between the braced points. In this case reduced in X direction but not reduced in Y direction.

Note that Not only bracing, but a Deep member w/ moment connection to the major axis of the column can also break the  unbraced length.
When we talk of unbraced length it might be helpful to visualize it as weather the member brace point, support or a Moment connection introduces a point of inflection at the location or a point of zero lateral displacement.
A brace introduces a point of zero displacement ( zero relative displacement if it is a relative brace). A Moment connection causes the moment diagram to change sign at support location and both reduce the unbraced length of the member. The single curvature is broken into double.







er_moinuddeen wrote:
dear all,
              plz guide me with regards to the subject
1) can we directly support the purlins only on Rafter(rcc/steel)without the steel truss, for a purlin span of 12 Meter.
2) what is the correct method of calculating the unsupported length when there are members(bracings) in one direction(x) and no members(bracings) in other directions(y) as for the frame in our project whose pics are attached, because according to authors n IS code its the distance between floor top n bottom of the shallowest member
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er_moinuddeen
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PostPosted: Fri Jun 14, 2019 6:46 am    Post subject: regards Reply with quote

Thanks Mr. Mandeep for ur generous response , but i have a little confusion as to what u mean by "maximum un braced length between the braced points".
as regards to the staad frame tat i have attached, is it 2.667(w.r.t second level) or is it 1.829+1.829+2.667+2.324+32.324=10.973 in the direction where there r no beams. what i understand from ur description is, it is 2.667,but if u refer "limit state theory n design of rcc",by shah n kharve, on page no 565,Ex.11.2.4,he takes the full length in the unbranded direction, i have attached the pics below for the same. but when the same frame, when it is designed(for wind load) in rcdc soft it considers distance b/w nodes n calculates effective lenght factors based on stability index. so plz clarify



IMG_20190614_113817968.jpeg
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IMG_20190614_114005147.jpeg
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mandeep_singh_kohli
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Joined: 29 May 2019
Posts: 11

PostPosted: Mon Jun 24, 2019 10:18 am    Post subject: Re: regards Reply with quote

if their are no braced or beams in a particular direction then the column is not braced in that direction, so maximum un braced length by default is the full length.







er_moinuddeen wrote:
Thanks Mr. Mandeep for ur generous response , but i have a little confusion as to what u mean by "maximum un braced length between the braced points".
as regards to the staad frame tat i have attached, is it 2.667(w.r.t second level) or is it 1.829+1.829+2.667+2.324+32.324=10.973 in the direction where there r no beams. what i understand from ur description is, it is 2.667,but if u refer "limit state theory n design of rcc",by shah n kharve, on page no 565,Ex.11.2.4,he takes the full length in the unbranded direction, i have attached the pics below for the same. but when the same frame, when it is designed(for wind load) in rcdc soft it considers distance b/w nodes n calculates effective lenght factors based on stability index. so plz clarify
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