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Design of Long Span Trusses.
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thirumalaichettiar
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PostPosted: Thu May 07, 2020 1:50 pm    Post subject: Design of Long Span Trusses. Reply with quote

Dear Sefians,

In case of long span trusses the GLOBAL SHEAR can not be neglected. Since designing a truss for long spans say 50 m to 100m is not that simple as found in texr books.

I found the following text in the  page 27 of  book STEEL BUILDINGS IN EUROPE Single-Storey Steel Buildings Part 5: Detailed Design of Trusses

Global shear deformations are not, in fact, negligible in the case of trusses, since they result from a variation in length of the diagonals and posts.

The value of the reduced modulus of elasticity clearly varies depending on the geometry of the truss, the section of the members, etc. For a truss beam with “well proportioned” parallel chords, the reduced modulus of elasticity is about 160000 N/mm2 (instead of 210000 N/mm2).

Any comment on this or more explanation since while modeling in a software need to enter E value.

T.RangaRajan
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thirumalaichettiar
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PostPosted: Sat May 09, 2020 2:47 am    Post subject: Reply with quote

I read in the above reference that

For large member forces, it is a good solution to use:

1. Chords having IPE, HEA or HEB sections, or a section made up of twochannels (UPE).


2.  Diagonals formed from two battened angles.

The web of the IPE / HEA / HEB chord section is oriented either vertically or horizontally. As it is easier to increase the resistance to in-plane buckling of the chords (by adding secondary diagonal members) than to increase their to outof- plane resistance, it is more efficient to have the web horizontal, for chords in compression.


On the other hand, it is easier to connect purlins to the top chord if it has a vertical web.


It could be a good solution to have the top chord with a vertical web, and the bottom chord with a horizontal web.

The above are good guide for long span trusses. To young  and precising engineers can make use of the above guide while designing even if any SOFTWARE is used.


T.RangaRajan


Last edited by thirumalaichettiar on Sun May 10, 2020 3:49 pm; edited 1 time in total
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thirumalaichettiar
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PostPosted: Sat May 09, 2020 3:25 am    Post subject: Reply with quote

]Typical joints in welded building roof trusses[


You can browse google to get more.


T.RamgaRajan



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thirumalaichettiar
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PostPosted: Sat May 09, 2020 7:34 am    Post subject: Reply with quote

As per the clause 7 of IS 875-Part 3(2015) the INTERNAL PRESSURE COEFFICIENT are:

1. If the opening is o to 5% -+0.2 & -o.2


2. If the opening is 5 to 20% -+0.5 & -o.5


3. If the opening is above 20%  -+0.7 & -o.7


If the % of opening is say 8% or 13% is it possible to


interpolate the values?


T.RangaRajan
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es_jayakumar
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PostPosted: Sun May 10, 2020 6:26 am    Post subject: Reply with quote

Sir,
For permeability value between 5% & 20% , the set of values is [+0.5 & -0.5]. How can we interpolate them  for any intermediate value between 5% & 20% ?

E S Jayakumar
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thirumalaichettiar
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PostPosted: Sun May 10, 2020 12:40 pm    Post subject: Reply with quote

es_jayakumar wrote:
Sir,
For permeability value between 5% & 20% , the set of values is [+0.5 & -0.5]. How can we interpolate them  for any intermediate value between 5% & 20% ?

E S Jayakumar


Er.Jeyakumar,

That is what my question?

if u know kindly explain.

May be we can first take the valor fro 0 to -5 interplaote  and again fro 0 to +5 >
May be wrong or wright must be ponyed out by experts in the forum?

T.RangaRajan.
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es_jayakumar
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PostPosted: Sun May 10, 2020 1:10 pm    Post subject: Reply with quote

Sir,
As per my understanding, for any permeability value between 5% and 20%, we have to try for the worst effects (maximum load) by taking first Cpi = +0.5 and then Cpi = -0.5, to see which one governs the design. In other words, we have to take Cpe+/- Cpi, that produces maximum pressure. Please see the attachment.

Regards,
E S Jayakumar



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nurat
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PostPosted: Sun May 10, 2020 2:40 pm    Post subject: Reply with quote

Dear Sir,

Please see that C.G. Lines of intersecting members ( detail A ) falls within the Joint to avoid any additional moment at the Joint. Other experts can comment on my statement please.
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thirumalaichettiar
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PostPosted: Sun May 10, 2020 11:28 pm    Post subject: Reply with quote

Er.Jeyakumar,


After going through your load on 31.5 meter truss I noticed that there are so intersections of vertical and diagonal members in the bottom chord. My point of view is

1. Since the bottom chord is subject to tension,
2. and the SLENDERNESS RATION as per IS-800-2007 -Table 3(v)        is 350  and (vi) is 400.

In the above is it possible to make long length in the bottom chord. I mean the number of diagonal and vertical may be arranged in such a way that the length will be good enough for resisting the forces as well as the SLENDERNESS RATIO.

This is my view in the case of long span trusses.

T.RangaRajan.
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es_jayakumar
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PostPosted: Fri May 15, 2020 5:37 pm    Post subject: Reply with quote

es_jayakumar wrote:
Sir,
As per my understanding, for any permeability value between 5% and 20%, we have to try for the worst effects (maximum load) by taking first Cpi = +0.5 and then Cpi = -0.5, to see which one governs the design. In other words, we have to take Cpe+/- Cpi, that produces maximum pressure. Please see the attachment.

Regards,
E S Jayakumar

Please find the attachment, which is connected to this  (ASCE)



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