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wind loads on Octagonal Hip Roofs with common Crown

 
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sidhaivam
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PostPosted: Thu Dec 31, 2015 8:51 am    Post subject: wind loads on Octagonal Hip Roofs with common Crown Reply with quote

Dear Friends,

Being new to the site , I ended up posting in the wrong place initially. Corrected since.

Need your experienced views/opinions/suggestions on the fwg problem
I present the problem stmt as under:-

1. Design an Octagonal Metal Roof with 1:15 pitch and central Crown.
2. Dimensions : sides of the Octagon = 9m
3. Height of the roof at Eaves =4.2m
4. RCC Circular Columns are at each apex of the octagon and are 900mmDia
5. Basic Wind Speed is 47m/s and Bldg is in EQ zone IV.
6. The walls of the Octagonal hall are in Brick masonary and have 18% Openings.
6. No columns are allowed in the octagon anywhere else.

This Bldg is in the ground floor and the floor below (UG) houses a larger Bldg. Now I have the following issues

1. For the wind loading external coeffs should I use the Table 6 and table 4 of IS 873-III( mono-sloped Roofs) or use Table 4 and Fig 2 of IS 875-III( Circular standing Conical Roofs).
2. What type of truss is well suited for this type of situation given the octagonal hips and common crown and the large column free space present.
3. Suggestions about the connection/member choice at the crown.


I att the screen shot of the staad model for ease of understanding the issues

Thanks in advance in anticipation of your valuable views



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rajprayas2000
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PostPosted: Mon Jul 26, 2021 4:43 pm    Post subject: Reply with quote

Can you please share me the stadd file of the designed roof truss?
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vikram.jeet
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PostPosted: Tue Jul 27, 2021 2:47 am    Post subject: Reply with quote

Octagonal metal roof

1.0 Metal roof slope , generally , not flatter than 1:10.
2.0 All trusses will share loading equally ,collectively.
3.0 From aesthetic point of view , a steeper slope seems more pleasant with red Khaparail type metal sheet . But it is just a personal view.
4.0 Steeper slope will have other benefits like more rise for each of eight trusses and economy in steel pipe / steel members.

5.0 It is not a Monopole roof case.
6.0 calculate external pressure coeff  from table as well internal pressure coeff  of I think 0.5p. But I will check from code .
7.0 Trusses roof  to be protected from wind uplift .
8.0 columns to be designed for  net tensile force and Moments from walls under wind case.

Will check back from code and revert.
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vikram.jeet
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PostPosted: Tue Jul 27, 2021 3:09 am    Post subject: Reply with quote

Wind load on roof sheeting
External pr coffee as per Table 6
Internal pr coff.  as per 7.3.2.2 , it is 0.5p  for 5 to 20% openings
Add

Wind on walls
External as per table 5
Internal 0.5p
Force coff as per table 25 for octagon shape.
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vikram.jeet
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PostPosted: Tue Jul 27, 2021 8:06 am    Post subject: Reply with quote

For such a column free hall of size around 25m, the height of structure needs  below spring level i.e rcc col top level needs to be more than 5.0 m.  Rise of truss at center is in addition to this.
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vikram.jeet
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PostPosted: Tue Jul 27, 2021 8:48 am    Post subject: Reply with quote

Type of truss :
The truss is A type . Truss to be divided into panel points with spacing about 1.25m to 1.45m,  lesser spacing if lap is required in sheeting , otherwise about 1.45m.
Verticals at each panel point . Diagonals starting from center top towards bottom chord .
Purlins at all top panel points.
Bottom runners at alternate bottom panel points.

Truss to be designed for
DL + LL
DL + wind load
Under wind case permisdible stresses can be increased , but check  latest Code .
Trusses are generally designed for 2/3 rd of Live Loads
But purlins to be designed for 100%LL.
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vikram.jeet
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PostPosted: Tue Jul 27, 2021 9:04 am    Post subject: Reply with quote

TYPE OF CONNECTION AT COLUMNS

For analysis on Staad , one end as hinged support and other end as
Roller support ,i.e.  supporting allowing horizontal movement in span direction. Input modelling to be done accordingly.

CONNECTION AT SITE .
The plates with stiffners provided at end of trusses at bottom of bottom chord - top chord junction point are called Truss SHOE PLATES.
Truss shall rest on top MS plates of rcc columns with at least 4 nos anchor bolts pre-embeded into column concrete on each plate.

Truss SHOE plate on one side shall have normal circular holes as per anchor bolt dia. , But Truss SHOE plate on other end shall have OVAL SHAPED SLOTTED HOLES ,to allow for temp movement.
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vikram.jeet
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PostPosted: Tue Jul 27, 2021 10:13 am    Post subject: Reply with quote

Provision of EAVES BEAM AT EAVES LEVEL

In model Rcc eaves beam is not seen . It is advisable to tie all rcc columns . If rcc beam is not admissible to owner/ Architect , the the Purlin over column must be a heavier steel section OR provide TWIN PURLUN AT COLUMNS .
The benefit to tie all columns with rcc beam  at eaves level is to avoid cantilever action of rcc columns  in vertical direction due to wind on walls .  Even steel beams with proper fixity on columns also serve same purpose.

The situation of wind on cladding walls transfered to columns  dealt in similar way as to wind load on water tank staging as regards manual analysis.
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rajprayas2000
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PostPosted: Sun Aug 01, 2021 6:37 am    Post subject: Reply with quote

I want Stadd Pro template for reference purpose only. Can you send it?
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