PRE-FABRICATED STEEL STRUCTURE DESIGN TUTORIAL: PART 3

Seismic/Earthquake Load Calculation

According to BNBC 2006, page 10631,

T=Cn(hn)^(3/4)
=0.083*5.32^3/4

=0.29 sec

C=(1.25*S)/(T^(2/3))
=(1.25*1.5)/(0.29^2/3)

=4.28

V=(ZICW)/R
Dhaka is in zone 2, so, zone coefficient is 0.15
=0.15*1*4.28*W/6
=0.107W

W=Total seismic dead load

=Rafter weight (assume=0.5KN/m)*frame length+Column weight (assume=0.5KN/m)*nos of column*length of column+(purlin weight+sheet weight)*frame length

=0.5*11.66+0.5*2*5.32+0.66*11.66

=18.90 KN

So, V=0.107*18.90

=2.02 KN

According to BNBC 2006, page 10634

V-Ft=2.02 KN

So, Seismic load, F=2.02 KN

PRE-FABRICATED STEEL STRUCTURE DESIGN TUTORIAL: PART 2

Live Load Calculation
According to BNBC 2006, page 10592, Table 6.2.4,  Live load (LL) is 1 KN/sq. m
A reduction can be done according to BNBC 2006, page 10597, Table 6.2.7
Tributary area for a frame
=6.1*12.18 sq. m
=74.30 sq. m
For this tributary area, the reduction factor, R is = 0.69
So, Live load is
=(0.69*1) KN/sq. m
=0.69 KN/sq. m
Applying bay spacing
=0.69*6.1 KN/m
=4.21 KN/m

Wind Load Calculation
According to BNBC 2006, page 10616, for this building,
L=12.18 m
B=18.29 m
h={4.570(Clear height)+0.5(Assumed rafter depth)+0.25(Purlin depth)} m
=5.32 m
So,
L/B=12.18/18.29=0.67
h/B=5.32/18.29=0.29
h/L=5.32/12.18=0.44
For a height 4.5 m where wind ward wall load is applied uniformly (see figure 1) ;[For a height 4.5 m wind ward wall load is always applied uniformly]
qz(4.5m)= CcC1CzVb² [Ref: BNBC 2006, page 10607]
Value of Cz = 0.368 for Exposure A
=47.2*10ˉ⁶*1*0.368*210²=0.77 KN/m²
For a height 5.32 m where wind ward wall load is applied trapezoidal (see figure 1)

Figure 1

qz(5.32m)= CcC1CzVb² [Ref: BNBC 2006, page 10607]
Value of Cz = 0.394 for Exposure A
=47.2*10ˉ⁶*1*0.394*210²=0.82 KN/m²

Force,
Wind ward wall, Pz(4.5 m)=CGCpeqz*Bay Spacing=1.654*0.8*0.77*6.1=6.22 KN/m
Wind ward wall, Pz(5.32 m)=CGCpeqz*Bay Spacing=1.620*0.8*0.82*6.1=6.48 KN/m
Wind ward roof, Pz=CGCpeqz*Bay Spacing=1.620*0.9*0.82*6.1=7.29 KN/m
Leeward roof, Pz=CGCpeqz*Bay Spacing=1.620*0.7*0.82*6.1=5.67 KN/m
Leeward wall, Pz=CGCpeqz*Bay Spacing=1.620*0.59*0.82*6.1=4.78 KN/m

Selection of column spacing

It is necessary to select last/end column center to building outer distance. If a normal shed building is to be constructed which has brick wall of certain height (it may be 8 ft to 10 ft or more), the spacing is kept according to the thickness of the brick wall. The suggestion is following.
1. for 10 in (around 250 mm)  thick brick wall, the distance is kept 475 mm. See figure 1
2. for 5 in (around 125 mm) thick brick wall, the distance is kept 350 mm. See figure 2

Figure 1

Figure 2

Spacing of steel column, bay spacing will be kept in between 25 ft to 30 ft, member such as beam, column, purlin, girt etc should be kept under 30 ft for easy transport. In case of mezzanine floor or any other floor it is economical to keep column spacing in between 20-22 ft.

Standard practice in column rotation
Column rotation is kept according to following figure 3. This figure is only for I-shaped column of single storied pre-fabricated steel building.

Figure 3

But in case of the building having floors/floor column rotation is kept according to the following figure 4.

Figure 4

To construct a steel building it is necessary to keep unit in mm. And for easy construction the spacing should be round in number. Standard practice to round the numbers in 100, 500 and 1000. For example if clear height of a building is 18 ft, in mm unit it is 18X304.8=5486.4 mm, but it should be kept 5500 mm.
In a frame it is standard practice to keep column spacing in between 50 ft.
As maximum clients are non technical they give you out to out (O/O) dimension of their building. So this article may help you to give column spacing.

PRE-FABRICATED STEEL STRUCTURE DESIGN TUTORIAL: PART 1

In this tutorial I want to teach practical design. so my tutorials are project based. I have planned to discuss different building. Those are one storied building pitched roof building, two storied building having a pitch roof, multi storied building etc.

My tutorials will contain load calculations, design by software, drawing guideline, practical design experience share etc.

At first, I will design a single storied Pre Fabricated Steel Building.

Figure 1

Building Information

Building Dimension: 60'X40' (See column layout)

Figure 2

Clear Height: 15' (see typical section)

Wall System: 5 inch brick wall and wall sheet (see typical section)

Figure 3

Roof slope: 1:10

Wind Bracing: Roof and wall bracing

Roof cladding: 0.47 mm thick steel sheets (color coated)

Wall cladding:0.47 mm thick steel sheets (color coated)

Building Location: Dhaka, Bangladesh.

Loads on all buildings are applied in accordance with BANGLADESH NATIONAL BUILDING CODE (BNBC)-2006

Bay spacing (c/c distance of main frame): 20' (6100 mm). See Article "Selection of column spacing"

Here single frame analysis is done.

Load Calculation

Dead Load Calculation

There are three types of dead load will be applied in this structure.

1. Self weight of the main structural member (column & rafter)

2. Roof sheet weight

3. Purlin weight

Self weight of the main structure will be calculated by the software (Staad.pro V8i).

Roof sheet weight is calculated by the following:
=(0.47/1000)*(7850/0.82)*(9.81/1000) KN/sq. m.
=0.044 KN/sq m.
Here, 7850 is unit weight of steel in kg/cu. m.
When sheet is folded by machine it's width is generally decreased 18 %. That is why it is divided by 0.82.

Purlin weight is calculated by the following:

Let us purlin section is according to figure 4. Nos of purlin in a frame is =(40*304.8)/1200=10(around), So, spacing of purlin is =(40*304.8)/10=1219.20 mm=1.2192 m.

Figure 4

So, weight of purlin is
=[{(18*2+60*2+250)/1000}*(2.5/1000)*(7850*9.81/1000)*(1/1.2192)] KN/sq m.
=0.065 KN/sq m.

So, Total weight of sheet and purlin is
=(0.044+0.065) KN/sq m.
=0.109 KN/sq m.
Applying bay spacing
=(0.109*6.1) KN/m
=0.66 KN/m

[TO BE CONTINUED]

Boss Suggestion Regarding Short Column & Concrete Strength

Where designing a RCC Base Column the minimum size is 16”X16”

In practical while using stone chips the f’_c is found 3500 psi (1:1.5:3)

While using Brick chips the f’c is found 3000 psi (1:1.5:3)

While using Brick chips the f’c is found 2500 psi (1:2:4)

Boss Suggestion Regarding Connection Plate Bolt Spacing

In a connection plate minimum bolt-bolt distance is 2.5d to 3d. And edge distance is 1.5d

Boss Suggestion Regarding Shop Drawing 1

1. In rafter, generally connection plates are provided after 8m distance for transportation easiness.

2. Purlin-purlin distance should be maximum 1300 mm or 4 ft.

For Fabrication Drawing following drawings are done,

a. Proposal drawing

b. Erection drawing

Anchor bolt layout

Anchor bolt details

Steel column layout

Rafter layout

Sections

Purlin layout

Wall girt layout

Roof sheet layout

Wall sheet layout

c. Fabrication/ shop drawing

1.       Individual details of

a.       Column

b.      Rafter

c.       Purlin

d.      Accessories