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CLOSE THIS BOOKRoof Truss Guide - Design and construction of standard timber and steel trusses (BASIN - SKAT, 1999, 187 p.)
7 WORKED EXAMPLE STEEL TRUSS
VIEW THE DOCUMENT7.1 The task
VIEW THE DOCUMENT7.2 How to proceed

Roof Truss Guide - Design and construction of standard timber and steel trusses (BASIN - SKAT, 1999, 187 p.)

7 WORKED EXAMPLE STEEL TRUSS

7.1 The task

The primary school building of Chapter 5.1 needs to be covered by a steel truss and micro concrete tiles (MCR).

The building is located in a tropical but non-cyclonic area without snow load. Architectural considerations have determined the roof slope to be 30° in a double pitch design.

7.2 How to proceed

1. Shape of the roof:

®

double pitch




Design span of trusses:

®

5.60 + (2 × 0.075) = 5.75 m


5.60 + 1.80 + (2 × 0.075) = 7.55 m




Loads (S'):

®

We assume the load as commended


in this guide for an MCR roof: 1.0


kN/m2

2. Select truss from guide:

Which one of the three options do you want to use:

a) tube truss (lowest weight but requires gusset plates for the connection of the members)

b) angle bar truss (ease of fabrication but slightly more steel required than with tube truss)

c) rebar truss 300BJ08 (for 6m span) and 300BJ10 (for 8 m span) with ridge purlins (6 m span); since the roof structure will not be visible from inside the classrooms (covered by a ceiling) and the loft is not being used, the rebar truss is not a preferred option.

We decide to use the tube trusses:

chapter 6.9.2, Double Pitch Roof, Tube Truss, 6 m span
chapter 6.9.3, Double Pitch Roof, Tube Truss, 8 m span

3. Compare design span with span of the selected sample truss and adjust distance between trusses:

Truss (1):

span of standard truss

= 6.00 m

standard distance

= 2.50 m

design span

= 5.75 m

= 2.72 m

Truss (2):

span of standard truss

= 8.00 m

standard distance

= 2.50 m

design span

= 7.55 m

= 2.81 m

4. Compare your total design load (S') of the roof with the total load of the sample truss.

Trusses (1) and (2):

total load of sample truss

= 1.0 kN/m2

total design load

= 1.0 kN/m2

® no change in distances between trusses and the distances to be applied are:

Truss (1): 2.72 m
Truss (2): 2.81 m

These distances between trusses are the max. distances and they have to be compared with the dimensions of the building. To achieve and equal distribution of trusses, the distance can be reduced, but never increased. In our example the room on the left would require two trusses with a distance of 2.00 m between the trusses.

In case the design load had been calculated to be 1.25 kN/m2, the following procedure would have to be applied:

Truss (1):

total load of sample truss

= 1.0 kN/m2

total design load

= 1.25 kN/m2

Distance (A)

= 2.72 m

= 2.18 m

Truss (2):

total load of sample truss

= 1.00 kN/m2

total design load

= 1.25 kN/m2

distance (A)

= 2.81 m

= 2.25 m

5. Adjust the design of the sample truss to your exact requirements!

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