Archive for the ‘Diagnosis’ Category

Diagnosis

August 16, 2009

1. What are the Loading acting on the roof? The gravity dead load were the self weight of the trusses system and the roof covering(roof sheets) Impose live load were the wind(reversal in direction),temperature effects and loading derived from rainwater.
2. Determine the load path i.e flow of forces from the point of origin to the foundation .

Gravity Load Distribution

We must consider not only what loading conditions might effect a structure, but also how those loads are transfered from their point of origin through the structure to the ground. Strucutral design decisions concerning the structural framing system will dictate this flow of forces. The most direct path from the point of loading to the ground will create the most efficient structure. The continuity of this line of transfer is essential. However, this direct path is not always possible. As a matter of fact, due to the nature of live loads this path is constantly changing.

Wind Load

The essence of wind load distribution through a structure from one structural component to another, and how the structure resists this load can be investigated by assuming a uniformly distributed wind load acting on any one face of the structure at any one time. Normally a wind load design requires a separate analysis of wind from two perpendicular directions, such as wind from the north or south and then from the east or west.
The portion of the load going to the roof tends to cause the roof to move laterally; this lateral movement is resisted by the end walls. The movement of the end walls is prevented by their connection to the foundation.

Temperature Effect


Space trusses are subject to thermal expansion and contraction due to changes in temperature and thus may be subject to axial loads if restrained. Potential temperature effect must be considered in the design especially the span is large.

• Load path is a chain


o It is only as strong as its weakest link.
o The roof acting diaphragms and end walls are links in the chain.
o The connections between the roof, end walls, and foundation are additional links.
o These additional links serve as the connective points that complete the chain.
o The connections are just as important as the diaphragms and end walls.
o The loads imparted on a building must successfully pass through all of these elements in order to reach the ground and effectively resist applied forces.
o In other words, the load path or chain must be continuous and complete. There can be no weak links in the load path chain.

To have an effective horizontal force-resisting system, there must be a continuous load path.

* To be effective, each part of the horizontal force-resisting system must be adequate and properly connected to the other parts in the system.
o Roof diaphragm must be strong enough to safely resist the any horizaontal loads
o Connections from the roof diaphragm to the end walls(acting as shear walls) must be strong enough to transfer the force to the shear wall.
o End walls at the base of the roof structure must be connected to the footing
o Footing must be adequate

3. From the pictures and along the load path determine the location of weak chain link(failures) (bolts, base plate, buckling of components, snapping of ball joints etc) that lead to the collapse of the roof.
4, List down weak chain link which will be probably the causes of collapse.