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Case study: Cantilever Structures and Bolting Techniques

posted Jan 15, 2018, 5:27 AM by jeffery jim   [ updated Jan 15, 2018, 5:29 AM ]

Floor at Indonesia's stock exchange collapses, more than 70 injured. (Read more

1. After going through certain audio visual evidences, the structural collapse involves a walkway which does not form part of the initial design of the mezzanine floor.

2. The additional work structure as constructed most likely constructed/added upon request by the client when there is a need for additional (public) traffic flow within the floor. Thus, the structural engineer designed a cantilever slab using steel element (I-Beam) which are bolted to the reinforced concrete beam (and the free end is actually not supported by architectural's aesthetic fitting.)

3. The failure started from one particular design node which was supported by bolts. The reason for failure cannot be determined or established at this point of time but it is believed that bolts failed due to overburden.

4. After scrutinizing some photos, we can established that the failure is not due to bolt failure but anchoring method and design philosophy. We can establish that it is due to its low pull-out strength instead of tensile, stripping or shearing because from the bolt anchoring hole photos and the slight deformation of the gusset plate on the I-Beam.

5. This caused instability when rotation occurred due to insufficient moment capacity of the support to restrain bolts from being pulled out (when it was philosophically designed as fixed support). For this to happened, we can assume that slight tensile deformation did happened close to the gusset plate when reacting with high vertical stress.

6. Due to its design which is highly rigid (since the other end of all beams are interconnected/linked by tie-beam), a single movement at bolted-end generates reaction at the tied-end which propagates reactions (Fx, My) accordingly and caused chain reaction of instability to the whole structure.

7. Lastly, another crucial design which was not considered by the consultant is the role of group bolts analysis which can reduce pull-out stress on 4 anchoring holes by half with 8 anchoring holes.

** This is an educational article and should be use as it is