CROSS Safety Report
Excessive deflection of spliced steel beam
A long-span primary steel beam deflects excessively at its splice during concreting of the floor slab.
Key Learning Outcomes
For civil and structural design engineers:
- The designer must consider and understand how standard bolt hole tolerances may manifest themselves in the assembly of their structures and the potential impact on deflections
- Consider specifying High Strength Friction Grip (HSFG) or Tension Control (TC) Bolts if the impact of bolt slip could be problematic
For steelwork connection designers:
- Use your experience to query the designer on the need for HSFG or TC bolts in a given situation
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A correspondent discusses a new five-story steel-framed building with concrete floors cast on permanent metal decking. A 14 m span primary beam, which occurred on every floor, had a bolted splice with flange and web plates near its quarter point. During concreting of the deck slab excessive deflection was reported at the positions of these splices at every level.
The reporter stated that the deflection of the beam was 55 mm at the splice and the gap between the bottom flanges increased to 16 mm wide. The reporter confirmed that the beam was designed to the relevant local codes of practice.
The degree of deflection was obvious and remedial work was necessary. The original splice detail is shown in Figure 1 and it was decided to strengthen this by welding on additional members as shown in Figure 2 to increase both stiffness and strength.
The reporter says that the connections were computer-designed but that additional consideration should have been given when designing the critical connections.
Expert Panel Comments
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The reporter has only provided limited information, and therefore, to some extent, the reason for the deflection can only be surmised. A recorded deflection of 55mm at the quarter point, under the wet concrete load, is indeed excessive.
The standard clearance around a bolt hole is (usually) 2mm, although this can be more for larger diameter bolts. For beam connections carrying moment with standard dimension tolerance bolt holes, and a set-up gap between beam segments, the connection can act as a pin if the bolts slip until the slack is taken up. This may account for the increase in the gap between the bottom flanges observed by the reporter. Thus, the angular distortion capable of occurring at the joint is proportional to the amount of bolt slippage. It is also possible that some deformation of the connecting plates could have occurred which would increase the rotation further.
With the use of laser-controlled machines commonplace in steelwork fabrication, the dimensional control of bolt holes in steelwork can be very accurate. Therefore, the potential for bolt slippage, and its impact on serviceability limit state design, must be considered for critical connections.
The designer should consider if non-slip bolts, such as High Strength Friction Grip (HSFG) or Tension Control (TC) Bolts, are required within any joint to prevent rotation or movement. The reporter is correct to suggest that additional considerations should have been given when designing these critical connections. A computer design would not have solved this problem; the likely error is one of concept, not numbers. The concept, which assumed no slip, was poor and on-site this was exacerbated because the problem was repeated over five floors.
the concept was poor and on-site this was exacerbated because the problem was repeated over five floors
How the remedial solution was designed, and its overall purpose, cannot be determined from the information provided. Additional concrete may have been poured during construction to make up a level surface following the beam deflection, thereby increasing the required ultimate load to be carried by the connection. The connection may also have been found to be under-designed following back-checks after the deflection was observed. Clearly, any strengthening works employed needed to be installed in-situ with respect to the site constraints and the existing connecting plates and bolts, thereby leading to a somewhat unusual final connection detail.
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