Steel scaffolding is of skeletal form structures, which has been widely used as temporary structure in support of formworks, fresh concrete, workers etc. during concreting or other temporary construction. However, there have been a number of collapse cases in China, Hong Kong, Taiwan and United Kingdom. This is possibly due to the fact that designers have little information describing the actual causes of scaffold support system collapse, so it is difficult for them to design such as temporary support for safety in the construction field. Often, construction engineers rely on their own experience in view of the lack of guidelines.
Design of steel scaffolding is complex due to the uncertainty in assuming a correct value of effective length or K-factor. There are many factors affecting the overall stability and the carrying capacity of the scaffolding system. For instance, the semi-rigid connection between the modular units and the base plate, eccentric connection of the system and the imperfection of the individual member and the overall system have significant effect onto the overall stability of the scaffolding system.
This research proposes a refined, robust, practical and efficient second-order analysis procedure for structural design of steel scaffolds. Unlike the conventional second-order analysis for plotting of bending moment diagrams and then using the design code to check the member strength, it directly checks the strength of members and the whole structures by checking member sectional strength. Using the proposed method, it is not necessary to assume an effective length and only analysis of the structure allowing for various second-order effects and considering condition for violation of sectional strength is required. The sectional strength, allowing the instability due to the effective length of a member or the P-δ and the P-Δ effects, is the yardstick for load capacity determination. The member individual buckling strength does not need to be checked against the design codes. The concept of effective length is abandoned when using this approach which compares well with experiments for buckling strength analysis of small to medium size scaffolding systems.
This research focuses on the use of notional force to simulate the imperfections of an individual member and the system and works out the corresponding value for notional force. The value is verified with the tested failure loads. The research also focuses on the method of applying the notional force to a scaffolding frame with restraints at top and at bottom. The research recommends that the notional force is applied at the mid-height in order to create a system imperfection with maximum magnitude at mid-height of the scaffolding system. The proposed method simplifies significantly the analysis and design procedure whilst giving a practically acceptable limit.
The research formulates an equation for the section capacity check, allowing for the P-Δ and the P-δ effects so that individual local sectional and member buckling strength checks are not needed. As the second-order effect related to member slenderness is simulated, it is not necessary to assume the effective length for a member for buckling strength check.
The research also covers the investigation of the connection stiffness of the scaffolding jack base with a view to studying the effect of jack base plate on the overall stability of the scaffolding system and its effect on the buckling strength of a steel scaffolding system. An experimental investigation was conducted to measure the moment versus rotation with different value of axial load for the commonly used scaffolding systems in Hong Kong.
In short, the originalities of this research are summarized below:-
a) Development of a novel practical design method for scaffolding systems, which is considered as unavailable by even eminent researches in structural stability.
b) Consideration on allowance of imperfections is emphasized and quantified.
c) The effect of semi-rigid connections of scaffold is investigated.
d) Application of section capacity check to design of scaffolds.