Composite slab is one of the most important of floor system that is used in the construction of multistory steel structures. Different types of loads which are imposed on the slab are discussed.
The types and definition of construction loads discussed here is mainly based on the specifications and recommendations of British Standard and Eurocode.
Types of construction loads, their magnitudes and various factors considered for calculations are discussed in this article.
Following topics regarding Construction loads on composite slabs are discussed:
1. Why it is important to evaluate loads imposed on composite slab during its construction?
2. Construction load types
- Construction loads imposed during concreting
- Construction loads exerted after concreting
Fig.1: Concrete Composite Slab
Why it is important to evaluate loads imposed on composite slab during its construction?It is considerably crucial to define and assess construction loads on the composite slab in order to prevent slab failure during construction. If such failure is occurred, the not only does the lives of labors and worker will be lost but also it would be considerably costly. Therefore, the description and computation of construction loads is invaluable.
Types of Construction Loads on Composite SlabsTypes of construction loads imposed on composite slabs are categorized into two major classes:
- Construction loads imposed during concreting, and
- Construction loads exerted after concreting.
Construction Loads Imposed During ConcretingThe construction loads on the composite slab during concreting is composed of the weight of worker, fresh concrete, pipelines, formworks and load bearing elements, weight of small equipments, and impact forces. The construction loads that structural designer should account for include the weight of concreting staff and their number should not be greater than of 6 men and only four of them should be around the outlet of the pipeline. With regard to load of formworks and small equipments, they can be computed and accounted for easily during the design. As far as concrete load is concerned, it is specified that height of concrete should not be greater than knee level above decking and the designer consider the load of concrete that is poured accordingly. This will ensure that excessive impact load is not imposed. The weight of the pipeline considered by structural designer is the weight of 150mm pipe full of concrete. This load need to be properly distributed over larger areas using suitable means such as timber. This measure is recommended to prevent local damages of decking. Another load encountered during concreting and considered by designer is a cone of heaped concrete with height of 20cm and with the base of 100cm. It is suggested that the outlet of the pipeline is moved regularly to prevent too much heaping. It is necessary to control the discharge of concrete when skip is employed for concreting. It is likely to place extra concrete if decks and steel beams deflect, especially when the slab is required to be finished at a specified level. The placement of extra concrete needs to be conducted with consultation of structural designer to check whether such extra load is permitted according to the design or not.
Construction Loads Exerted After ConcretingLoads that may be imposed on the composite slab include bags of fire protection, skips of debris, pallets of blocks and other equipments. If these loads are no more 1.5KN/m2, then it is considered that extra loads are not imposed on concrete and hence it would not impair undesired effect on recently placed concrete slab. However, if such loads are larger than 1.5KN/m2, then the strength of the concrete need to be considered and such loads must should not be imposed till the concrete achieve about 75 percent of its strength. If the composite slab is load prior to the age of 28 days, then the strength of concrete should be estimated though testing concrete whether it is cylinder or cube samples. In the design, objects that comprise construction loads after concreting are placed on pallets which are positioned on supporting beams. Examples of construction loads along their magnitude that provided by British Standard are shown in Table-1. Table-1: Construction Load Examples and Their Magnitude Used in Design
|Items||Loads on the composite slab after concreting, KN/m2|
|Concrete block||1 m high pallet of blocks applies a load up to 10|
|Bricks||1 m high pallet of bricks can exert a load of over 15|
|Bags of fire protection||1 m high pallet of bags can be equivalent to a load of 2.5|
|Bags of cement||A standard pallet of these weighs 12|
|Items||Heavy loads on the composite slab after concreting|
|Generators||Welding generators can apply a load of 50 KN|
|Fork lift truck||Fork lift trucks can exert a load up to 100 KN, not including their live load|
|Crane counter weights||Each counter weight is marked clearly with the value of its weight|
|Mobile access platform||The potential loading imposed by any mobile access platforms used to install services, finishes, etc. should be checked.|
- BS EN. Eurocode 1 - Actions on Structures Part 1-6: General Actions -Actions During Execution. European Committee for Standardization. Brussels, p. 21-24. 2005. (1991-1-6:2005).
- J W Rackham, G H Couchman, S J Hicks. Composite Slabs and Beams using Steel Decking: Best Practice for Design and Construction. The Metal Cladding & Roofing Manufacturers Association in partnership with The Steel Construction Institute. Ascot, p. 81-83. 2009. (P300).