SiSC has been established to help the small and medium size members comply with the Management of Health and Safety Regulations by giving those that subscribe the availability of direct competent health & safety advice.
Publications are available to buy or download here.
A new £1 million 'Target Zero' project is looking at five major building types (schools, warehouses, offices, supermarkets and mixed use) and will generate fully-costed solutions demonstrating how to achieve the three highest BREEAM ratings and meet the changes to Part L of the Building Regulations.
Below is an introduction to the topic Design. You can also search for your specific topic using the Search box at the top of the page or click on any of the following keywords and phrases:Methods of design; loadings; limit state design, partial load factors.
Every construction project starts with a client defining a need. The client could be an individual, but is more likely to be a company, local authority or department of national government. There are many different kinds of projects, which normally fall into three categories.
The first, and most common, category is factories, office blocks and shopping centres. The second involves a large amount of building services and includes computer centres and process plant. The third category – civil engineering projects – includes bridges, power stations, dams and roads.
The aim of the design process is to ensure that the structure is capable of resisting the anticipated loading with an adequate margin of safety and that it does not deform excessively during service. Due regard must be paid to economy which will involve consideration of ease of manufacture, including cutting, drilling and welding in the fabrication shop and transport to site. Under CDM requirements the designer has an obligation to consider how the structure will be erected, maintained and demolished.
Historically, engineers have been accustomed to assume that joints in structures behave as either pinned or rigid to render design calculations manageable. In ‘simple design’ the joints are idealised as perfect pins. ‘Continuous design’ assumes that joints are rigid and that no relative rotation of connected members occurs whatever the applied moment. The vast majority of designs carried out today make one of these two assumptions, but another alternative is possible, which is known as semi-continuous design.
Simple design is the most traditional approach and is still commonly used. It is assumed that no moment is transferred from one connected member to another, except for the nominal moments which arise as a result of eccentricity at joints. The resistance of the structure to lateral loads and sway is usually ensured by the provision of bracing or, in some multi-storey buildings, by concrete cores.
In continuous design, it is assumed that joints are rigid and transfer moment between members. The stability of the frame against sway is by frame action (ie by bending of beams and columns). Continuous design is more complex than simple design therefore software is commonly used to analyse the frame. Realistic combinations of pattern loading must be considered when designing continuous frames. The connections between members must have different characteristics depending on whether the design method for the frame is elastic or plastic.
The principal forms of loading associated with building design are:
To cater for the inherent variability of loading and structural response, engineers apply factors to ensure the structure will carry the loads safely. Design used to be largely based on an allowable stress approach. The maximum stress was calculated using the maximum anticipated loading on the structure and its value was limited to the yield stress of the material divided by a single global factor of safety. Serviceability deformations were calculated using these same maximum anticipated loadings. However, this approach gave inconsistent reserves of strength against collapse. The method is now superseded by a limit state approach in which the applied loads are multiplied by factors, capacities and resistances are determined using the design strength of the material. Limit states are the states beyond which the structure becomes unfit for its intended use. BS 5950-1 is a limit state design standard.
The values of the partial safety factors given in the Standard, which vary from load case to load case, reflect the probability of these values being exceeded for each specified situation. Reduced values of the partial safety factor are given when loadings are combined, as it is less likely that, for example, maximum wind will occur with maximum imposed load. This can be seen from Table 2 of BS 5950. The part of this table relevant to buildings not containing cranes is reproduced below.
The ultimate limit state (ULS) concerns the safety of the whole or part of the structure. In buildings without cranes, the principal load combinations which should be considered are:
| Type of building and load combination | Factor γƒ |
|---|---|
| Dead load | 1.4 |
| Dead load with wind load and imposed load | 1.2 |
| Dead load when it counteracts the effects of other loads | 1.0 |
| Dead load when restraining sliding, overturning or uplift | 1.0 |
| Imposed load | 1.6 |
| Imposed load acting with wind load | 1.2 |
| Wind load | 1.4 |
| Wind load acting with imposed load | 1.2 |
| Storage tanks including contents | 1.4 |
| Storage tanks empty, when restraining sliding, overturning or uplift | 1.0 |
| Exceptional snow load (due to local drifting on roofs ) | 1.05 |
Serviceability limit state (SLS) corresponds to the limit beyond which the specified service criteria are no longer met. Serviceability loads are generally taken as unfactored imposed loads, there are some exceptions. Further guidance is given in Clause 2.5.1 of BS 5950-1:2000. Serviceability criteria include deflection, vibration and durability.
Further detailed information is given in:
The Handbook of Structural Steelwork
Steelwork Design Guide to BS5950-1 : 2000
![SteelConstruction.org - All the information you need about steel construction - Go to home page [Accesskey '0']](/templates/images/logo_site.gif){kind=logo}