Primary structure

[edit] Introduction

The term primary structure can be loosely defined as all of the components in the building that are necessary in order to keep the structure standing.

[edit] Primary and secondary structural frames

A primary structural system is typically designed by a structural engineer. It is sometimes referred to as the primary structural frame.

The International Building Code (IBC) determines how structural frames should be protected from fire in a manner that preserves the stability of the building based on their relationship to essential elements of the structure. Regarding regulation for the purposes of required fire resistance, the IBC divides structural frames into two categories: primary (essential to the vertical stability of the building under gravity loads) and secondary (structural members that are not directly connected to columns or supporting gravity loads). Provisions regulating primary structural frame members generally are more restrictive than those regulating secondary members due to the increased loading conditions and lack of redundancy associated with them.

According to International Code Council’s Significant Changes to the International Building Code 2021 edition, primary structural frame members include:

1. Columns.
2. Structural members having direct connections to the columns, including girders, beams, trusses and spandrels.
3. Members of the floor and roof construction having direct connections to the columns. This may include concrete slabs that span horizontally between supports and can sometimes be used as floor and roof systems.
4. Members that are essential to the vertical stability of the primary structural frame under gravity loading. This includes bearing walls or sheer walls which are assemblies rather than single components. These can include structural panels that can resist lateral forces (such as wind and seismic loads) that could act on them.

The following components are considered secondary structural members and not part of the primary structural frame:

1. Structural members not having direct connections to the columns.
2. Members of the floor construction and roof construction not having direct connections to the columns.
3. Bracing members that are not designated as part of a primary structural frame or bearing wall.

[edit] Primary structures and multi-storey steel construction

In multi-storey buildings, there are many factors that can influence the design of the primary structure. This can include:

• Floor layout. An open floor plan may be desirable in situations where it is necessary to create flexible interior space.
• Cladding systems. The choice of cladding system may include components that are attached to the primary structure of a building to form a non-structural external surface.
• Planning requirements. Height restrictions in an area can influence plans for multi-storey buildings. Site conditions can prevent certain types of foundation systems and their locations.
• Building service integration. Effective integration of services can dictate decisions regarding primary structure design.

[edit] Modern Methods of Construction

The term primary structural systems is also used in reference to Modern Methods of Construction, Introducing the MMC definition framework, published by the Ministry of Housing, Communities & Local Government, which became the Department for Levelling Up, Housing and Communities in September 2021. The term is used in relation to the seven defined categories for MMC.

For more information, see Pre-manufacturing components (non-systemised primary structure).

[edit] Related articles on Designing Buildings

• Cladding for buildings.
• Concept structural design of buildings.
• Off-site prefabrication of buildings: A guide to connection choices.
• Pre-manufacturing components (non-systemised primary structure).
• Structural principles.
• Structural systems for offices

[edit] External resources

• Gov.uk, Modern Methods of Construction, Introducing the MMC definition framework.
• International Code Council, Significant Changes to the International Building Code 2021 edition.