ncci simple construction” - concept and typical frame.pdf

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NCCI: "Simple Construction” - concept and typical frame arrangements

SN020a-EN-EU

NCCI: " Simple Construction” - concept and typical framearrangements

Introduces the concept of 'simple construction' for multi-storey buildings. Lateral stabilityis provided either by stiff cores or by braced bays; global analysis is elastic and joints are

nominally pinned.

Contents

1. Introduction 2

2. Design: Beams 2

3. Design: Columns 2

4. Design: Connections 2

5. Design of stiff cores or braced bays 3

6. Typical frame arrangements 3

7. Basis in EC3 6



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1. Introduction

1.1 GeneralThis design method is applied to frames where either bracing or stiff cores provide strength

and stiffness to resist lateral forces and ensure lateral stability. The method is simple to use in

design; and leads to economical structures, see SS047. Its use in the UK and elsewhere has

made a substantial contribution to the market dominance of steel frames in low rise

construction.

The sway stiffness of the frame is considered in SN004, SN028 and SN047.

1.2 Global Analysis

The method uses elastic analysis and assumes pinned connections at a nominal eccentricityfrom the column face that is defined in SN005. The introduction of pins at each connection

makes the structure statically determinate, simplifying the task of size selection for both

columns and beams.

2. Design: Beams

Beams are designed assuming they are simply supported because of the use of ‘simple’ joints

(‘nominally pinned’ joints) as EN 1993-1-8 § 5.1.1(2) and Table 5.1 and § 5.2.2.2.

3. Design: Columns

Columns are designed assuming they must resist both the axial compression and, where

appropriate, a nominal moment from the connection to the beams. See SN002, SN012 and

SX010.

In accordance with EN 1991-1-1 § 6.2.2(1), the design of column in buildings should assume

that floors are uniformly loaded, so pattern bending need not be considered.

4. Design: Connections

As discussed below, the classification of the joints as ‘nominally pinned’ may be justified on

the basis of experience of previous satisfactory performance in similar cases. Therefore, it is

advisable to use traditional joints and normal geometry. SN013, SN014, SN015 and SX012

provide this information for ‘simple’ end plank connections and SN016, SN017, SN018 and

SX013 for fin plate connections.



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5. Design of sti ff cores or braced bays

Stiff cores are commonly formed by reinforced concrete walls around the staircases and liftshaft areas. A typical plan and elevation is shown in Figure 6.1.

Stiff panels are most commonly formed by steel cross-bracing. Cross-bracing arrangements

need to be carefully arranged to allow for doorways. Typical plans and elevations are shown

in Figure 6.2, Figure 6.3 and Figure 6.4.

6. Typical frame arrangements

A

A

1

1

2

A - A

1. Concrete core walls

2. Doorways

Figure 6.1 Concrete core surrounding stairs, lifts, service shafts etc.



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Figure 6.2 Plan showing a stiff core of cross braced panels

Figure 6.3 Frame restrained by stiff panels not grouped as a core



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Figure 6.4 Elevation showing typical bracings



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7. Basis in EC3

This tried-and-tested approach to braced frame design complies fully with the Eurocodes.Key points are addressed below:

1. ‘Nominally pinned’ connections

EN 1993-1-8 § 5.2.2.1 (2) states that ‘A joint may be classified, on the basis of experimental

evidence, experience of previous satisfactory performance in similar cases or by

calculation based on test evidence’. There is substantial experience of previous satisfactory

experience of these connections. They had been used for many decades in most low-rise steel

frames in the UK and elsewhere.

2. Treatment as braced frame

EN 1993-1-1 gives no definition of a braced frame ENV 1993-1-1 § 5.2.5.3 previously stated

that ‘A frame may be classified as braced if its sway resistance is supplied by a bracing

system with a response to in-plane horizontal loads which is sufficiently stiff for it to be

acceptably accurate to assume that all horizontal loads are resisted by the bracing system’.

This is the case in frames where the beam to column joints are ‘simple’ joints.]

3. Resistance to horizontal forces The stiff cores/braced bays should be designed to resist the entire horizontal loading from

external actions and frame imperfections using EN 1993-1-1 § 5.3.2. SN028 demonstrates

that imperfections may be considered by using horizontal force coefficients of 0.5% of

vertical loads.

4. Second order effects As discussed in SN001, stiff cores and panels sized for resistance to ultimate limit state forces

may lead to frames with α cr < 10 in which case they must be designed considering the effects

of deformed geometry (second order effects). SN028 demonstrates that using horizontal force

coefficients of 2,5% of vertical loads and limiting the stresses in the bracing will always

ensure α cr >10, giving a simple design process by avoiding second-order analysis.



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Quality Record

RESOURCE TITLE NCCI: "Simple Construction” - concept and typical framearrangements

Reference(s)

ORIGINAL DOCUMENT

Name Company Date

Created by C M King SCI Oct 2005

Technical content checked by A S Malik SCI Oct 2005

Editorial content checked by

Technical content endorsed by thefollowing STEEL Partners:

1. UK G W Owens SCI 23/5/06

2. France A Bureau CTICM 23/5/06

3. Sweden B Uppfeldt SBI 23/5/06

4. Germany C Müller RWTH 23/5/06

5. Spain J Chica Labein 23/5/06

Resource approved by TechnicalCoordinator

G W Owens SCI 31/8/06

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