MULTI-STOREY STEEL STRUCTURES

Cont.

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STRUCTURAL ELEMENTS OF MULTI-STOREY STEEL BUILDINGS

Columns carry important vertical axial loads, and bending moments ; their shape depends on: - magnitude of the loading (axial loads are predominant); - the technology for assembling at site the prefabricated units on the height of the

building ( columns cross section may be changed at every 3-4 storeys, increasing downwards to the bottom of the building);

- other economic arguments. The steel elements are hot rolled profiles I, H, or other shapes obtained by welding, opened

or closed sections.

Typical sections for columns and the variation on the height of the building

Connections of the prefabricated units of the columns made at the building site: welded or bolted

1. Columns

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The base of the columns

• Specific features of the columns of multistorey structures: Axial loads are heavy at the foundations level; If the structure has a bracing system (commonly the case) the bending moments are rather

small; • The base plate is thick (30...100 mm); the surface is very well polished also the head of the

column (the cross section) for making the contact tight; • The base plate is cast in concrete with the help of the anchorage bolts; • Other elements for strengthening the base plate are: ribs, diaphragms, cross pieces, the details

being common with those used for the base plate of the columns of the industrial buildings.

Base plates of the columns for the high-rise buildings (generally hinged or taking very small moments)

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2. Beams and girders of the structural frame

• The joints between the columns and the girders are hinged or fixed (rigid); • The beams sustain the floor and are supported by the girders (the details of supports are designed

considering the effective height of the storey and the thickness of the floor itself); • Both beams and the girders are elements in bending; the static system is usually continuous; • If the loads are important or the span of the girder is rather big lattice girders are preferred. • Beams are designed as continuous, the redistribution of the bending moments reducing the

maximum values of hogging moments with about 15% (important reduction of material are possible also by using class 1 and 2 sections) .

• If spans are big lattice girders or trusses with parallel chord are used to carry the loading from the floor above.

Redistribution of the bending moments in the case of the continuous beam

8

lqM

2

0

0201 M2

1M;M

11

8M

22

21 ql1057.0M;ql0779.0M In elastic:

In plastic:

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3. Structural connections between girders and columns of the plane frame 1. Simple method for classification: a) the joint takes only the reactions from the girders b) the joint takes the reactions and the bending moments

2. More accurate analysis of the connection between columns and girders based on a simplified linear behaviour – linear variation of stiffness with respect to rotation, (although the behaviour is not linear) give a realistic classification:

a) from the point of view of the stiffness for the rotation: articulated joints; rigid joints; semi-rigid joints;

b) from the point of view of the capacity of the connection: articulated joints; full strength joints; partial strength joints;

c) from the point of view of the design of the connection itself: welded connections; bolted connections;

d) as constructive method applied: strongly stiffened; stiffened; not stiffened

Moment resistant MRd – rotation curve; Sj – stiffness of the connection

Strongly stiffened Stiffened Not - stiffened

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Classification of the structural connections in relation with the curve moment-rotation and their components: a-rigid joints; b-semi-rigid joints; c- articulated joints

Different design details of articulate, semi-rigid and rigid joints

a b c

c

According to SR EN 1993-1-8, the joint must be verified with respect to the resistance of all the concurrent components and the failure mechanisms.

The joint is made by the basic elements (column and girders) and the other elements that take part of the connection, like: end plates, splices, ribs and diaphragms, other additional plates, welding and/or bolts.

The resistance of the joint is determined on the basis of the resistances of all these parts, taken separately after their behaviour and their failure mode are put in evidence.

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FLOORS AND WALLS SYSTEMS USED IN MULTI-STOREY STEEL STRUCTURES

Common solutions for floors are:

- reinforced concrete cast in-situ floors,

- prefabricated units assembled at the building site, generating a system of composite structure;

- sheeting decks on which light concrete is put in order to insure the rigidity;

- corrugated steel plates welded to the top flange of the steel beams (the steel plate is stiffened).

The steel sheeting may collaborate with the reinforced concrete or may be just a shuttering element for the cast concrete on site.

Solutions for the floors of the multi-storey buildings- steel beams and reinforced concrete slabs, cast at site or

prefabricated: 1- beam; reinforced concrete; 3- steel sheeting; 4-

prefabricated unit in reinforced concrete; 5- light concrete prefabricated elements with holes; 6- monolithic concrete

Solutions for floors- steel decking with cast in site reinforced concrete slab at the top; a- steel used only for shutting; b,c-

composite structural floor with horizontal and vertical studs

Types of steel sheeting and decking for the steel floors

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Internal walls The most common solution consists in light prefabricated units in one or more layers-a rigid part, the phonic and thermal insulation and the finishing coating; a rigid frame and elastic restraining elements at the top and bottom part insure a good reaction to the horizontal displacements of the whole structure (1/100h…1/150h, h being the storey height); External walls Generally the solution is the curtain walls, developed on 2...3 storeys height; the so-called second order structure is in fact made of frames with horizontal beams- transoms and vertical members-mullions, inside of which a multi-layer flat element is obtained. A rigid layer takes the loads from the wind pressure and protects from rain transferring these horizontal loads to the frame of the wall. The wall framing transfers these loads to the main structural frame by the means of the joints. The thermal, waterproof insulation and the finishing coating layers determine a wide variety of these systems

Wind action on the external walls: a- frames with rigid joints; b- hinged joints; c- columns and reinforced concrete floors

FLOORS AND WALLS SYSTEMS USED IN MULTI-STOREY STEEL STRUCTURES

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HORIZONTAL MAXIMUM TRANSLATIONS AT THE TOP OF THE BUILDING

Horizontal displacements produced by wind (or seismic) actions must not affect the general stability and the comfort of habitants (serviceability restrictions): δ =< δa;

The fundamental period of vibration of the structure must not have to be the same with that induced by the wind gusts because of the resonance phenomenon; the acceleration of the movements of the structure must not exceed the limits of 0.5m/s2 (≈0,5g);

The total displacement of the structure δ is determined by making a sum of the static and dynamic displacements (δs, resp. δd);

The sway of the current floor is necessary to be determined in order to avoid the deterioration of internal walls.

Ha

800

1...

200

1 sd 5.0

ds

Horizontal translations at the top of the high buildings and the effect of the dynamic actions

(cumulative maximum deflections)

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FIRE PROTECTION OF MULTI-STOREY BUILDINGS WITH STEEL STRUCTURE

•General design measures 1.Every storey is provided with fire protection walls (in reinforced concrete)-the stairs and the lifts are marked with fire protection walls; 2.The evacuation has to be provided in the shortest possible time (in the World Trade Centre the evacuation of the 55000 persons was designed to be completed in 5 minutes); 3.Storing a minimum quantity of water at every level (at the World Trade Centre this is 18500 l); •Special protection against the fire action of the steel structural elements •The systems adopted for protection must consider: 1.The critical temperature at the surface of the steel element; 2.The fire rate class (minimum period of time of resistance of the element against fire); 3.The cross section of the structural element (due to the ratio between the perimeter and the area of the section). ! The critical temperatures for the steels S235 and S355 are 560oC and 580oC. If the provisions of structural strength of the redundant structures are taken into account, the critical temperatures increase: 650oC for S235 and 670oC for S355.

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Solutions for fire protection for the columns of high rise buildings; a, b- internal columns; c, d- columns and mullions externally placed in the facades

-Ducts for the building service circuits placed inside the box section of the column (prefabricated

elements for fire protection around the column); -Encasing the beams and girders in concrete and

other insulating layers

CONSTRUCTIVE DETAILS FOR PROTECTION OF THE STEEL STRUCTURAL MEMBERS

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THE BRACING SYSTEM ACCORDING TO EUROCODE 3 SPECIFICATIONS

All the systems of braces used to stiffen the multi-storey steel structures are based on some fundamental aspects:

- the bracing system is designed to take all the forces coming from direct external loading and the effects of the imperfections of the system;

- the bracing system is designed to take in addition the effects of vertical and horizontal forces acting on the main structural elements (because the bracing system insures their stability); it takes also the equivalent forces due to initial imperfections of the structural elements;

The bracing system is a plane girder, fixed in foundations (at the ground level). The connection between any vertical bracing and the adjacent columns is obtained with longitudinal elements - girders that are considered with infinite stiffness ► the horizontal forces acting in the joints of the vertical bracing system at a certain level will determine translations of the ends of the columns identical with the translations of the joints of the bracing elements.

The elements of the bracing system are designed considering the combination of internal forces from the effect of horizontal and vertical forces directly applied on the bracing system from external actions and from horizontal bound forces between the bracing system and the adjacent row of columns. The last ones are determined by the P- effect and supplementary equivalent forces corresponding to global imperfections of the whole structure.

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P- Effect

The P- effect

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1

iii1ii1ii VtgVDDsinDDH ii VH

ii1i1ii DDH

iiiii DDH 11

Horizontal translations of the structural joints

under horizontal and vertical actions

The joints of the bracing system will have horizontal translations that vary linear on the height of the structure. The force necessary to fix the elements in the connection will then be (the angle is very small);

and at the level i the whole force acting on the bounded connections will then be:

•The real behaviour is that as the variation of the horizontal translations is in fact different from level to level. The effect P- will then be described by the horizontal force at the level i:

•Then the real total force acting at the level i will be:

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RECOMANDATIONS FOR THE DESIGN OF MULTI-STOREY STEEL STRUCTURES TO SEISMIC ACTION ACCORDING TO EN 1998-1; P100-2006

Classification of the steel structures from seismic point of view As an important design aspect for these structures is the dynamic behaviour, the classification of the structural types is based on the consideration of the dissipation of the energy coming from earthquakes of any other dynamic action (wind, explosion, impact). If the dissipation rate is a classification criterion then we may consider the multi-store structures into the following categories: • Frames with rigid connections subjected mainly to bending from horizontal forces; the dissipation zones

are placed in girders close to the connection with the columns in the potential plastic hinges, these areas being subjected to cyclic bending.

• Frames with concentric braces - braces in X or in tension only; the dissipation zones are placed in the members that are designed as braces and are subjected to cyclic tension; - braces in V taking the seismic effect both by braces in tension and in compression; - braces in K for which there is no dissipation of energy (q=1) because the connection between the brace and the column suggests the formation of the plastic failure mechanism (plastic hinge) on the column.

Types of frames for multi storey steel structures: a)- rigid connections; b)- concentric braces, in X or diagonal; Concentric braced frames with V braces

a, b,c)- V centred braces; d) K braces

d

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• Eccentric braced frames to which the bracing elements resistant to horizontal forces develop axial internal forces but the seismic links are the dissipative elements; the hysteretic energy is dissipated through cyclic bending or cyclic shear;

Eccentric braced frames

• Structures with reinforced concrete cores or walls for which the dissipation of the energy is performed by these elements in cyclic shear.

Structures with reinforced concrete cores or walls

• Combinations between the moment resistant frames and braces (dual structures)

Dual structures with centric diagonal braces in X

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