1) Multi-storey steel structures use steel columns, beams, girders, and bracing systems to support vertical loads and resist lateral forces. Columns vary in cross-section depending on load and may be welded, bolted, or have base plates anchored in concrete. 2) Beams and girders are designed to bend and can be continuous or use lattice girders for large spans. Connections between columns and girders vary from articulated to rigid. 3) Floors commonly use steel beams with cast-in-place concrete slabs or prefabricated decking. External walls are often curtain walls comprising mullions and transoms.

1. MULTI-STOREY STEEL STRUCTURES
Cont.
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2. STRUCTURALELEMENTSOFMULTI-STOREYSTEELBUILDINGS
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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3. Thebaseofthecolumns
• 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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4. 2.Beamsandgirdersofthestructuralframe
• 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
lq
M
2
0


0201 M
2
1
M;M
11
8
M 
2
2
2
1 ql1057.0M;ql0779.0M In elastic:
In plastic:
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5. 3.Structuralconnectionsbetweengirdersandcolumnsoftheplaneframe
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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6. 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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7. FLOORSANDWALLSSYSTEMSUSEDINMULTI-STOREYSTEELSTRUCTURES
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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8. 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
FLOORSANDWALLSSYSTEMSUSEDINMULTI-STOREYSTEELSTRUCTURES
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9. HORIZONTALMAXIMUMTRANSLATIONSATTHETOPOFTHEBUILDING
 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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10. FIREPROTECTIONOFMULTI-STOREYBUILDINGSWITHSTEELSTRUCTURE
•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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11. 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
CONSTRUCTIVEDETAILSFORPROTECTION
OFTHESTEELSTRUCTURALMEMBERS
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12. THEBRACINGSYSTEMACCORDINGTOEUROCODE3SPECIFICATIONS
 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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13. P- Effect
The P- effect
H
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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14. 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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15. • 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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