lecture 2 s.s. iii continuare design of steel structures - faculty of civil engineering iaşi

Lecture 2 continuare

STEEL INDUSTRIAL BUILDINGS

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Steel Industrial Buildings

SPECIFIC BUILDINGS FOR INDIVIDUAL PROCESSES

UNIVERSAL (FLEXIBLE) DESTINATIONS

steelworks with Martin kilns or with converters, rolling mills, workshops for coke production, etc.

various kinds of industrial activities developed or modified/changed during the service life of the building: workshops, wear-houses, showrooms.

1. CLASSIFICATION

2. SPECIFIC FEATURES OF INDUSTRIAL BUILDINGS

Regular shapes in plan: - rectangular, -oblong several rectangular interconnected.

- mono-pitched or duo-pitched roof in transversal plane - small slopes of the roof: under 300

-single storey buildings (rarely two stories or more)

Plane shape of the industrial buildings: separation in blocks between them tolerance distances being used

The type of the industrial building The limit braced block 1) The limit length of a building 2)

Heated 90 m 230 m

Non-heated and with exothermic processes 75 m 200 m

Trestle bridges (portal frames and conveying

bridge for outside activities)

50 m 130 m


Distances between the blocks of buildings

1) Limit braced block - the maximum distance between the end of the building and the axis of the vertical bracing;

2) Limit length of a building - the maximum length of a building between two tolerance distances along the building.

Position of the tolerances between blocks in the industrial buildings

• Observations

• Steel structures may adopt bigger tolerance distances than structures in reinforced concrete, due to the superior characteristics of the steel. Deformations are easier to be absorbed at the level of mechanical connections; also, the cycles of loading having as a result the development of stresses in plastic domain, safe from exploitation point of view, are consuming important quantities of energy (high dissipative structures);

• Steel columns are placed at bigger distances and flexible enough to deform under the variations of temperature;

• In the case of a structure that combines columns of reinforced concrete with steel rafters or trusses, constructive conditions for the reinforced concrete elements must be considered;

• For structures that carry steel crane girders tolerance distances must be considered up to 120 m.

• Between two units (braced blocks = rigid blocks) the tolerance distances imposed by the seismic provisions are 1…2 m.


• Transversal section of the industrial buildings

The structure of the building is obtained from several transversal plane frames having one or more spans placed at certain distances, named bays.

Two running transversal frames determine the basic module of the building.

Modern industrial buildings have an in plane layouts with dimensions ranging from 10.00 m up to about 40.00 m for spans and 4.00 m up to 15.00 m for bays.

The transversal frame is the main structural system of the building consisting in columns and girders interconnected and its design depends on the dimensions imposed by the technological processes - equipment, devices and various systems for lifting and transportation both horizontally and vertically inside the building.

• The roof of the industrial buildings are mono-pitched or double pitched, the slope being usually small (3...5%).


Structural System of the Steel Industrial Buildings

VARIOUS NON-STRUCTURAL ELEMENTS:

STEEL, MASONRY, AND GLASS ROOF DECKING;

WALLS SHUTTING MADE OF STEEL SHEETING OR SANDWICH PANELS

–TRANSVERSAL -THE SPAN FRAMED STRUCTURE - LONGITUDINAL - THE BAY

For buildings equipped with cranes the crane girders may be structural elements

STRUCTURAL ELEMENTS COLUMNS

RAFTERS (STEEL GIRDERS OR ROOF TRUSSES)

Modern Industrial Steel Buildings Heavy or Light Industry

Structural Solutions for the Transversal Frame

c)-three spans frame equipped with travelling cranes; d) two central spans frame with two auxiliary sides frames with lower heights; e) -several transversal frames

a)- one span with double pitch roof truss; b) central double pitched frame with transversal skylight and two side with mono-pitched frames

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Steel roof trusses for industrial buildings


Steel columns for industrial building: a) simple sections and b) compound sections

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Transversal frame for buildings equipped with crane travelling on runway longitudinal girders: 1- crab

Top part of the steel columns with two different sections

Different profile shapes of steel sheeting used for roofing:

a)- corrugated; b)- with trough; c)- deck trough; d) – deck trough

with tray; e) – build-up deck


The roof cladding may be placed directly on the top chord subjecting the bars to compressive forces and bending moments; if the roof is heavy and/or the span and the bay are big the stresses increase substantially. The design solutions are: - to diminish the length of the bars at the top chord by introducing intermediary struts; - to strengthen the cross sections of the bars at the top chord to cope with combined

stresses from N and M including the stability verifications.

In order to avoid these inconveniences we use purlins placed on the top chord in the truss internal joints


Roof truss for wide spans and heavy loading: a) constructive solutions; b) hinged joint between truss and column a

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Layout and Design of the Bracing System

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braces designed only for tension braces designed to tension:

-if the braces are articulated in j:

-If the braces are not articulated in j:

-diagonal i’k’ is compressed and out of service.

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braces designed for compression

Upon the transversal frame act horizontal and vertical loads in its plane; the frame is not able to take loads normal to its plane, longitudinal to the building.

The stiffness of the whole structure is insured by the longitudinal elements placed between the plane frames.

The bracing system insures a spatial character and a corresponding stiffness and is designed to provide the following requirements: a spatial collaboration between all the transversal frames, small deformations of the structure under horizontal transversal actions, stability during the assembling stage, reducing of the effective lengths of the structural elements in compression and

bending, taking over the horizontal loads due to wind actions and the surge effects due to

cranes. The following structural elements are usually braced: the roof trusses; the skylight (if it exists and has relevant dimensions); the columns; the crane girders (if the building is equipped with cranes).


Braces for the roof truss They divide themselves in the following distinct categories: - horizontal longitudinal bracing at the bottom chord of the truss, their position being along the structure in the external panels determined between the joint in the supports of the trusses and the next joint by certain longitudinal elements (rods). The destination of the horizontal bracing is to take the horizontal reactions from the top part of the intermediary columns that carry the longitudinal walls. If the truss is fixed to the column, then the bottom chord of the truss being in compression, has to be provided with bracing in the second panel. The positions of the bracing in the case of multi-spans structures when both the heights of the columns and the lift capacities of the cranes have small or medium values are presented and also for big heights and heavy lift capacities.


Bracing systems at the bottom chord of the truss: a) - ties are placed in the truss joints; b)- ties are placed intermediary between the truss joints; 1- transversal bracing; 2- longitudinal bracing; 3-ties

Longitudinal bracing system in the plane of the roof: 1- at the top; 2- at the bottom; 3- intermediary column


- horizontal longitudinal bracing at the top chord of the truss, are mostly used for the case when the height of the columns and the lift capacities are small. They are placed in the same positions only they are part of the roofing system as plane lattice girders, their flanges being the purlins. If the roofing system is not able to sustain wind action alone, then the following conditions are necessary to be verified:

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The longitudinal horizontal bracing of the truss reduce the deformations of the structure at the roof level from the effect of the important horizontal transversal loads.

The static scheme for design of the horizontal longitudinal bracing is a continuous lattice girder, the internal supports being the structural joint of the transversal frame between the column and the rafter.


Different layouts of the bracing system at the top and bottom chord of trusses: a) braces in the boom in case of fixed connection between the truss and the column; b) braces in case of light cranes and small heights

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- horizontal transversal bracing from the bottom and the top chord of the truss are placed normal to the longitudinal axis of the structure, in its end bays, also in the bays next to the tolerance distance between adjacent blocks and at every 50...60 m along the structure. Some of the purlins are parts of the system of horizontal bracing (under the skylight the purlins are missing so rods have to be placed). The role of horizontal transversal bracing at the bottom chord of the truss is to insure its lateral stability. The bracing system at the top chord of the truss takes the horizontal reactions coming from the top of the intermediate columns of the gable walls (reactions coming from the wind action on gable)


Design of the internal members of the horizontal transversal bracing

In design and computation of internal forces the bracing system is assimilated with a lattice girder with parallel chords.

-vertical bracing of the marginal and central struts of the truss are placed in the bays containing horizontal transversal bracing and intermediary at 3-5 bays along the structure (in order to insure the stability during the assembling stage).

They may also be placed under the supports of the skylight and in the case of the overhead travelling cranes at the joints of the bottom chord of the truss.


Vertical bracing system of the struts of the truss: marginal and central

Braces of the columns: - Vertical bracings of the columns are placed in the axis of the columns longitudinally along the structure. In the end bays, temperature effects must be taken into account and only a flexible rod is placed at the top part of the column. Generally, the bracing is placed both at the top and at the bottom part of the column in a bay situated in the middle of the longitudinal axes, or if the building is long, at L/3, L being the longitudinal dimension. - horizontal bracings of the columns are used in the case when the bays are very big their purpose being mainly to reduce the effective length of the intermediary columns belonging to the structure of the longitudinal walls; they also transfer a part of the horizontal forces due to wind action on walls to the structural columns. In choosing the cross sections of the bracings the governing verification relationship is for limit slenderness, their cross sections resulting rather small.


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lecture 2 s.s. iii continuare design of steel structures - faculty of civil engineering iaşi

Engineering

blocks of buildings

modern industrial buildings

steel rafters

transversal plane frames

observations steel structures

b c d e steel roof trusses

duopitched roof

running transversal