capacity design

NSCC2009

1 INTRODUCTION

Seismic loads and explosion loads belong to the family of exceptional actions which demand the best structural performance of the global structure and of its details. Deliberate conceptual design considering the structural response including dynamic effects has to be performed. The key is-sue is the control of the nonlinear response allowing for the dissipation of energy by plastic defor-mations and the prevention of sudden failure of vital members or details.

The control of the nonlinear structural response requires a clear identification and definition of structural parts intended to develop plastic mechanisms and of members and details which must provide sufficient resistance in order to allow for the formation of the intended mechanisms. This concept is called capacity design, where the resistances depending on the structural response con-cept are designed and verified in relation to each other.

2 INVESTIGATED STRUCTURE

2.1 General characteristics The investigations and design against exceptional loads have been performed for the main building belonging to new gas and steam power plants erected in Karst, Norway. Figure 1 shows the main

ABSTRACT: The contribution describes the common requirements and differences between design concepts and capacity design rules for structures required to resist seismic actions and a blast wave caused by an external explosion. As exceptional actions like accidental explosion are hardly to predict, various scenarios need to be investigated and evaluated with regard to the provision of maximum redundancy and robustness. Aiming at the limitation of effective explosion loads ventilation mechanisms can be provided e.g. by controlled failure and opening of the faade or roof decking at a certain level of overpressure. This concept requires also the consideration of capacity design rules by controlling the collapse of the faade ele-ments and preventing failure of the main structure or its parts due to unintended overstrength of these elements. This concept was used for the design of a power plant in Norway which is situated in a seismic zone and adjacent to a refinery which was identified as a possible source of the explosion load.

Capacity design concept for resistance to exceptional loads

B. Hoffmeister1 1Institute for Steel Structures, RWTH Aachen University of Technology, Aachen, Germany

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structure before and during installation of the faade. The dimensions and key characteristics of the steel structure are: Dimensions L W H: 54m 29m 25m Moment resisting frame with fixed column bases (transverse), bracings (longitudinally) Roof structure constructed of truss girders

Figure 1. Steel structure of the main building and installation of wall claddings

2.2 Design situation and requirements The power plant is located in a seismic zone characterized by the effective peak ground acceleration of ag = 0.28g (including an increased importance factor corresponding to a return period of 10000 years) and subsoil of type A according to NS 3491. Although not necessarily required for the seis-mic verification (the selected behaviour factor was approx. 1.5) for redundancy reasons the main steel structure was designed to provide energy dissipation. This included the capacity design of the roof girder to columns connections as well as of the column bases and their anchorages to founda-tions and thus allowing for developing plastic hinge mechanisms in the frame.

The scenario concerning the exceptional blast load was based on the assumption of an explosion in the adjacent refinery causing a blast wave with a free field pressure of 15 kN/m and duration of 200 ms (Figure 2). Additionally the reflection coefficients depending on the angle of incidence had to be taken into account (Figure 2). The reflection coefficient for the front and side wall (angle of incidence approx. 45) was estimated to 2 resulting in a peak surface pressure of 30 kN/m.

200 ms

15 kN/m

approx.blast wavedirection

Impulse

t Figure 2. Simplified representation of the blast wave and assumed distribution of the pressure on the walls

The basic requirement which governed the design was the prevention of a collapse of the main

steel structure and of a disintegration of main structural members. There were no requirements or restrictions with regard to: Serviceability conditions (limitation of deformations) plastic deformations were acceptable Damages to light faade elements Operability of the plant after the explosion

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3 MAIN STRUCTURE DESIGN PHILOSOPHY

3.1 General considerations The investigated provisions against explosion actions have been performed according to the guid-ance given by NS 3490 and Eurocode 0 and 1 for exceptional design situation.

The estimation of the structural response and of the action effects needed to consider the particu-lar characteristics of the explosion action: Short duration of action Dynamic response of the main structure and of structural parts Main action effects are delayed with regard to the load as they depend on the response of the

structure (Figure 3).

t

blast load structural response (hyper-elastic)

max. deflection

Figure 3. Time functions of the blast load and the elastic structural response (schematically)

The verification of the resistance of the UMC building (main building containing turbines and

generators) against blast load was performed after the design and erection of the main steel con-struction. Thus the procedure had to consider existing conditions and structural properties and to be performed in a manner of an assessment rather than in a manner of design.

In order to know the redundancy and reliability of the structure it was necessary to estimate the structural performance under extreme conditions. This allowed to assess the required safety margins in order to prevent global collapse. The first attempt was performed under the assumption of a full acting blast load (worst case condition) and provided information on the utilisation of the main structural members as well as the probable failure path so far relevant. In a second step the influ-ence of the faade behaviour (controlled collapse) was taken into account. The expected loads have been compared to the ultimate resistance of the main structure and used for the estimation of avail-able safety margins.

According to the assessment philosophy the verification procedure performed for the UMC build-ings consisted of the following steps: assessment of the performance of the main structure under worst case conditions (full blast

load acting at the building); determination of critical structural parts and of the corresponding structural performance (duc-

tility, stability, capacity of connections); identification of structural parts providing sufficient or not sufficient resistance to the worst

case load; in case the resistance to the worst case is not sufficient: detailed investigation of the behaviour

of secondary structural parts (here faade) and its influence on the load transfer to the main structure;

determination of the action effects (internal forces) considering the influence of the behaviour of secondary structural elements;

verification of critical parts determined above for the modified load conditions; evaluation of additional effects of the behaviour of secondary elements on the performance of

the main structure; judgement of the overall safety of the structure under blast load conditions.

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3.2 Comparison to other load cases The external blast wave was compared to the other laterally acting loads i.e. wind load and earth-quake load. The comparison of blast load to wind and seismic actions shows similarities and differ-ences with regard to the time dependency, load patterns and structural response.

roof mass

blast wind earthq.

blast wind earthquake

elastic limit

deflectiondeflection

deflection

force forceforce

dissipatedenergy

dissipatedenergy

Figure 4. Load patterns of blast, wind and earthquake (horizontal component only) and the corresponding responses of the structure including nonlinear ductile behaviour

The following table summarizes the common characteristic and the differences between the three

types of loading and the typical corresponding response of the structure.

Table 1. Comparison of load and structural response characteristics for blast wave, wind and earthquake load Blast Wind Earthquake load time

very short duration, load decreases rapidly

quasi static, constant load assumed

short duration, load changes over time

load pattern

uniform distribution, unidirectional action

uniform distribution, unidirectional action

non uniform distribution (mass dependent), bi-directional action

response non-linear, mainly unidirectional

linear elastic, unidirec-tional

non-linear cyclic behaviour

It has to be noted that although the blast load pattern is similar to the wind, to some extend the

dynamic response of the structure to the blast is similar to the seismic response in terms of modal shape and frequency.

3.3 Response of the main steel structure The objectives of the investigation of the dynamic response of the main structure were the determi-nation of the dynamic amplification factors in comparison to the blast load when assumed to act statically and the assessment of ductility demands for the structure and its structural elements. The amplification factor was determined using a simplified time step analysis of SDOF elastic substitute systems. The ductility demands were calculated from the energy equivalence of the elastic response and the non-linear response with given elasticity limit (Figure 5).

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elastic limit(=1)

deflection

force

dissipatedenergy

"elastic"energy

ductility demand

1

x

x

Figure 5. Ductility demand derived from the equilibrium of elastically stored energy and dissipated energy

The dynamic amplification factor (relation of the hyper elastic structural response to the peak

pressure) for the entire structure yielded a value of approximately 1; the amplification factors for some structural elements (e.g. vertical beams) were partially significantly higher.

Figure 6. Vertical beams in the front faade with the highest ductility demand (flexural bending, full blast load, left picture) and main truss frames loaded by the blast wave (right picture)

The maximum ductility demands determined from the investigations reached values of 4,5 for the

response of the entire building and 7 for the response of single structural elements. These values are comparable to ductility demands of a highly ductile seismic resistant structure. Also the global re-sponse of the frame in transversal and longitudinal direction was similar to the response of the structure to seismic actions, thus the capacity design rules for the main structure were applicable to the blast load. An exception are the vertical beams of the front faade which need to resist very high flexural bending moments and corresponding shear forces at their bases and at the connections to the roof level. These details have been verified according to capacity design rules under the assump-tion of a full plastic hinge in the span of the beams.

4 RESISTANCE OF CLADDINGS

4.1 Ventilation concept The concept of ventilation through the faade aimed at the limitation of the maximum load being resisted by the light weight elements and thus limiting the loads transferred to the main structure. In either case the light gauge elements had to resist the loads resulting from the basic load situations (i.e. wind and snow); the verifications of sufficient resistance to the ordinary load cases has been done using design resistance values and appropriate load combinations.

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The limitation of the ultimate resistance is governed by the expected resistance values which are much higher than the design values and higher than the characteristic values. In order to esti-mate a realistic load level at which the light gauge elements fail test results have been evaluated from reports prepared for the achievement of approvals (Hilti). These expected resistances have than be compared to the action effects at various levels of overpressure in order to determine the governing failure mode and the corresponding load which had to be applied for the final verification of the main structure.

The concept applied to the main building was based on the control of the failure of the wall clad-ding elements at a certain load level and thus allowing the blast wave to pass through the build-ing. In order to achieve the intended ventilation effect the wall cladding elements had to open, that means they had to disintegrate at least partially from the supporting main structure. The me-chanism governing this failure mode was different from the failure mode governing the normal de-sign, where the bending moment in span of the elements was the crucial ultimate limit state.

In order to ensure the intended disintegration of the wall cladding elements numerical investiga-tions considering large deformations (membrane effect) have been performed. The control of the maximum load level has been realized by the provision of connections such that the tensile force in the elements induced by the membrane effect leads to a shear failure (rupture) of the connected light gauge panels.

4.2 Failure modes The ultimate behaviour and ultimate resistance of thin walled profiles is influenced by a number of parameters: development of membrane action formation of plastic hinges at the intermediate supports number of fasteners ultimate resistance and deformation capacity of fastening under shear action (resulting from

membrane action) ultimate resistance of fastenings under tension (uplift action)

The ultimate resistance is generally governed by two failure modes: exceeding of capacity of fastenings due to shear action, rupture of the fastenings at intermediate support (uplift), which immediately leads to a total

failure of the panels at the end supports (valid for panels over two spans). The overpressure loads were determined depending on the ultimate resistance of the panels by

nonlinear calculations considering large deformations (theory 3rd order) and the development of nonlinear behaviour due to formation of local plastic hinges and exceeding of the ultimate shear re-sistance of the fastenings.

4.3 Ultimate resistance of the wall claddings The faade of the Norwegian building consisted of several components (Figure 7): Steel columns supporting the entire faade, Z-profiles (t = 3 mm) supporting the faade elements (flashing), Fasteners fixing the Z-profile to the steel columns, Fasteners fixing the faade element to the Z-profile, The faade element (t = 0,88-1,50 mm).

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Figure 7. Details concerned by the capacity design of claddings and panels used for wall claddings Formation of a plastic hinge in the faade element or the plastic deformation of the Z-Profile

could not be considered as leading to the ventilation effect. Only discontinuation (rupture with opening of the wall) provides the intended effect. Under the blast pressure load the maximum bear-ing capacity of the faade element is achieved by membrane action. The membrane effect develops in the following steps (see Figure 8): The Z-profile unfolds by plastic bending due to the shear force and the eccentricity of the

connecting fasteners, The unfolding leads to an additional length of the membrane (4 cm per Z-profile) which needs

to be considered, The faade member bends and forms together with the unfolded Z-Profile the membrane

shape, The faade elements is in tension and has its highest resistance to the pressure action, In this state the bearing forces to be resisted by the fasteners shall be determined. Pressure re-

sulting in forces higher or equal the maximum resistance of the weakest part of the connec-tions define the limit value of the pressure which can act on the main structure.

HE 1000 L

4

0

,

5

54,5 22 70

~20

1

,

5

3

V

V

N

N

failure due to shear

caused by axial force N

(membrane effect) Figure 8. Development of the membrane effect considered in the calculations of the faade

Depending on the sheet thickness of the light weight gauges the expected ultimate resistance to

axial forces have been determined 21 kN per panel for t = 0,88mm and 33 kN per panel for t = 1,25mm. The corresponding pressure was dependent on the span of the cladding elements. Fig-ure 9 shows an example of the calculation results.

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X

Z

21.44 21.44

68 9

4.15

21.44 21.44

57 8

4.15

21.44 21.44

79 10

4.15

21.44 21.44

911 12

4.15

21.44 21.44

810 11

4.15

21.4421.44

11 4

4.15

22

21.4421.44

24 5

4.15

21.44 21.44

46 7

4.15

21.4421.44

35 6

4.15

21.44 21.44

1012 13

4.15

21.44 21.44

1719 20

4.15

21.44 21.44

1618 19

4.15

21.4421.44

182021

4.15

21.4421.44

20223

4.15

21.4421.44

192122

4.15

21.44 21.44

1214 15

4.15

21.44 21.44

1113 14

4.15

21.44 21.44

1315 16

4.15

21.44 21.44

1517 18

4.15

21.44 21.44

1416 17

4.15

2

20.07

7.57 20.07 7.57

Figure 9. Ultimate state of the wall element governed by the tensile force and corresponding pressure

From the comparison of the ultimate limit states of the wall panels to the corresponding distrib-

uted loads achieved by the calculations the maximum external overpressure transferred to the main structure has been determined to approximately 10 kN/m which provided a significant reduction of load compared to the full blast load of 30 kN/m.

The external overpressure of 10 kN/m has been used for the second stage of the verification of the main structure against collapse. Under consideration of the ventilation effect major parts of the steel structure remain in elastic range; the ductility demands for the most critical parts (vertical beams in the front faade) have been reduced significantly.

5 CONCLUSIONS

The investigations have shown that it is possible to achieve a sufficient resistance and robustness of building subjected to exceptional actions like explosion by applying the fundamental rules of per-formance based conceptual design and by observing capacity design rules for the details.

The capacity design rules applied to the main steel structure are very similar to those known from the seismic design. They provide sufficient robustness to cover uncertainties concerning the load as-sumptions and the performance of crucial elements.

The concept of ventilation was realized by applying indirect capacity design limiting the resis-tance of the claddings and proving the resistance of the supporting structure against conservatively assessed resistance values of the elements intended to collapse.

6 REFERENCES

[1] EN 1990, 2002, Basis of structural design [2] EN 1991-1-7, 2006, Actions on structures, General actions, Accidental actions [3] FEMA 426, 2003, Reference Manual to mitigate potential terrorist attacks against buildings [4] Hilti Untersuchung ENPH3-21 L15, Dokument 42/87, 1987, Tragfhigkeit des Hilti-Setzbolzen

ENPH3-21 L15 auf Stahl der Qualitt Fe 510 Statische Querzugbeanspruchung (not published) [5] NS 3490, 2004, Design of structures, Requirements to reliability (in Norwegean) [6] UFC 4-023-03, 2005, Design of buildings to resist progressive collapse [7] E.R. Vaidogas, 2005, Explosive damage to industrial buildings: assessment by resampling limited exp erimental data on blast loading, Journal of Civil Engineering and Management

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