Study structural roof
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INTERNATIONAL SCIENTIFIC CONFERENCE CIBV 201530-31 October 2015, Brasov
THE IMPROVEMENT OF BEHAVIOR TOVERTICAL FORCES FOR STRUCTURALROOF ELEMENTS BY RECONFIGURING
THE STATICS SCHEME. CASE STUDY.
TF. GALATANU1 D. TAUS1 I. TUNS1
Abstract: The service level of safety is a main indicator for a building’sstructural performance. The non-fulfilment factors of this criterion aremultiple. An important component is constituted by the increase of the loadsintensity. In this context, the case study presented in this paper wasconducted on a industrial building “type ground floor” and highlights thegrowth of strains in the roof trusses elements, higher than the initial oneswitch ware considered, as a result of modification of prescriptionscontained in the norms for vertical loads evaluation.In this situation, the fulfilment of the service safety to vertical loads wasconsidered to be ensured by trying to redistribute the strains as a result ofchanging the supports.
Key words: steel truss, strain, vertical load, restrain, beam.
1 Department of Civil Engineering, Transilvania University of Braşov
1. Introduction
The concept of safety in exploitationrefers to a whole structural assembly andfor an element or for a steel structure it isstudied through the close-up view of thenotions of bound state and the assurancelevel. In the present context, where a bigpart of the already built foundation hasbeen designed and executed previous to theadoption of the new design rules in civilengineering, the question arise about theway of implementation of the safety levelin exploitation for the structures designedafter old norms.
1.1. General presentation data
The construction representing the focus
of the case study belongs to a group ofbuildings with the end use of „ spaces ofproduction”, specific to the beneficiary'sarea of activity.
The inquired construction area is tooledup with bathrooms intended for the surfacetreatments specific to certain helicoptercomponents, resulting due to the chemicalsubstances used during the technologicalflow, in an aggressive environment for theconstruction elements that define theproduction space.
1.2. Presentation data of the inquiredconstruction
The inquired building, entitled „Surfacetreatments” is part of a production coveredmarket, with a type ground floor.
Proceedings of The International Scientific Conference CIBv 201584
It has three spans of 24 m and 20 bays of6m. Structural assembly has the followingcomponents: isolated foundations,reinforcement concrete columns by precaston site, transversal truss beams with anopening of 24,0m, with bearing on
concrete column and on truss with openingof 12 m, alternating. The surface elementsof the roof are coffer type of 1,50 x 6,00m,mounted at the superior bloom level of thetransversal metal beams (Figure 1).
A
B
C
D
1 2 3 4 5 6 7 8 9 10 11 12
5×12.00m
24.0
024
.00
24.0
0
5×12.00m
Reinforced concrete columsSteel truss beams-12mSteel truss beams-24m
Fig. 1. Structural elements arrangement
1.3. Roof elements description
The steel trusses with a bay of 24 metershave the rails with an integrated sectionformed by two U profiles supported withsmall boards. The shape of these trusses istrapezoidal. The steel trusses with a bay of12 meters are of rectangular shape withparallel chords [1]. This cross beamrepresents a prop for the truss of 24 metersand in turn it props up on thereinforcement concrete columns (Figure2).
Taking into consideration the building'send use, namely „bathrooms for thesurface treatment” of helicoptercomponents, at the roof level there hasbeen a steam release with a considerable
corrosive substance content[2]. Among thesubstances heavily corrosive present in thebuilding, there were: azotic acid,hydrofluoric acid, hydrochloric acid,sulphuric acid [2].
Fig. 2. Structural system for roof
TF. GALATANU et al.: The improvement of behavior to vertical forces for structural roof … 85
2. The case study’s description
The investigated building presentsdifferent degradation stages for thestructural and non-structural elements andmany forms of manifestation of these,according to the materials, the location inthe building, the type and transportmechanism of the aggressive agents.
The degradation state investigationfollowed the level of depreciation of thephysic-mechanic characteristics of thestructural elements, in order to determinethe level of safety in exploitation theseelements provide to the buildings structure,mostly following the endurance, stabilityand stiffness demanding fulfilments.
The investigation processes targetedstructural elements, mostly roof trusses,and consisted in: the visual investigation ofthe degraded areas; the verification of thestructural geometric dimensions usingmeasurements; running non- destructivetests like spectral analysis; runningdestructives tests like traction on extractedspecimens; local scraping.
The findings detached during theinvestigations, the results obtained after themeasurements, the laboratory and on sitedeterminations are presented for the roofstructures elements in this study.
The investigations performed for the rooftrusses revealed a relative good state of thecomponent elements, without obvioussection reductions produced by corrosion(Figure 3), noting some points and stains
unsearchable by the rust in the steelstructure, which penetrated only theprotection layer, weld cords faultyexecuted, the absence of some bracings,deposits of dirt as special on the elementsintersections, on the superior side of thehorizontal elements, and on the supports.
On the trusses supports was noticed thatnot all the nuts wore adequate tight, thelack of some nuts, rusted screws, dirtdeposits and support defects of the metallictrusses ends.
By clearing the surface, wasn’t noticedany texture modifications in steel’sstructure and any geometricalmodifications of the structural elements.
Fig. 3. On site measurements of theelements geometry
The spectral analysis result of thematerial highlighted that the chemicalcomposition of the initial steel was kept,Table 1.
The chemical steel composition Table 1
Measuredvalues
C% Si% Mn% P% S% Cu%
Specimen 1 0,159 0,198 0,76 0,02 0,0025 0,171Specimen 2 0,149 0,35 0,85 0,028 0,0030 0,245
Using the traction tests on specimensextracted from the steel structures elements(Figure 4), the steel’s mechanic
characteristics used for the trusses weredetermined. The specimens were extractedaccording to the technical norms and their
Proceedings of The International Scientific Conference CIBv 201586
dimensions respected SR EN 10025[4].
Fig. 4. Steel specimens for traction tests
The obtained results showed asatisfactory behavior of the steel, without a
significantly deterioration of the elasticand mechanic properties (Figure 5).
After the analysis of the characteristiccurve of the material obtained on steelspecimens a adequate behaviour can beobserved. The steel has a 30-35%elongation and a rate between themaximum stress and yielding stress of1,35.
Forc
e[kN
]
Deplacement[mm]
Fig. 5. The load- displacement curve
To consider the materials ageing effect, a10% reduction of mechanic characteristicsof the steel of the structural elements wasconsidered.
2.2. The structural analysis of themetallic roof trusses
With the purpose of obtaining the safetylevels, therefore of the level ofaccomplishment of the resistance andstability demanding for each elementcomponent of the metallic roof trusses,both buildings were modelled using theautomatic analysis program Axis VM 9.
For the roof steel trusses having a spanof 24,0 m, representing the transversal’sframe beams, two distinct situations wereconsidered:
The trusses are considered freelysupported beam; The trusses are considered continue
beams on three equal spans of 24,0 m;The purpose followed was to obtain a
comparative analysis of the two distinctivesituations and so to establish a solution ofconsolidation which will involve minimalfinancial costs and the necessity tointervene to a minimum number ofelements.
For designing the trusses, U shapesections and steel plate were used and thearticulated grip in the end of the elementswas built with double brackets and weldingcords.
The geometry of the elements ispresented in Figure 6.
TF. GALATANU et al.: The improvement of behavior to vertical forces for structural roof … 87
y y
z
z
z
z
yy
y y
z
z
a) The upper chord cross section;b) The lower chord cross section;
c) The cross-bracing cross section;
Fig. 6. The cross sections of the elements
For every element of the steel truss witha traction strain were established[3]: The value of the plastic strain in the
raw cross section:
0,,
M
yRdplRdt
fANN
(1)
where:A = area of the raw cross section;fy = yielding stress;γM0 = partial coefficient of safety for the
resistance of the cross sections indifferentof the sections class;
For every compressed element of thesteel truss were established [3]: The value of cross sections resistance
to compression:
0,
M
yRdc
fAN
(2)
The value of cross sections resistanceto buckling:
1,
M
yRdb
fAN
(3)
where:χ = the reduction factor for the buckling
form considered;γM1 = partial coefficient of safety for the
resistance of elements to buckling;The trusses schematization and the knots
numbering is represented in Figure 7 andstrains obtained for simply supported beamand continue beam on three equal spansare presented in Table 2.
1500 3000 3000 3000 3000 3000 3000 3000 1500
2000
700
2700
Fig. 7. The numbering of the trusses knots
Proceedings of The International Scientific Conference CIBv 201588
Mechanic characteristics, strains and insurance coefficients for the 24,0 m span rooftrusses Table 2
Nr.
cr
t.
Elem
ente
Elem.
typeSlenderness coeff.
The designforces
resistance ofcross sections
Freely supportedbeam Continue beam
Maxim strains
Deg
ree
of sa
fety
NR
d/NEd
Maximstrains
Red
uctio
nfa
ctor
Tens
ion
Buc
klin
gN
b,R
d
Tension
Com
pres
sion
Nb,
Ed
Tension
Com
pres
sion
Nb,
Ed
Deg
ree
of sa
fety
NR
d/NEd
λy λz χ Nt,Rd Nt,Ed Nt,Ed
[kN] [kN] [kN] [kN] [kN] [kN]
0 1 2 3 4 5 6 7 8 9 10 11 12 13
1 1;3
The
uppe
r cho
rd
46 81 0.599 1409 845 0 432 1.96 -545 0 2.58
2 3;5 46 81 0.599 1409 845 0 1026 0.82 0 352 2.40
313;1
5 46 81 0.599 1409 845 0 1081 0.78 0 6091.39
415;1
7 46 81 0.599 1409 845 0 496 1.70 -82 7411.4
5 5;7 47 84 0.599 1665 999 0 1387 0.72 0 786 1.27
611;1
3 47 84 0.599 1665 999 0 1436 0.70 0 937 1.07
7 7;9 47 560.785
4 1665130
8 0 1561 0.84 0102
4 1.28
8 9;11 47 56 0.785 1665130
8 0 1607 0.81 0110
3 1.19
91;4
Supp
ort’s
diag
onal
54 34 0.785 724 569 -713 0 1.02 -853 0 0.85
1017;1
8 54 34 0.785 724 569 -824 0 0.88 -817 0 0.89
11 3;4
The
diag
onal
46 36 0.843 724 611 0 621 0.98 0 742 0.82
1215;1
8 46 36 0.843 724 611 0 612 1.00 0 600 1.02
13 3;6 68 36 0.712 470 335 -429 0 1.09 -534 0 0.88
1415;1
6 68 36 0.712 470 335 -424 0 1.11 -418 0 1.12
15 5;6 57 38 0.785 574 451 0 657 0.69 0 519 0.87
1613;1
6 57 38 0.785 574 451 0 411 1.10 0 405 1.11
17 5;8 89 370.569
8 385 219 -254 0 1.52 -342 0 1.13
1813;1
4 89 370.569
8 385 219 -249 0 1.55 -244 0 1.58
19 7;8 76 39 0.662 470 311 0 246 1.27 0 332 0.94
2011;1
4 76 39 0.662 470 311 0 241 1.29 0 236 1.32
TF. GALATANU et al.: The improvement of behavior to vertical forces for structural roof … 89
21 7;10 94 39 0.539 385 208 -97 33 3.98 -177 0 2.17
2211;1
2 94 390.539
9 385 208 -101 35 3.81 -88 43 4.37
23 9;10 99 41 0.512 385 197 -36 68 2.91 0 156 1.27
24 9;12 99 41 0.512 385 197 -131 2 2.95 -152 0 2.53
25 2;4
The
low
er c
hord
22 13 0.984 1193117
5 0 0 OK 0115
7 1.02
26 4;6 43 75 0.662 1193 790 777 0 1.53 0 251 3.15
27 6;8 1193 0 1246 0 0.96 -451 0 2.64
28 8;10 1398 0 1507 0 0.93 -780 0 1.79
2910;1
2 1398 0 1561 0 0.90 -898 0 1.56
3012;1
4 1398 0 1557 0 0.90 -896 0 1.56
3114;1
6 1193 0 1300 0 0.92 -673 0 1.77
3216;1
8 1193 0 838 0 1.42 -257 0 4.64
3318;1
9 22 13 0.984 1193117
5 0 0 OK 0 642 1.83
The graphic representation of the resultsafter the processing and interpretation of
the results is represented in Figure 8.
Fig. 8. The graphic comparative representation of strains in trusses elements
Proceedings of The International Scientific Conference CIBv 201590
2.3. Improvement solutions of structurebehaviour of the roof steel trusses
From the comparative analysis of thedata presented in Table 2 results thebenefit effect for the axial strains forsome elements by the modification ofstatic scheme from the freely supportedbeam to continue beams on three equalspans of 24,0 m.
The configuration of the static schemewas obtained by insuring the continuityconnection between the chords of thetrusses. The configuration of the staticscheme leads to a diminution of the axialstrains obtained for the vertical loads to60% for the upper chord and currentdiagonals and 80% to the lower chord. Anexception is the support’s diagonal, who’sstrain is increasing after the change of thestatic scheme.
Choosing this solution implies a fewinterventions on the mechanicconnections in the end of the elements,allowing the transmission of the axialstrains from one chord to the other for twosuccessive trusses, as well as thetransmission of the axial strains from thelower chord to the concrete column.
The solution of improving the rooftrusses for vertical loads by changing thestatic scheme leads to the modification ofthe strains in the component elements inintensity and in direction, as is shown inTable 2.
For the purpose of restoring theresistance capacity of the steel trusses, afew intervention measures can be adopted:
• changing the static scheme;• increasing the cross sections;• changing the mechanic connection at
the end of the elements;• prestressing some elements, etc;The modification of the static scheme
does not ensure the complete restoration ofthe bearing capacity. For this reason it hasto be applied simultaneous with other
consolidation solutions. In this case studythe enlargement of the cross section wasapplied.
4. Conclusion
Consolidation of component solutions torestore full bars bearing capacity aimedminimal intervention measures carried outunder exploitation, without interruptingthe production process. Followingprocessing and interpretation of dataobtained from the case study made thatreconfiguration statics scheme by themodification truss steel solution "simplysupported" with "continuous beam" leadsto redistribution favourably internalforces by vertical loads and nature strain,registering reduce calculation valuesapprox. 60%.
Through this type of operation hasensured substantial reduction interventionworks and time to execution.
References
1. Sedmak A., Radu D., - „Truss BeamsWelded Joints – ManufacturingImperfections and StrengtheningSolutions”, Journal Structural Integrityand Life – Vol 14, No.1 (2014) pp.29-34, ISSN 1451-3749
2. Contract research no. 1506/2014:”Research regarding the evaluation ofdegradation for buildings exposed bydestination surface treatments ofchemical aggression operatingenvironment. Identify and proposesolutions for rehabilitation.”Transilvania University of Braşov
3. ***SR EN 1993-1-1: Design of steelstructures-Part 1-1: General rules andrules for building. ASRO Romania
4. ***SR EN 10025-1: Hot rolled productsof structural steels. Part 1: Generaltechnical delivery conditions. ASRORomania.