Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

NUMERICAL ANALYSIS OF THIN-WALLED COLD-FORMED STEELCOLUMN-BASE OVER-ROOFING

SOLUTIONS

A. FLORICEL 1 G. ZAGARI 2 V. UNGUREANU 1

Abstract: In an increasingly developing world, in which sustainability hasbecome not only an option but more of a demand, over-roofing of existingbuildings, together with over-cladding form an optimum complete retrofittingsolution. In this context, the paper is mainly focused on numerical analysis ofcold-formed steel solution, in both semi-rigid and rigid solutions used foraddition of new storey. Also, for ease in erecting, an option of column-baseconnection with chemical anchorage was analysed.

Key words: light steel structure, small building system, over-roofing, over-cladding, retrofitting solution.

1 Department of Steel Structures and Structural Mechanics, Politehnica University of Timisoara, Romania.2 MODELING Department, University of Calabria, Italy.

1. Introduction

More and more buildings nowadays arebeing subjected to the process ofrefurbishment and conversions. Evenmore, taking into account the sustainabilitycriterion which has become a veryimportant factor for construction works,there is an increasing tendency of choosingrefurbishment of existing buildings ratherthan building new ones [4].

According to the European Commission,buildings are responsible for 40% of theEU energy consumption and 36% of CO2emissions. This is the reason whyimproving the overall energy efficiency ofbuildings is crucial for the overall resourceefficiency of the European economy [3].

Over-roofing is defined simply as theinstallation of a new roof on an existing

building, creating new habitable spaces.However, over-roofing is frequentlycombined with over-cladding, thus,forming a complete retrofitting solution.

The main advantages for over-roofing, inview of RFCS Project RFSR-CT-2007-0043 [8], include: heat reduction byimproving thermal insulation,enhancement and modification of theoverall building appearance, creation ofnew space for habitable use or buildingservices, arrestment and overcoming ofdeterioration of the existing roof and ofleakage problems (by a new pitched roof),avoiding disruption that is inevitable in thereplacement of an existing roof.

From structural point of view, over-roofings add an extra weight to the existingbuilding, and thus, rechecking the load-bearing capacity of the original structure is

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a demanded measure for safety reasons, asto decide whether or not consolidationmeasures are needed. Nevertheless, inseismic countries, as Romania’s case, thisprecaution measure becomes more of ademand, and traditional buildingtechniques cannot be always used for thistype of restructuring. Thus, in order tominimize the weight of the new storeystructure added above, engineers choose touse steel based solutions because of theirlightness in comparison with othermaterials (concrete, brick) (see Figure 1).

Fig. 1. (a) Over-roofing of a single storeyindustrial building and (b) view of the new

building façade [8]

Even more frequent, nowadays,construction companies turn to lightweight steel sections due to their numerousadvantages: very good strength to weightratio, fast and easy erection andinstallation, fully recyclable, adaptabilityand flexibility to architectural design, and,last but not least, proven optimal solutionfor seismic areas, due to their lightness.

However, some aspects regardingstructural issues must be controlled,namely stability problems due to theirslenderness and complex detailingconnections [5].

Moreover, over-roofing and over-cladding of existent buildings using thesekind of light weight solutions becomes anoptimal retrofitting technique as thesesmall building systems are a prefabricatedproduct that can be easily erected on sight,piece by piece (see Figure 2).

Fig. 2. (a) Modular assembly ofprefabricated components and (b) view of

the small building system module [5]

Therefore, what was designed initially asa modular structure for industrial purposescan also be transformed into an over-roofing, or a storey extension of an alreadyexisting building.

Referring to current building stock inEurope, there are about 196 millions ofdwellings corresponding to about 160millions of buildings, while new buildings

(a)

(b)

(a)

(b)

A. FLORICEL et al.: Numerical Analysis Of Thin-Walled Cold-Formed Steel Column-Base… 77

represent yearly about 1.5% of the buildingstock [9].

According to data from the 2002 Censusand, there are about 493,000,000 sqm. ofbuilding floor area in Romania, of which86% are accounted for by residentialbuildings [3]. Moreover, over 71% ofexisting building stock in urban areas iscomposed of collective large prefabricatedconcrete panel houses. These largeprefabricated blocks were built in threemain periods, i.e.: (a) 1962-1975; (b)1975-1982; (c) 1982-1989 [2].

The current major problems of thesebuildings can be categorized to: problemsregarding thermal comfort, ventilation,energy consumption; problems caused bythe small inhabitable area and interiorsrigid partitions; problems concerning thepublic space, accessibility, lack of parkingareas, elevators, space for socialinteraction etc.

Therefore, in present context, there is alarge variety of geometries, volumes andaesthetics for the over-roofing and over-claddings even in the boundaries of thesame neighbourhood, degrading the overallvisual image. Due to this lack of ratifiedmethodology, errors in the execution ofthese over-roofings are unavoidable.

2. Over-Roofing Solutions Based onIntensive Use of Steel

The proposed solution for over-roofing,presented in Figure 3, was considered forthe main block typology built inTimisoara, Romania in the period 1962-1989, i.e.: IPCT 744 specific to the period1962-1975 – cold-formed steel sections(CF);

In pinned solution (see Figure 3a), thecolumns (two back-to-back lipped channelsections) are connected to the baseconcrete slab through a rectangular steelplate, but in this case a steel box wasneeded to connect webs of the profiles too.

The second case scenario, i.e. the oneusing rectangular hollow section, was notpresented here because is similar to thefirst case scenario. The connection is doneby bolts, both for webs as well as at thebase. In both cases, the column-baseconnection is strengthened by an angleprofile placed under the last floor slab andconnected to the steel plates above by arow of two bolts, and respectively to thelast floor wall also by two bolts (see Figure3).The rigid column-based connections arepresented in Figure 3b.

Fig. 3. Proposed connections for steelover-roofing: (a) semi-rigid; (b) rigid

The design of the structures subjected toover-roofing is composed of two stages: (i)verification of the current state of theoriginal structure and (ii) verification ofthe structure after intervention (includingthe new-added steel structure).

The initial concrete structural systemwas made of precast panels assembled onsite. The project has 3 longitudinal and 8transversal, concrete diaphragmscomposed of standardized precast concretepanels. The interior panels are singlelayered elements of 14 cm thickness, inB250 (equivalent to C16/20) concreteclass. The exterior panels are composed ofthree specific layers, with differentfunctions: the load bearing layer (12 cm),the thermal insulating layer (6 cm) andprotection layer (6 cm), respectively.

The state of the original structure and therefurbished one was done through 3D

a) Cold-formed semi-rigid section (CF)

b) Cold-formed rigidsection (CF)

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analyses using ETABS computer code, byusing shell finite elements (see Figure 4).

Fig. 4. 3D view of representative commonblock typology T744R

The building was checked according tothe actual standards, proving to be resistantto all loading combinations. Theseverifications also showed that, byremoving the top thermal insulation layersfrom the terrace, the adding of asupplementary over-roofing floor wouldnot affect the capacity of existing concretestructure.

The buildings were verified according tothe actual standards, proving to be resistantto all loading combinations. The seismicload was accounted through a spectralanalysis using the ground accelerationvalue ag=0.20g and a control periodTc=0.7s.

These verifications also showed that, byremoving the top thermal insulation layersfrom the terrace, the adding of asupplementary over-roofing floor wouldnot affect the concrete structure’sresistance or stability. Thus, steel was thematerial chosen for this over-roofing study,due to its lightness, reversibility and cleansite.

The over-roofing systems were analysedby including in the already built models inETABS, the new steel structures as shownin Figure 4. The over-roofing was designed

in accordance with the followingstandards: (1) EN 1990-1-1-2004 "Designcode. Basis of structural design"; (2) EN1991-1-1-2004 "Design code. Evaluationof load actions on structures"; (3) EN1993-1-1-2004 "Design of steelstructures"; (4) EN 1998-1-1-2004 "Designof structures for earthquake resistance".For the seismic analysis of the over-roofing system, a behaviour factor q = 1,specific to non-dissipative structures waschosen.

One type of solution for over-roofingbased on intensive use of steel was chosenfor numerical studies: cold-formed steelprofiles. Therefore, pinned/semi-rigid andrigid column-based connections werefurther investigated, by using numericalanalyses, in order to optimise this solution(see Figure 5).

Cold-formed steel members are efficientin terms of both their stiffness andstrength. Additionally, because the basesteel is thin, even less than 1 mm thickwhen high strength steel is used, themembers are lightweight. The use ofthinner sections and high strength steelleads to design problems for structuralengineers which may not normally beencountered in routine structural steeldesign.

Cold-formed steel design is dominatedby two main problems, i.e. (1) stabilitybehaviour, which is dominant for designcriteria of thin sections, and (2) connectingtechnology, which is specific and stronglyinfluences the structural behaviour anddesign detailing. Last years, the seismicperformance (3) of cold-formed steelstructures started to be examined [5].

4. Numerical Analysis

In order to prove the efficiency of thecolumn-base connections chosen for thisstudy, numerical simulations wereperformed using FE software ABAQUS

A. FLORICEL et al.: Numerical Analysis Of Thin-Walled Cold-Formed Steel Column-Base… 79

6.7 [1]. Semi-rigid and rigid baseconnections using cold-formed steelprofiles (CF) will be detailed in thefollowing, as presented in Figure 5.

(a) Semi-rigid CF (b) Rigid CF

Fig. 5. Cold-formed steel column-baseconnections

The numerical models (see Figure 5a and5b) are also analysed in both cases, assemi-rigid and rigid connection. Thecolumn is composed of two back-to-backcold-formed lipped channels of 300 mmheight and a 2 mm thickness. Two rows ofM16 bolts were used to connect the websof the profiles via a steel box section of 6mm thickness of the walls and 150 mmheight, placed between the two sections ofthe column, composing the semi-rigidsolution. The geometrical characteristics ofthe base plate and the angle profiles are thesame as for the previous case. For the rigidconnection, supplementary stiffeners andbolts were added to connect the flanges ofthe profiles.

Elasto-plastic material models were usedfor all elements. SHELL elements of S4Rtype, with 4 nodes, reduced integration, 6DOF per node were used for modelling thecold-formed steel sections, while for allother sections, BRICK elements of C3D8Rtype, with 8 nodes, reduced integration, 6DOF per node, were used. In order toobtain accurate results the dynamic explicitsolver was used. Initial convergenceproblems were issued by a preliminary step

of preload and the actual load of 200 mmdisplacement was imposed.

Also, the concrete was considered in theanalysis as elastic-perfect plastic, as shownin Figure 6 [10]. The option used tosimulate concrete plasticity was ConcreteDamaged Plasticity.

Fig. 6. Elastic-plastic model of concrete

The following results were obtained: (1)the first connection was modelled as semi-rigid but presents mostly a pinnedbehaviour; the maximum stresses werereported in the upper flanges of the sectionon the direction of loading, but also aroundthe holes of the webs (see Figure 7a); (2)for the rigid solution, the plastic stressesachieve maximum values both in theflanges and the webs; a web cripplingphenomenon occurs (see Figure 7b); (3)the concrete floors did not reach plasticity,not even in full loading conditions; thestresses are still in the elastic range,presenting only local effects at bolt-holeinteraction zone, as shown in Figure 8.

(a) Semi-rigid CF (b) Rigid CF

Fig. 7. Distribution of stresses in the basedconnections

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Fig. 8. Strains in concrete on bolt hole

The force-displacement curves obtainedfor each case scenario are presented inFigure 9.

0

10

20

30

40

50

60

70

0 20 40 60 80 100

F [kN]

Displacement [mm]

Rigid CFS

Semi-rigid CFS

Fig. 9. Force-displacement curves for thesemi-rigid and rigid cold-formed steel baseconnections

The above force-displacement curvesshow, as expected, the semi-rigid solutionhas smaller loading capacities than therigid one, but the intention was to reducethe local effects on the existing structure.The stress level on the existing concretefloors and the additional walls are smalland do not introduce supplementarystresses on the existing structure.

Taking the study further, a chemicalanchorage system based on epoxy resinwas numerically investigated for these twocold-formed solutions in order to offer abetter solution for the comfort of the

occupants. This solution was chosen fortesting as it gives minimum disruption tothe last level apartment owners and alsofor minimum concrete damage.

Generally speaking, there is a largevariety of chemical anchor types availableon the market. As a basic description theyare basically a steel screw positioned in apredrilled hole in the concrete material.The connection between the steel screwand concrete is assured by pouring glue-based material (resin) (see Figure 12).According to most used design methods, atultimate tensile force of these chemicalanchors the concrete or bond should failseparately. This assumption is also used incertification regulations for steel post-installed anchors for concrete e.g. ETAG[6].

The two cold-formed models were re-designed in the same finite elementsoftware, with chemical anchorage. Mostgeometrical characteristics were preserved,with a few minor adjustments of boltdistance. However, the steel box sectionconnecting the two back to back cold-formed sections were replaced by a moreadequate and practical steel sheet of 2 mmthickness. Nevertheless, the angle profileswere not necessary in this case as theanchors were embedded in concrete (seeFigure 13). Also, the same elasto-plasticmaterials were used in designing theassembly. A particular attention was givento the resin material in which the anchorsare embedded, as documented elasticproperties were given to the material inorder to simulate the exact effect. Dynamicexplicit analysis was used with a lateraldisplacement force of 200 mm. The initialaxial preloading was still necessary for thesemi-rigid section in order to simulate real-life loading conditions.

A. FLORICEL et al.: Numerical Analysis Of Thin-Walled Cold-Formed Steel Column-Base… 81

(a) Semi-rigid CF (b) Rigid CF

Fig. 10. Cold-formed steel column-baseconnections with chemical anchors

The following results were obtained: (1)the semi-rigid model presents maximumstresses around web bolt-holes and inupper section flanges; moreover atmaximum loading conditions the flangeson the direction of loading are crushed; (2)the base-plate of the column presents largedeformations in the tension zone (seeFigure 11a); (3) for the rigid model, thestiffeners do not allow section rotation, sothe flanges are crushed and a phenomenonsimilar to web crippling appears (seeFigure 11b); (4) the concrete floor in bothmodels shows some amount of plasticity atfull loading conditions and mostly locallyon bolt holes; however, the columnsections failed much earlier beforeconcrete reaches its plastic limit (seeFigure 12).

(a) Semi-rigid CF (b) Rigid CF

Fig. 11. Distribution of stresses in thecolumn-based connections with chemicalanchors

(a) Semi-rigid CF (b) Rigid CF

Fig. 12. Distribution of stresses in concretewith chemical anchors

The force-displacement curves obtainedfor each case scenario are presented inFigure 13.

Fig. 13. Force-displacement curves for thesemi-rigid and rigid cold-formed steel base

connections with chemical anchors

The above force-displacement curvesshow, as expected, the semi-rigid solutionhas smaller loading capacities than therigid one, but the intention was to reducethe local effects on the existing structure.The stress level on the existing concretefloors and the additional walls are smalland do not introduce supplementarystresses on the existing structure.

The scope of the numerical program is abetter understanding of the behaviour ofthese types of column-base connectionsand to find the optimum solution for thiskind of over-roofing systems, by limitingthe stresses to the existing concrete panelbuilding. Based on this numerical study

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several types of these connections will beexperimentally investigated.

5. Conclusions

The study presented in this paper showstwo possibilities of connecting steel-intensive over-roofing to the existingconcrete buildings. The numerical analysesshow the light steel-intensive solutions areideal systems for over-roofing the existinglarge precast concrete panel buildings dueto their lightness, reversibility and cleansites; also, they can adapt to existingstructural systems and several structuraltypologies can be thought. The numericalanalyses show that the cold-formedconnections can withstand a considerableamount of loading, for this kind of fourclass section. However, the main concernremains to minimize the structuralintervention on the existing concretestructure. A better solution for the comfortof the occupants seems to be theconnection by chemical anchors. Theresults show that the column behaviour ismostly the same as for the first solution.The concrete displays some plasticity atfull loading conditions, mostly on boltholes; however, cracking does not occur asthe column section has already failedbefore concrete reaches its ultimate limitstate. Experimental tests will be done inorder to validate the proposed solutions.

Acknowledgements

This work was partially supported by thestrategic grant POSDRU/159/1.5/S/134378(2014) of the Ministry of NationalEducation, Romania, co-financed by theEuropean Social Fund – Investing inPeople, within the Sectoral OperationalProgramme Human ResourcesDevelopment 2007-2013.

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