Dean Čizmar

Reliability of Concrete Columns Under Vehicle Impact

This article focuses on vehicle impacts on the concrete columns of bridges and other structures. European regulations recommend minimum equivalent static design forces due to vehicular impact on members supporting structures over roadways. The value of this force is dependent only on the type of road (speed and mass of vehicle) and height of the impact. This type of calculation is deterministic and in principle cannot take into account all uncertainties that arise prior to and during vehicular impact with a concrete column. The impact probability, angle of the collision curse, vehicle velocity, vehicle mass and stiffness, and material and geometrical properties of the column are modeled in a probabilistic manner and the probability of failure is analyzed. This work compares deterministic and probabilistic modeling of vehicle impact, and concludes that deterministic modeling of vehicle impact can largely underestimate actions on columns during impact.

Arch Bridge Made Of Reactive Powder Concrete

Concrete is the most commonly used building material. It is used for buildings, industrial structures, bridges and dams. Every day the concrete is being improved, to achieve better characteristics, reduce price and to be environmental acceptable. In the introduction an historical overview of concrete is given - beginning in the 1950s when the compression strength was 40 N/mm2. Today this is the Industry standard. Concrete with compression strength greater than 40 N/mm2 is called high performance concrete (HPC). The first HPC concretes were made in 1960. Around 1990, Reactive Powder Concrete (RPC) started to appear. Their strengths go up to 800 N/mm2. Today, HPC is limited to bridges, and RPC is still very rarely used. In this work we present a method for making a RPC composite material with a compression strength of up to 170 N/mm2. Preparation and testing of material is performed in the laboratory of the Faculty of Civil Engineering in Zagreb. In addition to the mechanical properties, the durability parameters are also tested. Detailed concrete mix proportions are given in the article. The possibility that RPC could be used to construct an arch bridge over Bakar strait (Croatia) is analyzed. The bridge would have arch span of 432 m. Due to very high compression strength of RPC the span length may be increased, therefore reducing the total construction cost. RPC is also used to increase the resistance to freeze-thaw cycles, increase abrasion resistance, and reduce chloride permeability. Many large Adriatic bridges have experienced durability problems that are primarily the result of using standard concrete. Each of these durability enhancements provided by RPC decrease maintenance costs and lengthens the service life of a structure, which is vital for bridges in the Adriatic region.

The reactive powder concrete bridge across Bakar strait

Croatia has a several big arch bridges. Many of these bridges, only after 20 years, had serious durability problems. These problems are primary related to the fact that the protective layer to reinforcement is rapidly being destroyed. Winds that reach up to 250 km/h drift large amount of chlorides that destroy the bridge structure, primary the arch and the columns. Thick layers of concrete didn’ t solve this problem entirely. Reconstruction of this type of damage is very expensive and long lasting. In the article is presented the making of the RPC 200 composite material. Preparation and testing of material is conducted in laboratory of Faculty of Civil Engineering, University of Zagreb. Concrete mix proportions are given in the article. This type of material, except for great mechanical properties has better durability parameters. Article discusses the possibility that the future arch bridge over Bakarska strait (Croatia) is made of RPC.

Screw Connection in Reinforced Concrete Column Joints of Prefabricated Structures

Classic connection between prefabricated reinforced concrete columns on a foundation slab using the concrete plinth base can sometimes be very problematic, especially in limited construction conditions and / or foundation on sandy terrain where the groundwater is very close to the ground surface. By applying the screw connection between a prefabricated reinforced concrete column and the foundation structure using anchor bolts and column shoes, the overall height of the foundation is successfully reduced. The established connection between prefabricated reinforced concrete column and the foundation can immediately sustain the design force after assembly and it is considered to be a rigid connection that acts as an equivalent conventional connection with concrete plinth base, without any other additional support and welding. The successful implementation is possible with underground levels of structures where the inverted concrete plinth bases are implemented - the indentation in the foundation slab for the connection of prefabricated columns with base plate is avoided.Application of the screw connection also enables the design of the continuation of the column at higher altitude (column to column) or connection of the upper part the element with the monolithic structure. Screw connection is particularly suitable for use in seismic areas because the performed tests showed very good ductility of the connections under cyclic load.

COMPARISON OF THE ZOLLINGER SYSTEM AND CLASSIC TIMBER ROOF STRUCTURE

Odabir optimalnog statickog sustava kod projektiranja konstrukcije zadaca je svakog odgovornog projektanta. U ovom radu bit ce prikazana usporedba prostornog Zollinger sustava i ravninskog klasicnog drvenog krovista nad istom tlocrtnom povrsinom. Zollinger sustav građenja kratkim lamela patentirao je Friedrich Zollinger i primjenjuje se od 1918. godine, a valjkasti oblik krovista omogucuje bolju iskoristivost prostora i optimalniji prijenos opterecenja, sto ce u konacnici rezultirati ustedama u potrebnim kolicinama materijala za gradnju. Kod klasicnih drvenih krovista koja imaju siroku primjenu javlja se problem oblikovanja cvornih detalja, te je cesto potrebno povecavati dimenzije poprecnih presjeka elemenata radi optimalnog prijenosa opterecenja u spojevima.

Proračun spregnutih nosača drvo - beton

Spregnuti nosaci drvo-beton cesto se koriste kod rekonstrukcija objekata visokogradnje, a moguci su i kod mostova. Proracun takvih sustava je relativno kompleksan jer u nosivosti sudjeluju dva materijala razlicite krutosti i reoloskih svojstava. Dok je proracun trenutacnih deformacija jasno definiran Eurokodom 5, proracun dugotrajnih deformacija, koje su cesto mjerodavne, nije jasno određen. U radu je prikazan proracun spregnutog nosaca drvo-beton uzimajuci u obzir trenutacne i dugotrajne deformacije na relativno jednostavan i inženjerima u praksi prikladan nacin. Osim proracuna prema granicnim stanjima dan je i proracun nosivosti spojnih sredstava.Spregnuti nosači drvo-beton često se koriste kod rekonstrukcija objekata visokogradnje, a mogući su i kod mostova. Proračun takvih sustava je relativno kompleksan jer u nosivosti sudjeluju dva materijala različite krutosti i reoloških svojstava. Dok je proračun trenutačnih deformacija jasno definiran Eurokodom 5, proračun dugotrajnih deformacija, koje su često mjerodavne, nije jasno određen. U radu je prikazan proračun spregnutog nosača drvo-beton uzimajući u obzir trenutačne i dugotrajne deformacije na relativno jednostavan i inženjerima u praksi prikladan način. Osim proračuna prema graničnim stanjima dan je i proračun nosivosti spojnih sredstava.

Analysis of timber-concrete composite girders

Spregnuti nosaci drvo-beton cesto se koriste kod rekonstrukcija objekata visokogradnje, a moguci su i kod mostova. Proracun takvih sustava je relativno kompleksan jer u nosivosti sudjeluju dva materijala razlicite krutosti i reoloskih svojstava. Dok je proracun trenutacnih deformacija jasno definiran Eurokodom 5, proracun dugotrajnih deformacija, koje su cesto mjerodavne, nije jasno određen. U radu je prikazan proracun spregnutog nosaca drvo-beton uzimajuci u obzir trenutacne i dugotrajne deformacije na relativno jednostavan i inženjerima u praksi prikladan nacin. Osim proracuna prema granicnim stanjima dan je i proracun nosivosti spojnih sredstava.Spregnuti nosači drvo-beton često se koriste kod rekonstrukcija objekata visokogradnje, a mogući su i kod mostova. Proračun takvih sustava je relativno kompleksan jer u nosivosti sudjeluju dva materijala različite krutosti i reoloških svojstava. Dok je proračun trenutačnih deformacija jasno definiran Eurokodom 5, proračun dugotrajnih deformacija, koje su često mjerodavne, nije jasno određen. U radu je prikazan proračun spregnutog nosača drvo-beton uzimajući u obzir trenutačne i dugotrajne deformacije na relativno jednostavan i inženjerima u praksi prikladan način. Osim proračuna prema graničnim stanjima dan je i proračun nosivosti spojnih sredstava.

Investigation On Influence Of The Loading Rate On Concrete Cone Failure

The loading rate significantly influences structural response. The structural response depends on the loading rate through three different effects: through the creep of the bulk material between the cracks, through the rate dependency of the growing microcracks and through the influence of structural inertia forces. The first effect is important only at extremely slow loading rates whereas the second and third effects dominate at higher loading rates. In this paper, a rate sensitive model, which is based on the energy activation theory of bond rupture and its implementation into the microplane model for concrete are given. To investigate the importance of the rate of the growing microcracks and the influence of structural inertia, static and dynamic analyses were carried out. The results show that with an increase of the loading rate the pull-out resistance increases. The comparison between model prediction and test data for uniaxial compression failure of concrete shows that the model realistically predicts the influence of the loading rate on the compressive strength and initial Young’s modulus.

ANALIZA STANJA POJAVE PREKOMJERNIH VLAČNIH NAPREZANJAU ZONI SLJEMENA RADIJALNO ZAKRIVLJENIH (SEDLASTIH) GREDNIHLIJEPLJENIH LAMELIRANIH NOSAČA

Svrha rada je analiza ojacanja zakrivljenih lijepljenih lameliranih nosaca koji prilikom opterecenja od savijanja doživljavaju stanje naprezanja kod kojih su poprecna vlacna naprezanja posebno interesantna obzirom na ortotropno ponasanje drva.Prikazan je postupak proracuna uzdužnih i poprecnih naprezanja prema normi HRN EN1995 i prema ASTM normama. Nadalje, u radu se analizira postupak ojacanja sljemena s celicnim sipkama prema ASTM normama. Rezultati dobiveni prema normama se uspoređuju s numerickim analizama. Prema dobivenom se može zakljuciti da norme dobro aproksimiraju vrijednosti naprezanja no prilikom ojacanja treba biti vrlo oprezan jer se samo pomocu numerickih analiza može dobiti vrijednost preostalih naprezanja poprecnih na vlakanca nakon ojacanja. Na osnovu ovoga može se precizno izracunati nosivost nosaca i provesti optimizacija (povecanje razmaka između nosaca ili opterecenja).

Composite timber-concrete girders

Klasicno spregnuti nosaci od celika i betona vec se dugo upotrebljavaju u građevinskoj praksi. Međutim, posljednjih godina u svijetu je sve prisutniji jedan novi oblik sprezanja gdje se umjesto celika koristi drvo. Rezultat ove ideje su spregnuti nosaci od drveta i betona koji se uspjesno primjenjuju u mnogim konstrukcijama – kod mostova, kod međukatnih konstrukcija u visokogradnji te industrijskim objektima. Pored ovoga, sprezanje drveta i betona koristi se pri rekonstrukciji i sanaciji međukatnih drvenih konstrukcija, kako bi se udovoljilo suvremenim zahtjevima nosivosti, deformabilnosti i zvucne izolacije