Rakenteiden Mekaniikka

Alkusanat

Jarkko Niiranen — to, 16 elo 2018 16:55:24 +0300

Rakenteiden Mekaniikka -lehti juhli 50-vuotista taivaltaan viime elokuussa Vaasan yliopistossa järjestetyssä juhlaseminaarissa. Seminaarin esitelmien pohjalta kirjoitetuista lyhyistä artikkeleista koottiin lehteemme paksu erikoisnumero (Vol 50, Nro 3), jollaisia on itse asiassa vuosien varrella julkaistu lehdessämme lukuisia. Perinne jatkuu tänäkin vuonna: loppuvuodesta on tarkoitus koota erikoisnumerot sekä Teräsrakentamisen T&K-päivien (15.–16.8. Hämeenlinnassa) että Suomen mekaniikkapäivien (29.–31.8. Helsingissä) esitelmien pohjalta.

Lehden uutena päätoimittajana pyrkimykseni on muiltakin osin jatkaa vuosien saatossa hyväksi havaittujen periaatteiden ja linjausten mukaista julkaisutoimintaa. Lehteä on luonnollisesti myös tarkoitus tarpeen mukaan uudistaa. Osa uudistuksista tulee todennäköisesti kytkeytymään lehtemme uuteen verkkojulkaisualustaan, jona on vuoden ajan toiminut Tieteellisten seurain valtuuskunnan ylläpitämä, tiedelehtien toimittamiseen ja julkaisemiseen tarkoitettu Journal.fi-palvelusivusto. Sivustolla on tällä hetkellä noin 60 suomalaista tieteellistä lehteä ja vuosikirjaa ja se käyttää voittoa tavoittelemattoman Public Knowledge Project -yhteenliittymän kehittämää Open Journal Systems -järjestelmää, joka perustuu avoimeen lähdekoodiin ja on tällä hetkellä maailman yleisimmin käytetty julkaisujärjestelmä tieteellisessä julkaisemisessa. Kunhan järjestelmän käyttöönoton alkukankeuksista selvitään, tämän digitaalisen palvelualustan on jatkossa tarkoitus helpottaa lehtemme kirjoittajien, arvioijien ja toimituskunnan työskentelyä, mahdollistaa artikkelien nopeampi julkaiseminen sekä lisätä lehdessä julkaistujen artikkelien näkyvyyttä ja uskottavuutta. Lopuksi haluan vielä lehden puolesta kiittää edeltäjääni, professori Reijo Kouhiaa, joka toimi lehden päätoimittajana viimeiset yksitoista vuotta – toimittaen lehden volyymit 40–50 käsittäen yhteensä 228 vertaisarvioitua artikkelia – ja jatkaa edelleen lehden toimituskunnan jäsenenä.

Helsingissä, 1. elokuuta 2018

Jarkko Niiranen
Rakenteiden Mekaniikka -lehden päätoimittaja, akatemiatutkija, apulaisprofessori

Combination of permanent and variable loads

Tuomo Tapani Poutanen, Sampsa Pursiainen, Jari Mäkinen, Tim Länsivaara — to, 16 elo 2018 00:00:00 +0300

This paper concentrates on the combination of permanent and variable loads in the structural probability theory and its implementation in codes. In the current codes, the permanent and variable loads are sometimes combined independently, and sometimes they are combined dependently. We propose that, for the safest outcome in the standardized load estimation, the actual permanent and variable loads should be combined dependently without any load reduction. The load reduction arising from the independent combination leads to an unsafe design. For example, when the permanent and variable loads are both equal to 1, the combination load is 2 if the dependent combination is applied. However, the value predicted by the model for independent load combination is only ca 1.8. Although the load formation processes are independent, the dependent combination is applied since the load formation and the load combination are different processes. To support our view, we present arguments and examples based on probability theory, physics and statics and relate them with the current codes.

Experimental study of the dynamic indentation damage in thermally shocked granite

Ahmad Mardoukhi, Mikko Hokka, Veli-Tapani Kuokkala — to, 16 elo 2018 15:46:04 +0300

This paper presents an experimental procedure to study the effects of pre-existing cracks and damage on the rock behavior under dynamic indentation. To gain better understanding on the mechanism involved in percussive-rotary drilling procedure, a modified Split Hopkinson Pressure Bar device was used to carry out dynamic indentation tests, where rock drill buttons were impacted on rock samples with dimensions of 30 cm × 30 cm × 30 cm. Before the mechanical testing, the samples were thermally shocked using a plasma spray gun for periods of 3, 4, and 6 seconds. The plasma gun produces a powerful heat shocks on the rock sample, and even short exposures can change the surface structure of the samples and provide samples with different crack patterns and surface roughness for experimental testing. The effects of the heat shock damage on the dynamic indentation behavior of the rock were characterized with single- and triple-button indentation tests. The specific destruction work was used to characterize the effects of heat shocks on the material removal during dynamic indentation. The results show that the force-displacement response of the rock does not change much even if the rock surface is severely damaged by the heat shock, however, the destruction work decreases significantly. This means that the same loading removes more volume if the material surface is pre-damaged, and that the efficiency of the indentation process cannot be evaluated from the bit-rock interaction forces alone. The presented experimental framework can be a useful tool for the verification of numerical models where the rock microstructure and especially the microcracks are essential.

Modeling the stress effect on the measurement of magnetostriction in electrical sheets under rotational magnetization

to, 16 elo 2018 14:05:56 +0300

The magnetostriction in electrical steel under rotational magnetization is usually measured with cross-shaped samples. However, the inhomogeneity of the magnetization and stress in the sample might hinder the measured results. In this paper, we investigate this phenomenon by using a magneto-mechanically coupled energy-based model to simulate the sample in a single sheet tester measurement setup, and compare the simulations and measurements. The results show that some anomalies in the measured magnetostriction can be explained by the inhomogeneous magnetization in the sample and the form effect, which result in inhomogeneous stresses and thus affect the observed quantities. The validity of the model as well as the presented statements are ascertained through experiments on the single sheet tester. The backgrounds of the used modelization technique are also detailed.

Implementing model reduction to the JuliaFEM platform

Marja Liisa Rapo, Jukka Aho, Hannu Koivurova, Tero Frondelius — to, 16 elo 2018 13:59:57 +0300

JuliaFEM is an open source finite element method solver written in the Julia language. This paper presents an implementation of two common model reduction methods: the Guyan reduction and the Craig-Bampton method. The goal was to implement these algorithms to the JuliaFEM platform and demonstrate that the code works correctly. This paper first describes the JuliaFEM concept briefly after which it presents the theory of model reduction, and finally, it demonstrates the implemented functions in an example model. This paper includes instructions for using the implemented algorithms, and reference the code itself in GitHub. The reduced stiness and mass matrices give the same results in both static and dynamic analyses as the original matrices, which proves that the code works correctly. The code runs smoothly on relatively large model of 12.6 million degrees of freedom. In future, damping could be included in the dynamic condensation now that it has been shown to work.

Numerical studies on vibration propagation and damping test V1

to, 16 elo 2018 14:14:37 +0300

Earthquakes and aircraft impacts induce vibrations that propagate throughout the entire building and they need to be considered in designing SSCs (Structures, Systems and Components). Mainly linear calculation methods have been in use in design practice and the codes and standards consider damping ratios only for linear structural analyses. Induced vibrations, especially in damaged concrete structures, have not been studied extensively enough for optimization of structural frameworks and/or qualified systems and components. Experimental data on damping properties of damaged reinforced concrete are needed also for benchmarking analysis programs and methods.

Recently, within IMPACT project, a new type of test series considering vibration propagation has been carried out at VTT. The test target is a reinforced concrete structure with two parallel walls connected to a floor slab. The front wall is additionally supported by triangular shaped side walls which are connected to the floor slab too. The test structure is supported on elastomeric bearing pads, with back pipes effective mainly in compression and with bars effective in tension. In order to obtain information on vibration propagation in damaged concrete structure at different levels of damage grades the same structure was tested six times. At each time the mass of the deformable stainless steel missile was 50 kg. The hit point located in the middle of the front wall. The impact velocity was about 110 m/s in the first four tests (V1A-D) and about 60 m/s in the remaining two tests (V1E and F). In this paper, numerical results on tests V1A and V1F are compared with the corresponding experimental ones.

The calculated results, such as accelerations, displacements, their response spectra and strains, are compared with experimental measurements. Five finite element (FE) programs are used in computations: Abaqus, Europlexus, LS-DYNA, SOFiSTiK and an in-house code (IHC).

Most of the FE-codes in the present study use shell elements. In Abaqus and SOFiSTiK non-linear behaviour of shell section is modelled by dividing the cross section into layers. Reinforcements are also modelled as layers. In Europlexus and IHC, an alternative approach is adopted in which the non-linear behaviour of concrete and reinforcement is homogenized beforehand in the shell thickness direction obtaining relations between stress resultants and generalized strains valid for the shell section. In LS-DYNA, 3D solid elements for modelling concrete and beam elements for modelling reinforcements are used.

Equations of motion are integrated with explicit central difference time integration method, except in SOFiSTiK implicit integration method is used. Modelling and computations with the mentioned FE-programs are made independently of each other. Computations with LS-DYNA are carried out as blind exercises.

Consideration of the results from benchmarking point of view is still on-going. However it is evident that analysed results follow reasonable well test results in main design parameter level such as maximum displacements, accelerations and strains. Also frequency spectra are estimated reasonably well.