Limit State Design Code Research Articles

Non-linear stiffness characterisation – a practical framework

Limit state design codes such as Eurocode 7 require good estimates of displacement to be made for serviceability checks. It is known that the stiffness of overconsolidated soils and weak rocks is highly non-linear and this needs to be taken into account for reliable displacements to be calculated. This paper outlines a relatively simple means of characterising the undrained and drained non-linear stiffness of a wide range of soils and rocks. A series of practical applications are described where the calculated stiffness non-linearity is compared with high-quality field and laboratory test data, including back-analysis of full-scale structures. The non-linear stiffness function requires just four inputs: shear modulus at small strain; shear strength; failure strain; and elastic threshold strain. Applications include: checking advanced test data and calibration of non-linear constitutive models; derivation of field stiffness, including variations with depth; non-linear stiffness curves; and assessing the potential effects of stiffness anisotropy.

A Study on Live Load Deflection Criteria of Long-Span Steel Bridges

Under-clearance of bridges is one of the major considerations when planning and designing bridges. Especially for sea-crossing bridges, under-clearance is more important for safe passage of ships. Long span cable bridges with steel orthotropic deck usually show relatively large deflections and excessive deflections may be not only disadvantageous to the clearance plan, but also a problem in bridge serviceability. The clearance of sea-crossing bridge is designed with air draft from the waterline to top of mast, ship’s trim, psychological free space, sea level, height of wave and bridge deflection by live loads. At design stage, bridge engineers need a live load deflection limit to ensure adequate clearance. The limitation of live load deflection is also considered for ensuring the serviceability of vibration of bridges. For vibrational serviceability, the live load deflection limit is expressed as a ratio to the span or the formula by natural frequency. On the other hand, some standards specify vibration serviceability limit by peak or root-mean-square (RMS) acceleration. The purpose of this study is to propose new deflection limit criteria for long span steel cable bridges based on analysis and long-term measurement data. Structural analysis was performed on two steel suspension and three steel cable-stayed bridges with more than 200 m of main span under design live load model of DL24 and new live load model KL510 in limit state design code. Also, the deflection limit criterion was evaluated by probabilistic method using the deflection data measured for 1 year. The results of this study are expected to be useful data for bridge engineers designing long span steel cable bridges.

80 MPa급 콘크리트를 활용한 이중합성 거더의 수평접합면 구조거동에 관한 실험적 연구

Abstract The horizontal shear capacity when the flange of a steel girder is replaced with 80 MPa concrete is important for its structural safety. In this study, 6 specimens with different interface conditions were designed andfabricated based on the Limit State Design Code on Korean Highway Bridges and static tests were performed to measure the horizontal shear capacity. Not only the resistance factors of the stud shear connector, concrete and reinforcement, but also the surface conditions of the casing concrete and spacing of the horizontal shear reinforcements were used as the experimental variables. The experiments showed that the interfaces between the steelgirder and the concrete flange have stronger joint performance than those between the concrete flange and deck slab. To ensure the composite action in the plastic zone, the conservative horizontal shear reinforcement is more importantthan the roughness in the concrete face. Keywords : Composite action, Concrete flange, Horizontal shear, Interface condition, Static test

Reliability analysis of piles in spatially varying soils considering multiple failure modes

In most limit state design codes, the serviceability limit checks for drilled shafts still use deterministic approaches. Moreover, different limit states are usually considered separately. This paper develops a probabilistic framework to assess the serviceability performance with the consideration of soil spatial variability in reliability analysis. Specifically, the performance of a drilled shaft is defined in terms of the vertical settlement, lateral deflection, and angular distortion at the top of the shaft, corresponding to three limit states in the reliability analysis. Failure is defined as the event that the displacements exceed the corresponding tolerable displacements. The spatial variability of soil properties is considered using random field modeling. To illustrate the proposed framework, this study assesses the reliability of each limit state and the system reliability of a numerical example of a drilled shaft. The results show the system reliability should be considered for the serviceability performance. The importance measures of the random variables indicate that the external loads, the performance criteria, the model errors of load transfer curves and soil strength parameter are the most important factors in reliability analysis. Moreover, it is shown that the correlation length and coefficient of variation of soil strength can exert significant impacts on the calculated failure probability.

한계상태 설계법을 이용한 고강도 콘크리트 세그먼트 라이닝 설계

국내의 콘크리트 구조설계는 허용응력 설계법(ASD) 및 극한강도 설계법(USD)을 사용하고 있다. 허용응력설계 및 극한강도설계법은 경제적인 설계에 제한이 따르기 때문에 최근에 지하구조물의 설계에 극한한계상태 설계법의 채택을 다각적으로 검토하고 있는 상황이다. 이를 뒷받침하고자 본 연구에서는 일본 한계상태 설계법(LSD)에 의거한 국내 고강도 콘크리트 라이닝에 대한 발생 단면력을 산정하였으며, 이를 국내의 극한강도 설계법과 비교함으로서 경제적 설계결과 도출 가능성 및 국내 적용성을 분석하고자 하였다. 해석결과, 철근량 산정과 밀접한 관계가 있는 최대 모멘트 발생양상은 두 설계법 모두 유사하나, 발생 부재력은 일본 한계상태 설계법이 극한강도 설계법에 비하여 모멘트는 26.0%, 전단력은 26.7%감소하는 것을 확인할 수 있었으며, 이를 통하여 안정성이 확보된 조건에서 경제적인 세그먼트 설계가 가능함을 확인하였다. The concrete structural design in domestic has based on the allowable stress design (ASD) method and ultimate strength design (USD) method. Recently limit state design (LSD) method has issued and attempted to adopt in geotechnical design. Because ASD method and USD method have restriction in economic design. In this study, the generated member forces were calculated about high strength concrete segment lining based on japanese LSD code. And it compared with domestic USD code for identifying the economic design possibility of LSD and domestic applicability. In analysis results, the aspect of moment had generated similarly each other but the member forces of japanese LSD code were decreased (26.0% of moment and 26.7% of shear force) comparing with USD method. For that reason, possibility of economic segment design with stable condition were identified.

Geotechnical resistance factors for ultimate limit state design of deep foundations in cohesive soils

This paper investigates the ultimate limit state load and resistance factor design (LRFD) of deep foundations founded within purely cohesive soils. The geotechnical resistance factors required to produce deep foundation designs having a maximum acceptable failure probability are estimated as a function of site understanding and failure consequence. The probability theory developed in this paper, used to determine the resistance factors, is verified by a two-dimensional random field Monte Carlo simulation of a spatially variable cohesive soil. The agreement between theory and simulation is found to be very good, and the theory is then used to derive the required geotechnical resistance factors. The results presented in this paper can be used to complement current ultimate limit state design code calibration efforts for deep foundations in cohesive soils.

Geotechnical resistance factors for ultimate limit state design of deep foundations in frictional soils

This paper investigates the probabilistic nature of ultimate limit state failures of deep foundations in purely frictional soils (e.g., sands). In so doing, the theory required to predict both the probability of ultimate limit state failure and the resistance factors needed to avoid this limit state are proposed. The proposed resistance factors are functions of site understanding and failure consequence, and the theory leading to these resistance factors is validated via Monte Carlo simulation of a two-dimensional spatially variable random field. In both the theory and the simulation, a pile is assumed to be placed vertically at a certain position in the soil mass, and the soil is sampled at various distances from the pile to come up with characteristic soil properties (namely friction angle) for use in the pile design. Agreement between theory and simulation is found to be very good. The theoretical model is then employed to determine upper bound geotechnical resistance factors, which can be used to complement current ultimate limit state design code calibration efforts. An example of such a calibration is presented.

Experiments on limit state design of large stud shear connectors

For the design of shear connection in high shear areas of steel-concrete composite bridges, large shear studs can be an excellent alternative. Large studs can provide the convenience of removing concrete slab and the proper distribution of shear pockets for precast decks. Through the push-out tests on large stud shear connectors up to 30 mm, which are beyond the limitation of current limit state design codes, static and fatigue behaviors were investigated and comparisons with design equations were performed. Ultimate strength of the shear connection showed that the design shear strength in Eurocode-4 gives conservative values for large studs. Fatigue endurance obtained from the tests was slightly lower than the current S-N curve in Eurocode-4.

Probabilistic risk assessment for structures

The existing safety culture within structural engineering has been highly successful and is likely to remain so. However, it is argued herein that it is no longer completely sufficient. It lacks the ability to interface adequately with risk assessment procedures used for other facilities in society. Such procedures require complete analysis of all sources of risk and of the various potential consequences. These are consequences to society rather than simply ‘structural failure’. Moreover, the probabilities involved are estimates for expectations rather than the nominal probabilities used to calibrate limit state design codes. The theme is illustrated with two examples: one dealing with the safety assessment of an existing road bridge and the second with potential structural fatigue failure of a large pressure vessel for liquefied petroleum gas storage. These examples show that structural safety cannot, for major structures, be divorced from more general safety analyses. Some comments are made also about the World Trade Center and related risk rationality.

Probabilistic risk assessment for structures

The existing safety culture within structural engineering has been highly successful and is likely to remain so. However, it is argued herein that it is no longer completely sufficient. It lacks the ability to interface adequately with risk assessment procedures used for other facilities in society. Such procedures require complete analysis of all sources of risk and of the various potential consequences. These are consequences to society rather than simply ‘structural failure’. Moreover, the probabilities involved are estimates for expectations rather than the nominal probabilities used to calibrate limit state design codes. The theme is illustrated with two examples: one dealing with the safety assessment of an existing road bridge and the second with potential structural fatigue failure of a large pressure vessel for liquefied petroleum gas storage. These examples show that structural safety cannot, for major structures, be divorced from more general safety analyses. Some comments are made also about the World Trade Center and related risk rationality.

AN APPRAISAL, BY COMPARISON WITH EXPERIMENTAL DATA, OF NEW DESIGN PROCEDURES FOR ALUMINIUM PLATE GIRDERS.

Keywords WALES UNIVERSITY CARDIFF GIRDERS, PLATES, ALUMINIUM, DESIGN, PROCEDURE, METHODS, METALS, TENSION, FIELD, PREDICTION, COLLAPSE, LOADS, SAFETY, CODES, STANDARDS, LIMIT STATE, PRACTICE, FAILURES, STIFFENED, WEBS, EXPERIMENTAL, DATABASES, EXPERIMENTS, TRANSVERSE, ALLOYS, STIFFENING, STIFFENERS, LONGITUDINAL... Show All