The paper presents the method of probabilistic optimisation of load bearing capacity and reliability of statically indeterminate bar structures, and of coupling of members in kinematically admissible failure mechanisms (KAFM), which contain minimal critical sets of elements (MCSE). The latter are characterised by the fact that if only a single element is operational, the whole set is operational too. A method of increasing load bearing capacity and reliability of KAFM built from bars dimensioned in accordance with the code is presented. The paper also shows estimation of load bearing capacity and reliability of KAFM of the optimised structures containing elastic-plastic bars with quasi-brittle connections with nodes. The necessity of increasing connection of load bearing capacity and reliability in relation to bar reliability in order to prevent bars from being excluded from MCSE due to connection fracture is estimated.

The paper provides a solution to the problem of dimensioning decisive bars on the basis of the conditions of meeting the recommended reliability classes [9] of statically determinate structures composed of n members. A theorem was formulated:if a statically determinate structure composed of n decisive members is to attain the reliability greater than, or equal to, the recommended relia-bility p = 1 – q, it is necessary and suffi cient that the damage frequency sum qᵢ of decisive members is smaller than the admissible damage frequency q of thestructure: ∑qᵢ < q. On the basis of this theorem, s coeffi cients that recommend increase of the load bearing capacity of the decisive bars in a statically determinate structure constructed in order to meet the recommended class [9] of the structure reliability, are estimated and presented in a tabular form.

Natural airfield pavements divide into soil and turf pavements. Turf pavement is a soil pavement covered with a developed grass layer that reduce soil moisture level, thus increasing its' resistance and extending exploitation period. Natural airfield pavements are formed through appropriate ground preparation. This pavement should be constructed in such a way as to have sufficient load-bearing capacity, which directly affects the safety of flight operations by aircraft. The current research indicates that a significant part of natural airfield pavements in Poland does not meet the requirements for load bearing capacity and require reinforcing. The article provides an example of reinforcing the natural airfield pavement with a system of geogrids. The paper describes what research was performed in order to measure the load-bearing capacity of natural airfield pavements and analyses the obtained results.