For ASD, the IBC provides combinations and accounts for the probability of simultaneous combinations by reducing some of their magnitudes. All of these combinations are given in Section Notice these loads are not factored increased : they represent actual expected service loads. The engineer must check all applicable combinations to determine the combination that creates the highest stress.
Also note that some of the loads are reduced when grouped with dynamic loads wind, snow, roof live, earthquake. These reductions represent a rational method to balance the probability of nature with economics and safety. With ASD, the factor of safety is not connected to the load combinations. Since it is on the resistance side of the equation, it varies only with the type of force axial, shear, bending moment.
The LRFD specification accounts separately for the predictability of applied loads through the use of load factors applied to the required strength side of the limit state inequalities and for material and construction variabilities through resistance factors on the nominal strength side of the limit state inequality. The ASD specification combines the two factors into a single factor of safety. By breaking the factor of safety apart into the independent load and resistance factors as done in the LRFD approach a more consistent effective factor of safety is obtained and can result in safer or lighter structures, depending on the predictability of the load types being used.
Typically, each load type i. The one exception to this is earthquake loads, which are expressed at strength levels. The individual loads are then combined using load combination equations that consider the probability of simultaneously occurring loads. The resulting combined loads and load effects from LRFD combinations equations are given subscript of "u". A subscript of "a" is used to indicate a load result from an ASD load combination.
Particular to this text, a subscript of "s,equiv" is used to represent the result of a load combination that is the simple algebraic sum of all the individual load components. The resistance factor is denoted with the symbol f , and the factors of safety with the symbol W. We'll see how they are applied below.
The other issue that seems to be conceptually challenging for many engineers is that, since LRFD looks at the strength of members i.
Either the service loads must be factored or the ultimate loads must be unfactored if they are to be compared. This gets even more complicated when you consider the effect on load combination equations. One method for comparing loads is to compute a composite load factor CLF that is the ratio of load combination result P u or P a to the algebraic sum of the individual load components P s,equiv or P s,eq.
The load combination with the lowest CLF is the critical load combination. You can check this out in the figure below. In a way, it is a measure of the relative safety of what you are designing. You can check out that distance and the equation for beta below. The reliability index also got much better after the adoption of LRFD.
Check out the figures below based on a bridge specification hey, a structure is a structure. In the first figure you can see the range is much larger versus after LRFD, meaning that the values for beta were all over the place.
After we adopted LRFD, we were able to tighten up the value we used for each span. With LRFD we calculate nominal loads and nominal resistances. For loads, nominal loads Qi are multiplied by load factors Yi to obtain factored loads. Instead of lumping all loads we usually keep them separate. We then combine the load using statistically calibrated load combinations!
With all of that said, in the end, the ultimate goal of using LRFD is to prove this — that your resistances are greater than the loads. What do you think? Let me know! Your email address will not be published. Related CAD tips :. CADforum Home. CAD Videos. Web Links. SW Development.
CAD Shop. Latest Posts. Members Profile. You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum.
0コメント