Recommendation to scale factored action effects to determine shear capacity for load rating 1

[kww2008]

A few articles have been published recommending the use of a scaled load (action) effects to determine ultimate shear capacity for use with load rating to AS 5100.7. It is not clear from these articles why it is necessary to do that. Doing so will result in the strengths being different for design and load rating of a new bridge for a same vehicle (i.e. design vehicle is the same as nominated rating vehicle). Any input to this will be much appreciated.

 

[Settingsun]

IDS - referring to your quote from my post- I confused myself. Need V* to correspond to to Vu under the ULS design action effects, then spply the phi factor.

My comments were also assuming that simple extension of the longitudinal reinforcement per C8.2.8 is adequate. I don't have the appetite to reduce effective bending reinforcement for routine work. But that does clarify the intent of the paper/method in question, so thanks.

 

[IDS]

Need V* to correspond to to Vu under the ULS design action effects, then spply the phi factor.

Yes, I agree.

I think this raises an important point regarding the Caprani and Melhem paper. In their "naïve" approach they use V* as the target shear capacity Vu, but V* must be >= phi.Vu, which is why this approach may indicate a section with inadequate capacity is OK. To do the check without iteration the input shear and moment should be V*/phi and M*/phi, just as in checking combined moment and axial load the calculation is done with axial load/phi, and the resulting actions are multiplied by phi for the design capacity.

In my opinion the requirements for additional longitudinal steel and/or extension of the longitudinal steel are confusing and are inconsistent between AS 3600 and AS 5100.

In AS 3600 the alternative of extending the reinforcement is given as an alternative in Cl 8.2.8 (Proportioning longitudinal reinforcement), but it states there should be no sudden changes in the calculated tension force, which presumably means it can't be used when there are applied point shear forces.

In AS 5100 the procedure for extending the reinforcement is given in a separate clause (8.2.9), but it makes no reference to sudden changes in the calculated tension force, which presumably means it can be used when there are applied point shear forces.

I would be interested in knowing how others handle this in practice.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[kww2008]

I still think the use of an assumed monotonically increasing load not that of the rating vehicle (which is the reason why iteration is required in the recommended approach) requires further looking into. Since in the AS 5100 series, the MCFT-based shear capacity is calculated using equations with non-linear terms, understanding any new proposed usage requires further research. I will continue to look into this with my research collaborator.

 

[kww2008]

Please note the publication of an article relevant to the topic under discussions.

The link to the open access article is:

https://www.mdpi.com/2412-3811/7/11/156

 

[IDS]

Interesting article, and well worth a read, but I disagree with the main conclusion.

AS 5100.7 defines the Load Rating Factor as:
3.10: A ratio of the available bridge capacity for traffic load effects to the traffic load effects of a nominated rating vehicle (see Clause 14).

To find that you need to find the combination of effects (bending moment, shear, torsion and axial load) that cause the most severe combined effect, and then find the factor on the vehicle loads that would cause the combined effects to reach the section's Ultimate Limit State.

Where the capacity depends on the combined loads, such as combined bending and axial load, or combined bending and shear, then the loads need to be factored to get an accurate value for the capacity.

To do this it seems to me that the simplest approach is to plot an interaction diagram of section capacity, then plot all the load points for different vehicle positions, and for different sections with the same capacity. It is then straightforward to find the minimum load rating. This is standard practice for combined bending and axial load, and I don't see why you wouldn't adopt the same approach for combined bending and shear.

One other point: For checking the shear effect on longitudinal reinforcement in AS 3600 all the effects are combined to find the total force on the tension steel, and the applicable capacity reduction factor is the factor for bending capacity. In AS 5100.5 the steel capacity for longitudinal force due to shear is factored down by the shear reduction factor, which doesn't make sense to me, since the steel will be equally ductile no matter where the force is coming from.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/