Minimum Negative Flexure Concrete Deck Reinforcement (LRFD composite steel design)

Question:

Hello,


For the LRFD minimum negative flexure concrete deck reinforcement for composite steel beam design, it looks like MIDAS doesn't check for phi*fr as specified in LRFD 6.10.1.7. Per MIDAS manual,  "Under negative moment" reinforcement is checked against Ars greater or equal to 0.01Adeck. But per AASHTO LRFD 6.10.1.7, this is only required for whenever the concrete tensile stress under SERVICE II exceeds phi*fr rather than for every section that has negative moment (this section remained unchanged between LRFD 2012 & LRFD 2018). Please see attached for LRFD section 6.10.1.7 & MIDAS manual. Could you please verify whether MIDAS is checking for the phi*fr? If not, the reinforcement design will be too conservative in some cases. 


Another question is per AASHTO 8th edition LRFD 6.10.1.1.1d, "For calculating flexural stresses in the concrete deck due to all permanent load and transient loads, the short-term modular ratio, n shall be used". Is MIDAS using short-tem n for the concrete flexure stresses in this case?


Thank you very much!


Answer:


Hello,

I'll try to answer each question by part:
1)  Could you please verify whether MIDAS is checking for the phi*fr? If not, the reinforcement design will be too conservative in some cases.
A:  This check is being carried out.  A snapshot of report with this is shown below.

2)  Another question is per AASHTO 8th edition LRFD 6.10.1.1.1d, "For calculating flexural stresses in the concrete deck due to all permanent load and transient loads, the short-term modular ratio, n shall be used". Is MIDAS using short-tem n for the concrete flexure stresses in this case?
A:  This too could be noted from the image above.  For stress in deck, short term modulus is used.

Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS


Thank you for your timely response. There're two more questions regarding this issue:


1. From the screen shot of your report, under 6.10.4.2 it looks like a factor of "2" is missing. The AASHTO limits is 2*fr instead of fr. I checked the report from my own model, and it looks like my MIDAS CIVIL 2018 version has fixed the problem. Please see attached image "MIDAS 6.10.4.2 CHECK".


2. Back to the first question in my original message of AASHTO 6.10.1.7. In 6.10.1.7 the minimum negative reinforcement should be greater than 1% of deck area only when the concrete stress under SERVICE II exceed fr. In MIDAS report this checking is under strength limit, and it is not comparing the concrete stress against fr. Please see attached image "MIDAS 6.10.1.7" for more details. Please verify that whether MIDAS is checking the SERVICE II conc stress for the minimum negative reinforcement criteria, and if so, how is that reflect in the report?


Thank you very much!



Hello,

First of all, sorry I couldn't write back to you sooner.
Since the first one is clear, I'll just be focusing on second one.   The code says to check this for factored construction loads and service loads.
So, under strength, the factored permanent loads are used to check while under serviceability case, the serviceability load combination is used.  This could be checked quite easily in the attached report.  Kindly note that in this case, I've used the latest version (Civil 2019 v1.1).  If feasible kindly use that version.  If not, kindly let me know the exact version being used at your end and I'll check with that version.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/17/2018 10:45 PM (dklee@midasit.com)      Updated: 10/18/2018 8:16 AM (dklee@midasit.com)
Files   
23 Curved Steel Composite Design_methodA.xlsx
DataImage94.png
LRFD 6.10.1.7.JPG
MIDAS 6.10.1.7 CHECK.JPG
MIDAS 6.10.4.2 CHECK.JPG
MIDAS Manual.JPG