Check for Cross Section at Service Loads

Question:

Hello!

I have some concerns regarding the stresses given by MIDAS Civil on the PSC Check for Cross Section at Service Loads function of MIDAS.
1. In the figure,
.
why does MIDAS get the maximum of all stresses both in top and bottom? According to AASHTO LRFD 2012, only compressive stresses shall be checked with the ALW 24.8211 MPa and tensile stresses be checked with the stress -3.2088 MPa for moderate corrosion conditions.

2. I am a bit curious about the stresses that MIDAS produced for FB. For instance, take in consideration element 68 which is in the midspan.
FB that MIDAS Civil generated for element 68, -15.7872. I want to know how was MIDAS able to arrive on such value.
In most textbooks and my hand calculations, here is how FT and FB computed:



The above picture was taken from Nawy (2009) Prestressed Concrete:A Fundamental Approach.

Based on my manual computation (see attached, based on extracted forces from model), stresses at bottom fiber is only around -1 MPa (compression) versus 15 MPa(tension).. Why is there such huge difference?

---

Answer:
Hi,

1. According to AASHTO LRFD, tensile stress limit is 0.19 sqrt(f'c) for moderate corrosion conditions.


In your model, the corrosive condition is set as 'Moderate/Mild'.


2. The gLCB 227 which is the critical load combination for the element 68 consists of various load cases as below. In your hand calculation, only Dead Load(CS), Erection Load 1(CS), Erection Load 2(CS), Erection Load 3(CS), HL-93 (MV). Thus, the moments of your calculation are different from the program results.