Bending moment results of strap beam

[MUHANNAD.ABU_ABDO]

Hello, during analyzing & design strap beam connected two isolated footing i found the following results as in the pics below 

Max negative moment obtained from robot 77.92 t.m But the -ve moment obtained from hand calcs is 92.9 t.m

Max positive moment obtained from robot 41.4 t.m But the +ve moment obtained from hand calcs is 48.07 t.m

Why there are discrepancies in results ? 
Model file attached

Thanks in advance 

[Artur.Kosakowski]

Hi @MUHANNAD.ABU_ABDO

What is the static scheme you assumed for your hand calculations? What is the span length between the supports (was the real size of the foundation considered) ?

[MUHANNAD.ABU_ABDO]

I assumed the soil pressure is uniform stress under the footing and get the desired dimensions , strap beam not considered to exposed to soil pressure . all the assumptions considered in the model & hand calcs are the same 

[Artur.Kosakowski · Accepted solution]

Hi @MUHANNAD.ABU_ABDO

OK. Let me ask the question in the different way. Have you included the stiffness of the panels in your hand calculations or are you directly comparing the results for a beam on elastic soil with your more accurate model?

 

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[MUHANNAD.ABU_ABDO]

No i didn't include the panels stiffens in the hand calcs cuz this's gonna be complex little bit , strap beam will just carry the overturning moment , and the two footings are exposed to soil pressure ! in this case i couldn't assume the bar is elastic on soil ! is that wrong ? 

do u mean when i want to compare between hand calcs and robot i have to reduce the panels stiffens cuz the manual method assume all the overturning moment will directly transferred to the strap beam ! but this'not the exact behavior and the panels will take some moments ? 

[Artur.Kosakowski]

Hi @MUHANNAD.ABU_ABDO

IMHO it is exactly what you should do as proven by the test model shown on the picture I attached to my previous post.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[MUHANNAD.ABU_ABDO]

Thanks for your clarifications , Last question soil of elastic beam  should defined as t/m2 or t/m divided on the beam length ?

[Artur.Kosakowski]

Hi @MUHANNAD.ABU_ABDO

Force that cause a beam of a unit length with given width (e.g. 0.5 m) to move (downwards) by unit of displacement 

(t / m * m * m) * 0.5m [t/m2]

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.