AS4100 Bracket Steel Design Error

[Max-88]

Hello,

It seems that the steel design to AS4100 does not follow the requirements of section 5.6.1.1(b) Segments of varying cross-section for the design of a member with bracket.

Moa is not reduced by alfa_st as per 5.6.1.1(b)(ii)

Attached screen shot with calculated Moa, model and manual calculation for Moa.

cheers

max

[Artur.Kosakowski]

Hi Max,

I'm looking into it now and this is what I noticed so far:

dm value used by Robot is from the middle of the section rather than the minimal one. This should be corrected but this is not the factor responsible for such large difference of the value of Moa

J displayed in the note is incorrect. The value used (correctly) for the verification is from the current section and it is for the verified location along the bar 8:

J is 0.723e10-6 instead of 1.004e-6

The value of Mo you calculated seems to be wrong and this is what I think stands for the difference:

68218 Nm = 683 kNm 🙂 

[Max-88]

Hi Artur,

Honestly, I have used the value displayed by Robot as my intention was to check Mo 🙂

The steel design for bar 8 - in the model I posted - returns J = 1,004,049mm^4

That aside, have you considered all the rectangles in the section when calculating J? There are three flanges and a web 🙂

Nonetheless, I have subsequently done a calculation and found J=978,983.26mm^4

The computation is based on formula:

J=1/3 * Sum(b*t^3) for thin walled open section made up of rectangular elements.

Hope I got it right-ish as the result is not much different from value displayed by Robot.

cheers

max

[Artur.Kosakowski]

The values of Iw and J are calculated as for an I section instead as for a section withg 3 flanges. The same for slenderness of the web. I have asked for having them calculated as for a section with 3 flanges. 

[Max-88]

Hi Artur,

I am referring to steel design for member 2 in the model I have attached previously.

The critical section is outside the bracket/haunch.

I have manually calculated the reference buckling moment Mo and the value I have come up with is different from the value shown in Robot Detailed Results - despite using the very same values for E, G, le, I_y (minor), Iw and J.

Below my calculation showing Mo=430.7kNm. Calculations are done with Mathcad Express and double checked in Excel.

Robot displays Moa=448.44kNm. 

Is there an error or round-up in the computation / formula used by Robot? 

cheers

max

[Artur.Kosakowski]

Hi Max,

As part of this bar has got tapered section Mo you calculated is multiplied by alfa_st which causes the difference. If UB was on the entire lenght of bar 2 then:

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Max-88]

Hi Artur,

1. The critical section is at 4.42m from node 3, based on  Robot findings.

This section belongs to a segment of 4.8m long that starts from node 3 in the direction of node 4 - based on restraints assigned to type Rafter_1. 

Now this segment is of constant cross-section and no alpha_st applies.

2. Say the segment is of variable section though this would defy above.

The calculated alpha would be 1 (one) and Mo remain as I have calculated it.

alpha_st = 1.0 - ( 1.2 * r_r * (1 - r_s ) ) = 1.0 - ( 1.2*0.5*( 1 - 1 ) ) = 1.0

r_r = 0.5 for tapered members

r_s = A_fm / A_fc * ( 0.6 + 0.4 * d_m / d_c )

A_fm = A_fc <-- same flange

d_m = section depth at minimum cross-section = d_c at critical cross-section = 460mm

3. Where does Robot display alpha_st? 🙂

cheers

Max

[Artur.Kosakowski · Accepted solution]

Hi Max,

1. The critical section is at 4.42m from node 3, based on  Robot findings.

This section belongs to a segment of 4.8m long that starts from node 3 in the direction of node 4 - based on restraints assigned to type Rafter_1. 

Now this segment is of constant cross-section and no alpha_st applies.

Robot doesn't recognize internal bracings as divisions for section "shape" (tapered vs constant height). The bar is assumed as tapered reardless of their definition. 

2. Say the segment is of variable section though this would defy above.

The calculated alpha would be 1 (one) and Mo remain as I have calculated it.

alpha_st = 1.0 - ( 1.2 * r_r * (1 - r_s ) ) = 1.0 - ( 1.2*0.5*( 1 - 1 ) ) = 1.0

r_r = 0.5 for tapered members

r_s = A_fm / A_fc * ( 0.6 + 0.4 * d_m / d_c )

A_fm = A_fc <-- same flange

d_m = section depth at minimum cross-section = d_c at critical cross-section = 460mm

As I wrote in the post 2:

"dm value used by Robot is from the middle of the section rather than the minimal one. This should be corrected"

3. Where does Robot display alpha_st? 🙂

Currently it is not displayed.