## Stuck on this Frame analysis.

## Stuck on this Frame analysis.

(OP)

Stuck on a Frame analysis.

I keep getting a moment of 22.6 KNM whereas using the prokon frame analysis module I keep getting around 32.

I analysed the frame without the sway correction as of yet, but I genuinely do not feel it would amount to such.

I have tried introducing points of contraflexire between the rigid joints but can’t seem to match the software analysis.

Kindly let me know what I am possibly doing wrong.

I keep getting a moment of 22.6 KNM whereas using the prokon frame analysis module I keep getting around 32.

I analysed the frame without the sway correction as of yet, but I genuinely do not feel it would amount to such.

I have tried introducing points of contraflexire between the rigid joints but can’t seem to match the software analysis.

Kindly let me know what I am possibly doing wrong.

## RE: Stuck on this Frame analysis.

why isn't the moment 25.8/2*5 = 64.5 kNm ? (without the swaybrace)

"Wir hoffen, dass dieses Mal alles gut gehen wird!"

General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

## RE: Stuck on this Frame analysis.

## RE: Stuck on this Frame analysis.

Review your textbook to find out how to handle sidesway in frames.

## RE: Stuck on this Frame analysis.

But the load in CE and DF is much smaller because the moment is taken out by CD.

CD's loading is two moments (and the ends) with the same sign, reacted by a couple.

And the moment taken at C is dependent on the stiffness of CD ... the stiffness of CD is not only keeping the angle between CD and AE as 90 deg (though this is more a function of the joint design), but it is also stopping CD adopting an "S" shape.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"

General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

## RE: Stuck on this Frame analysis.

I have been using Hibbeler for my analysis, it works be great if you could point me to a specific book to use.

I do understand the moments at Joints where the bracing after would be greater.

I’m unable to wrap my head around how to break this frame into Free body diagrams set points of Contraflexure to be analysed.

## RE: Stuck on this Frame analysis.

From my understanding the stiffness K = EI/ L

assuming E and E are constant won’t that mean that K = 1/ L, so if the beam shown is 7.7 m

won’t that make it less stiffer?

## RE: Stuck on this Frame analysis.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Stuck on this Frame analysis.

this being a problem, the assumptions should be stated.

In my "real world", we might often assume a 60/40 split, and conservatively either split (40/60 or 60/40)

I believe the analysis assumes that joints C and D are not rotating (more than assuming the CD is perpendicular to AE).

"Wir hoffen, dass dieses Mal alles gut gehen wird!"

General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

## RE: Stuck on this Frame analysis.

I hesitate to recommend specific books, as I am not familiar with most of them. The Hibbeler text may be perfectly adequate for your purposes. The book I used prior to retirement is "Statically Indeterminate Structures" by Chua-Kia Wang, PhD, published by MGraw-Hill in 1953. If that is available, I recommend that.

## RE: Stuck on this Frame analysis.

You may be correct, but I would have to review Wang to see how it would be affected. If I remember correctly, in cases of sidesway, there are some corrections to be made to account for movement of joints. My guess is that the horizontal reactions would be pretty close to 50/50.

## RE: Stuck on this Frame analysis.

The moments in member EF and EC are around 30 KNm and in the CD member slightly higher at 32 KNm(from software analysis)

Regarding the behaviour of the frame under the lateral load, I understnd that the joints at C tend to open more (as compared to E) due to the pin support at A.

Hence to resist the joints opening a larger moment would be required at that joint.

How would we proceed with analysing the frame by hand?

## RE: Stuck on this Frame analysis.

That will get you closer to the software answer, the additional difference will be due to the axial deformation effects of the members. Another possible source of difference might be shear deformation if the software considers it. For textbook problems like this where the member properties are not intended to influence the results use very stout sections and make sure the results you are looking at do not include member self weights.

## RE: Stuck on this Frame analysis.

MDM for Joint C displacement (Mq1 model)

Assumes a moment for the upper column of 6EI delta,E / (3.5)^2 = 100 kN*m, fixed end moment for the lower column is scaled accordingly:

Note I only did 5 cycles of MDM so that will impact the final answers a bit.

MDM for Joint E displacement (Mq2 model):

Solve for the c1 and c2 constants in the joint load scaling functions:

Final Joint Moments:

From here you can see the top column moments aren't symmetric so the mid-height inflection point assumption of the portal method is not accurate for this case.

Analysis Program with large members to limit secondary effects:

## RE: Stuck on this Frame analysis.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"

General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

## RE: Stuck on this Frame analysis.

If the height is 5m and the span is 7.9m, FZ should be 25.8*5/7.9 = 16.3291; you found it to be 16.7532.

I don't know why there is a difference. (7.9 should be 7.7)

Using hand methods, the axial deformation was ignored in my day, but perhaps in some cases it should be considered.

## RE: Stuck on this Frame analysis.

Oh I read the span to be 7.7m.

## RE: Stuck on this Frame analysis.

To clarify my hand calc. did not include axial stiffness and in the software I used large members to make the impact of the axial deformation negligible I also disabled shear deformations.

## RE: Stuck on this Frame analysis.

@Celt83

Is there a reference book I can use as a guide to understand the sway model and the moments at the joints as you have mentioned, I will use this as a guide to develop my own personal excel sheet.

I am slowly understanding the frame behaviour thanks to all of y’all.

## RE: Stuck on this Frame analysis.

## RE: Stuck on this Frame analysis.

Consider the question as given, and its mirror image. Now add the two together.

We hav e net compression of 25.8 in the top member, and no forces anywhere else.

yet if we add the reactions for each pinjoint, at the left we have -12.46--13.33=0.7 kN

same at the other side -0.7 kN

Is this due to axial compression of the top member springing the posts apart at the bottom?

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Stuck on this Frame analysis.

The reason is that in the absence of compression, we are agnostic about where the force is applied to the top member, so split it into two and apply half at each end of the top member. Now cut the structure down the middle and we have two essentially identical structures taking into account various sign changes, hence the reaction force must be equal.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Stuck on this Frame analysis.

Interestingly enough it seems this structure is sensitive to the axial rigidity of the members when the load is applied to the corner nodes only.

Models:

@infinite/nearly infinite axial rigidity (W44x335):

@Soft axial rigidity (W8x10):

## RE: Stuck on this Frame analysis.

General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

## RE: Stuck on this Frame analysis.

So for the LH pin joint we'd get -12.2-(-13.6)=+1.4, and for the right -13.6-(-12.2)=-1.4

and top right agrees.

Cheers

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

Greg Locock

## RE: Stuck on this Frame analysis.

@celt83 I was able to understand most of it following the Hibbeler book and resource material found here :http://www.facweb.iitkgp.ac.in/~baidurya/CE21004/o...

Some final thoughts before I close the thread would be regarding the behaviour of elements under loadings and how these rotation or displacement formula were derived in the first place.

Thank you all.

## RE: Stuck on this Frame analysis.