Analytical Bolt Calculation
Analytical Bolt Calculation
(OP)
Hi eng-tips engineers!
I'm designing a bolted connection for a construction.
I'm quite unsure how to handle this statically indeterminate system shown on the picture below:
http://imgur.com/GNlr2G6
I know how to approach single bolt groups which are eccentrically loaded. This i usually do by moving the force to the center of the bolt group, and adding a moment.
However in the situation depicted on the image i dont know how to do this. One idea i had was to model the beam as being fixed in both ends (shown below), and then find the reaction forces at each end to the bolt group.
http://imgur.com/tcK9wYG
Would my idea work, or do you have other suggestions?
I'm designing a bolted connection for a construction.
I'm quite unsure how to handle this statically indeterminate system shown on the picture below:
http://imgur.com/GNlr2G6
I know how to approach single bolt groups which are eccentrically loaded. This i usually do by moving the force to the center of the bolt group, and adding a moment.
However in the situation depicted on the image i dont know how to do this. One idea i had was to model the beam as being fixed in both ends (shown below), and then find the reaction forces at each end to the bolt group.
http://imgur.com/tcK9wYG
Would my idea work, or do you have other suggestions?






RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
However, lets say that i do carry all the load in friction. And lets call the load P, pretension per bolt N, number of bolts n.
I could find an estimated required pretension by simply taking P=n*µ*N, i.e. N=P/(n*µ).
This is what i've done so far. However, i think that the friction also needs to overcome the moment. And then there's also the fact that due to the statically indeterminate nature of the problem, the inner bolts will carry a larger fraction of the load than the outer bolts and will hence need more pretension...
Am i making this more complicated than needed?
RE: Analytical Bolt Calculation
one approximation would be to say this is a fixed cantilevered beam, so the fasteners react the end moments (=PL/8) and the direct shear ... an easy enough calc. this is one extreme of the support reactions.
the other extreme is to consider this a pinned beam, so the fasteners react only the direct shear.
possibly the real answer is between these extremes.
Quando Omni Flunkus Moritati
RE: Analytical Bolt Calculation
I agree that it can be difficult to calculate precisely how much friction you have - but as in most other complicated situations that just mean you choose your unknowns conservatively - i.e. a low coefficient of friction.
However, and note that i'm no expert, i've been taught to never let the bolts carry in shear, and always make sure that shear is carried by the friction.
RE: Analytical Bolt Calculation
If this was an usual structural connection, the bolts would be assumed to carry equal loads, some aid from yielding is assumed. All this assumes that the bar is sufficiently sized for stability.
If you want real help, give more detail, loads size and type of bolt etc. and some evidence that you are indeed a structural engineer (think through what information people need to give an intelligent reply).
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Analytical Bolt Calculation
I think the keywords i needed to hear was yielding and thereby load redistribution.
The reason i haven't provided much detail about the problem, is that the design is really still up for discussion. However i know i'll be having a load situation as the one depicted.
To clarify which orders of magnitude we're in, the load in this situation will be ~600 kN, and the bolts will be somewhere between M30 and M40. The distance between the two bolted connections is ~2 meters.
RE: Analytical Bolt Calculation
I'd probably say that it's fixed for connection design, analyze the beam to the centre of the bolt group, then take the moment into the couples formed in each of the two orthagonal directions. I'd spread the vertical load evenly across all four bolts in the connection. Then I'd add all the vertical components and then take the sum of the squares to add in the horizontal components from one of the couples. Then I'd design the bolts to take the force in shear.
If you actually want to take it in friction the code way to do it would be to designate the connection slip critical and use the capacities given for the different faying surface conditions. You would then have to specify that the contractor is to pretension the bolts and prepare the faying surface as per whatever assumptions you made.
RE: Analytical Bolt Calculation
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Analytical Bolt Calculation
vertical shear on each bolt: V_vert=P/8
and approximating moment in each of the two connections: M=PL/8
which gives horizontal forces: V_horz=M*d
and then design again the force of V_resultant = sqrt(V_vert^2+V_horz^2) ?
RE: Analytical Bolt Calculation
Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis
http://www.engissol.com/cross-section-analysis-des...
RE: Analytical Bolt Calculation
you should notice that each fastener sees very different loads. if you're Sure about the load direction you can "optimise" the fasteners, so long as you re-do the bolt group ... making the less loaded bolts smaller increases the load on the more highly loaded bolts, so you probably won't gain much doing this.
reacting the moment equally at the four fasteners implies perfect placement, or in the real world a small amount of redistribution (imagine the moment is reacted by only two fasteners, 'cause the other two are slightly misplaced and don't bear up as the ideal design suggests).
fastener shear and plate bearing are most likely failures. the stresses in the plate need to be looked at too.
Quando Omni Flunkus Moritati
RE: Analytical Bolt Calculation
Right now i find the components from the moment as Rx = M*y/I_p and Ry = M*x/I_p.
With regards to optimising bolt placement and size, i cannot change these parameters unfortunately.
And you're spot on, on the perfect placement requirement. This is why i'd prefer to carry in friction. However it seems that friction is not viable, and hence the question is how many bolts will in reality carry the load. What does one usually estimate wrt this?
RE: Analytical Bolt Calculation
do the bolt group, find the loads on the fasteners, figure out the size of fastener required (from shear and bearing).
i realise your inexperience (which is fine, we all had to start sometime) ... but this is about a 1/2 hr calc.
Quando Omni Flunkus Moritati
RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
RE: Analytical Bolt Calculation
The only tricky part is that you will be putting in a "fake" eccentricity (e= M / V) that allows you to design for your end reaction moment.
RE: Analytical Bolt Calculation
There is ample precedent in the history of science for the overwhelming bulk of the scientific community strongly believing in imaginary entities postulated by a favovered theory. -Michael Behe
RE: Analytical Bolt Calculation
V = P/2 for symmetrical application
M = PL/6 for fixed ends
e = M/V = L/12
L = distance between centers of boltgroups.