continuous beam supported on springs 1

[EngMan40]

I have multi-span beam that is fixed on one end and is supported on 4 spring supports: |------3-----3-----3----3 the beam is not on ground. I want to analyze this beam with K value of the beam is known. Is there a theory or procedure to know the reactions and displacements of the springs for statically indeterminate beams, load is hydrau-static with maximum at fixed end.

 

[EngMan40]

BAretired: Concrete/neat grout (5000 PSI). I am not sure where the fixity point would be to be honest (FEM analysis showed moment inflection point below top of socket but not at fixed point with respect to that top). subsurface investigation was carried out and rock breaks are 8,000 PSI strength. what do you think?

 

[BAretired]

I would need to think about it a bit more. In the final analysis, the precise point of zero rotation of the pile and the variable load between that point and the top of rock is going to be a rather crude estimate at best. Perhaps that justifies the assumption of tiebacks as pins instead of springs.

BA

 

[Ron]

I would not approach this using springs, either. The pile has fixity, in my opinion. The tiebacks are propping a cantilever. Assuming you develop the strength of the tieback without soil mobilization, it is a pinned connection.

 

[IDS]

Ron - what do you mean by "develop the strength of the tieback without soil mobilization"?

I'd suggest doing the analysis with upper bound anchor stiffness (i.e. pinned supports) and lower bound stiffness, as a starting point.

If this is modelling loads due to excavation, with anchors placed as excavation proceeds, the results of any single stage analysis are going to be substantially different from what occurs in the actual structure, but if you cover the envelope of possible loads you will be OK.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[EngMan40]

The construction sequence is going to be: (1) drilling for the soldier piles and socket them into rock (2) install the lagging ( in this case sheet piles) (3) excavating the first stage and installing first row of tiebacks (4) excavation of subsequent stages and installing tiebacks. Since the socket is installed before excavation then The question is: am I conservative if I design the rock socket using results obtained from structural analysis of continuous beam fixed at one end with three or four pins? should I consider the effect of excavation and installation sequence in the analysis and increase the moment accordingly ?

 

[rb1957]

calling these props "pins" mean they rotate but don't compress, yes?

modelling as springs mean you're accounting for compression, yes? ... why not ?

fixity to the rock is one thing, fixity at the horizontal beam is something not being talked about (maybe 'cause you guys know) ... is it s single point attmt (ie pinned) or a multiple fastener (ie some moment capability) ?

if the choice is critical to the structure ... why not choose the most conservative ? by that i mean analysis the various models, and pick the most conservative result at each point (ie not consistently fixed or pinned, but which ever model produces the local maximum load) ?

Quando Omni Flunkus Moritati

 

[BAretired]

rb1957,
Tiebacks are usually cables anchored into the soil or rock and in this case, prestressed to 100 kips. They can be horizontal or angled downward.

Where the cable intersects the pile, it is hinged but the cable will strain with additional load. The total magnitude of strain in inches is expected to be P/300 where P is the cable tension in kips. Cables should be modeled as springs.

BA

 

[paddingtongreen]

The later information now makes some sense of the original question but I still have little experience of this type of work. The problem, of course, is that most of us are used to precise placement of members and loads, where this is an imprecise black art.

Just trying to think my way through this.

Drill holes, drop soldier piles in holes, place concrete in the rock to embed the pile.
Pile is not in contact with undisturbed soil, it is a cantilever sticking up from the rock. Is the hole flooded?
Excavate to first tieback level, place and pre-tension the tieback.
The pile is a cantilever being pulled towards the soil it is to retain. This soil must be dewatered or else it would pour in during excavation.

When the second level of tiebacks are tensioned, it will relieve the installed tension in the first level, and so on with each level. They probably go back and adjust with a torque wrench.

The retained soil still is not against the lagging but when dewatering stops, the gap will fill with water, I think.

I know this doesn't answer the question.


Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin