huangyhg

bearing pad design for horizontal loads
upasena (structural) 26 sep 08 13:44
i have a bridge with 4 elastomeric bearing pads on the
left bank with movement restricted along the axis of
bridge. the service horizontal force on one bearing
is 2300 kn (max),laminated bearing pad is 700mm x 600 mm x 83 mm thick.
to maintain fixity in the required direction i need 2 stainless steel dowel pins each of diameter 110 mm
with an additional rubber cap. the dowel us secured to the
masonry plate. the pins are 1200 mm long, approx. 600 in the pile cap and 600 in the bridge beam.
the diameter and length arrived at considering tensile
strength of the pin and bearing against concrete
my questions are " is this a feasible solution or an odd design? is there a better arrangement?
i shall thank an expert to express his opinion so that
i can improve my design. i feel 2 nos 110 mm dia pins
are somewhat bulky.
with kind regards!
upasena


2300kn is a large force. from your post it sounds like you are not allowing either end of the span to move with thermal expansion, which could explain the magnitude of the force you are trying to restrain. if this is the reason for the force, what is the reason you are not allowing movement at one end of the bridge? typically one end of the span is allowed to move.
depending on the grade of steel you are planning on using, 110mm pins may be realistic for forces of that magnitude. depending on bridge type and span, i may be concerned about introducing that much compression into the super-structure and ballast/retaining walls.
to:gwynn (structural)
dear sir, thank you very much for giving me a guidance on
this subject. the reason i am getting a high horizontal force is that the tractive forces accoding to bs codes.
this is an arch bridge. span 37.5 m, width 10.4 m
my lb bearings (4 nos) are fixed in the direction of
the axis of the bridge-say x-dirn, whereas rb bearings
are allowed to move in x-dirn. therefore total tractive
force combined with other loads give a hz force of 2300 kn
the bearings are placed under end cross beams which in
rest on the pile caps. piles/pile caps are designed to
take up hz force coming through 4 bearings
assuming that calculated hz force is justified, could you
please comment on my intension to insert 2 nos 110 mm dowel pins in two holes made in the rectangular bearing
pad.
or do you think part of the hz should be transferred to
the breast wall behind the end cross beam
======================================================
| b |g|e []brg |
| r |a|n |
| e |p|d |
| s |g|c []brg -|----->2300 kn
| t |a|r | <-lb rb->
| w |p|o []brg -|----->2300 kn
| a |g|s |
| l |a|s |
| l |p|beam []brg |
=======================================================
from your comment i felt that i should re-check the
analysis to see if i have overestimated the tractive
force
once again thank you very much for your valuable time.
i am sorry, i do not know how to attach a drawing to
this post
kind regards!
upasena

assuming the tractive forces are correct, your solution seems reasonable. not knowing the bs codes, the grade of stainless you plan on using, fatigue requirements or being familiar with common practices in the uk, i really can't comment further on that.
if the pile caps and piles have been designed for the loads, i see no reason to change where the load is going.
as a rough check, assuming 2000kn comes from the tractive forces, a cooper e-80 loading and 25% of the axle load as tractive forces amounts to roughly 45 axles needed to account for the 4000kn total load from your diagram. the number of axles needed would be reduced if you are including a dynamic load allowance in the service load, but would still require two trains travelling in the same direction with all of the tractive load going into two of four bearings. if you meant the total factored load under a limit states type design, the force is much more reasonable.
ps: my time isn't all that valuable. i'm on these forums after hours in part to expand my own knowledge.
to:gwynn
dear sir, your first suggestion was perfect. in my
re-analysis i found the error that i had fixed bearings
in the x-direction. when they are released the statics
are tallying perfectly and reactions are much lower.
the high value of hz forces was not coming from bs codes.
now i have no doubt.
thank you very much for your observation of high hz forces
and good suggestions made.
regards!
upasena

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