Bridge beam design

[BAGW]

HI All,

I am designing a bridge beam spanning between two piles which is 24" deep. The beam spans 5'-6" and factored load on the beam is 200 kips. I am checking the following,

1) Punching shear for the beam under the steel column
2) One way shear for the beam at a distance "d" away from column. Provide stirrups as plain concrete is not sufficient to resist one way shear.
3) Punching shear under the piles.
4) Torsion check assuming 3" offset for columns. As the beam is deep no torsional reinforcement is reqd.
5) Provide reinf for bending of beam btw piles.

Is there any other any other check that needs to be done. Is there any requirement at concentrated load location?

 

[Agent666]

I'd imagine you'll need to design the joints at ends and centrally for transfer of the appropriate design actions you've got going on due to the continuity.

 

[r13]

The beam is a one way structure, punching shear will not occur. The stirrups need to be placed over the entire beam, as there is a concentrated load in the middle. Design the beam with both fixed-fixed and pinned-pinned conditions, and provide flexural reinforcement accordingly.

 

[BAGW]

Agreed. The stirrups will be placed over the entire length of the beam.

Thanks for clarifying the punching shear requirement.

Also when checking the shear you check for Pu/2 correct and not Pu?

Also is it reqd to design for fixed fixed condition? Can we get away with pin pin condition. Design for wl2/8 and provide all reinf at the bottom and throw couple of bars on the top?

 

[r13]

Also, doesn't the column have shear at the base? You need to check the strength of the beam to hold the anchorage, and pile to beam connection.

 

[r13]

Yes, Pu/2 is correct. Theoretically Pin-pin condition is not required if you provide rebars per fix-fix condition. It is a conservative practice, considering the beam-pile connection is not a true fixed support. For short span one way beam like this, I would agree it is suffice to provide equal amount of steel on top and bottom per the negative moment requirement.

 

[BAGW]

Are you asking about the steel column? There is no shear at the base of steel column. This is a pure gravity column.

What do you mean by "strength of the beam to hold the anchorage"?

The rebars in the piles are extended into the beam as shown. I think that should suffice. How do you maske sure the connection is good? If I am assuming pinned connection, the bearing of the beam on the pile is a check and need not be a problem.

 

[r13]

Just a reminder, don't forget self weight of the beam.

 

[BAGW]

Yes. Beam self-weight is considered.

Could you please elaborate on beam to pile connection? and also the anchorage comment.

 

[r13]

Pure gravity column is a rare animal in the structural word. Usually it will have shar, and moment coupled with the shear. If that is the case, then the anchor bolts (embedment) will have to be designed to provide resistance for those forces, and transfer those forces to the concrete, in turn the concrete will have to have adequate strength to prevent the breakout of (hold) the anchors. For specific details, see code provisions for design of steel embedment in concrete structures.

If you consider fix-fix condition, then you need to ensure the reinforcing steel from the pile are fully developed into the beam - full tension development length measured from the top of pile into the beam, with a hook as necessary. For pin-pin condition, a merely 6" of steel from pile into the beam is considered suffice.

Your sketch of the beam-pile connection looked fine, but the pile now needs to be designed for the axial load and the moment to counter the beam end forces.

 

[Agent666]

I'd check your code in terms of turning out the column bars like shown. This is not allowed in most codes I'm familiar with. Draw a strut and tie model for how the forces in the joint are working, and you'll see that to anchor the diagonal strut you are required to turn the bars inwards.

There is no such thing as a gravity only column either in monolithic concrete design, there will be bending do to the continuity or eccentricity of axial loads. Review and design for this aspect.

You design the shear for the shear an effective depth out from the support, so it will be slightly less than the Pu/2 + 1/2 the self weight of the beam and half of any loads applied to the beam itself.

 

[r13]

Even for pure gravity column, I remember a small eccentricity is required when sizing the steel column, you shall consider it as a incidental load for the design of connection anchors. Also, without specific reference, I suggest to consider a shear force, at least equal to 2% of the column axial load, at the beam-column interface. You should use the moment (incidental load) and the shear force to determine the requirement of the anchor bolts (size, embedment length), and check the adequacy of the beam to hold the anchorage in place. You may wounded up needing a deeper beam, or more reinforcement at the anchorage location.

The last, don't forget to check deflection according to service criteria.

 

[BridgeSmith]

With reinforcing connecting the pile to the beam, you should either take the simple and conservative route and design the beam and piles for a fully fixed condition, or analyze the system (beam and piles) as a frame. There will be restraint at the pile to beam connection, unless you change the detailing to provide hinges at the pile to beam connections.

If it remains detailed as shown, the negative moment reinforcing in the top of the beam will need to be adequately developed at the ends of the beam.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Sorry for my slow though process, here is another catch. You beam well fall within the "deep beam" category, so the shear check shall follow provisions for deep flexural members. A strut and tie method mentioned by Agent666 is just.

I don't recall ACI prohibits the use of the 90° hooked bars (maybe it does for the seismic applications). If any problem, a 135°, or 180° bend shall do.

 

[BAGW]

Thanks for the inputs

If using strut-tie model I dont need top rebars. Designing the beam as pin-pin condition gives me nearly the same bottom bars as strut-tie model.

Technically I done even need to use strut-tie model as the entire beam ends up in "B" region where the classical beam theory still applies.

 

[BridgeSmith]

Your actual beam does not have pinned connections. You will not get a realistic approximation of the moments in the beam and piles unless it is analyzed consistent with its actual restraint conditions. If your analysis shows no reinforcing required in the top of the beam, it's likely because the restraint conditions are not realistic.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

I've designed pile head with pinned condition many times whenever lateral load is negligibly small, and project is not located in high seismic activity region.

 

[r13]

BAGW,

What is current code requirement for deep beam shear design? That is what you need to do.

 

[BAGW]

With strut tie method gives only bottom bars.

 

[r13]

If the beam shear strength is adequate after strut tie method, I'll simply reinforce the same amount of steel top and bottom, why to save 1 or 2 bars for an important foundation. The fixity of the connection should depend on whether the deflection of simply supported beam is acceptable or not. If acceptable, you may get ride of the hook tails, use straight bars only.