AASHTO loading interpretation for Pier design 3

[dgidwani]

For a highway bridge pier supporting simple spans on either side, should the pier be designed for one HS20 truck per lane (positioned right above the pier for maximum total reaction), or for the sum of the maximum reactions from the spans on two sides?

The latter implies two trucks being considered simultaneously, and AASHTO seems to require only one HS20 truck per lane on highway bridges. What is the generally accepted procedure, and what is the correct code interpretation?

Thanks for your help,
DG

 

[Qshake]

For the maximum load on the pier you would load each span simultaneously to arrive at the max reaction. This isn't to much of a strech to visualize assuming, of course, the spans will accomodate the trucks length.

Incidentally, using AASHTO you can't assume that the load cases which maximum superstructure stress will also be the load cases which provide max substructure stress. You have to evaluate them seperately or at least see if the superstructure case impacts the substructure the same way.

 

[koodi]

Is he saying that that the super structure is pinned at the pierwall? What does "simple spans either side mean"?

If the super structure is continuous over the pierwall, then at least my version of AASHTO, says one truck per lane.

There is no HS20 truck.

This is not a pipe.

 

[Qshake]

In the old days, my best days, it was very common to have multi-span bridges in which adjacent spans bear on a shared pier. Obviously, when continuous spans where introduced, fewer of the previous arrangements were constructed.

Multi-span bridges continue to be constructed today due to support settlement issues and stress reversal issues that are common with railroad structures.

Also, please keep in mind that you can one truck per lane and that each lane is 10 or 12 feet wide. As such, you can't really place a two 10 foot wide vehicles in a single lane. However, you can have multiple lanes loaded and there is a reduction for that case in AASHTO.

At the same time, you can have more than one truck in a single lane as well, not side-by-side, but end to end. This is what the lane load is intended to achieve.

KOODI

There is a HS20 truck but it is a design vehicle intended to produce maximum stresses which by way of the special automotive specifications cannot be reproduced for obvious reasons.

If it's not a pipe, then what is it?

 

[dgidwani]

Qshake,
About the end-to-end trucks. As you say, the lane loading is intended to cover that possibility. So, assuming one is considering the lane loading case, as required by AASHTO, then does one additionally look at the HS20 trucks back to back in each lane?. If so, what would be the minimum spacing between the front axle of the one and the back axle of the other?
Thanks for your responses.

Thank you too, Koodi.

 

[Qshake]

dgidwani,

The AASHTO lane load is 640 #/ft (for HS20 design vehicle) with one or two additional point loads applied when necessary to effect the maximum stress.

The uniform distributed loading takes into account the back to back nature of the vehicles and so the axle spacing etc is irrelevant for this case.

The point loads are applied:

One point load applied for reactions, shear and positive moment investigations.
Two point loads applied for maximum negative moments in continuous span bridges.

For the case of actual truck loading the axle spacing is taken to be 14'.

 

[mrbridge]

By asking about HS20 Loads, I assume you are using the LFD Edition and NOT the LRFD Edition of AASHTO. Listen to Qshake, but don't forget the Impact load.

 

[koodi]

If memory serves correct the HS20 lane load is not meant to represent multiple HS20 trucks. The reason that there is a multiple lane reduction is to reflect the improbability of two completely overloaded vehicles on the bridge at the same time in different lanes.

More improbable is a truck train consisting of the same overloaded trucks following one another 14 ft behind--fast enough that impact needs to be accounted for--I hope the brakes are fair.

Heh QSHAKE,
are you saying there is a Santa Claus?

 

[dgidwani]

It seems, Koodi, you're really concurring that there is no need to consider 2 HS20 trucks in the same lane.

To summarize, the maximum reaction on the pier beam should be computed by considering, in each lane, the larger of a)lane load of 640 lb/ft (times tributary length) plus the appropriate concentrated load (one load only on the centerline of the pier beam); and b)one HS20 truck placed to produce maximum (total) reaction on the beam.

Impact and multiple lane reduction to be applied per AASHTO.

Does that sound right to everyone?
Thanks

 

[Qshake]

No one wrote that the lane load was to represent two vehicles in the same lane. Thats crazy.

Moreover, the lane load does represent multiple trucks but end-to-end and not side-by-side. Read the post.

 

[rowe]

I believe Qshake is correct. The LFD std specs 3.11.4 specifies one truck load per lane shall be considered on the structure - "For continuous spans". There is no such limit for simple spans. Although I've not verified it, for reactions for a simple span, I assume that "Lane Load" will control over two or more "Truck Loads" on the same span(if you can fit 2 trucks on the simple span).

Therefore, for the original posted question, I would place a truck load on each adjacent simple span to determine the max reaction at the pier due to "truck loads" as Qshake replied. Lane loads may still control.

The specs (3.7.1.1)also state that Lane Loads are intended to be "equivalent to truck trains" (i.e. one truck followed by another, followed by another...)

Once you have determined the max. reaction due to one truck or lane load, then you need to place 12' lanes along the pier, placing the live load in each lane (reducing as allowed by spec 3.12). If you are designing the pier without the benefit of software that generates the live load/lane variations, I have conservatively used the max. reaction at a bearing and applied to pier cap in a way that maximized positive moment between columns (one load case) and maximized negative moment over columns (another load case).

 

[dgidwani]

Friends, 2 trucks per lane doesn't gel:
If its only one truck per lane for continuous spans, then why not for simple spans? How does the loading know whether the structure is simple or continuous?
If we had a pier that was designed to support 2 continuous spans of 75', and the spans needed replacement, going by this interpretation we wouldn't be able to replace them with simple spans, even of lesser span - the pier design wouldn't work.
The max. moment and shear tables (App.A) provided in the AASHTO have used only one truck no matter how long the span, even 300ft.
If 75' spans are considered, the max. reaction each side is 63.1 kips. If the max.reaction on both spans can occur simultaneously, that is 2 trucks in each lane, and that makes the reaction at the pier 126.2kips. For lane loading its only 74.0 kips (0.64x75 + 26.0), almost half.

 

[AndyZ]

Place 1 truck with the CG over the pier (you can also check with axles over each bearing if you are worried to check for absolute maximum load), or use the lane load including the point load as directed by AASHTO.

Usually for the span lengths where simple spans are economical truck controls. Use impact based on the average span length according to AASHTO. Check to see if the owner has special considerations in addition to AASHTO, like a special truck or a additional impact (Virginia DOT requires special loading for pier cap overhangs)

Place load in all lanes or any combination of lanes to maximize the load you are designing for, take reductions for 3 or more lanes as directed by AASHTO.

 

[Qshake]

dgidwani,

No one in the posts above are stating or inferring that two trucks per lane should be used! Where are you getting that from?

Not only is ROWE's post on point, ROWE backs up the statements with the correct AASHTO specifications.

The usual methodology for live load application is to:

1. Apply the appropriate truck loading (usually always controls (shear and moment) on short-medium spans and reactions for same)
2. Apply the appropriate lane loading (since lane loads will control in certain areas depending on span lengths)
3. Apply other design loading as necessary - this means Alternate Military for interstates and not rating trucks.
4. Use the maximums of 1, 2 and 3 to design various parts of the bridge superstructre.

It is clear that you don't design bridges for a living and you should consult with someone in your office who does. The travelling public deserves safe bridges and structures.

 

[CanyonRat]

I've been reviewing this thread and am finding it very interesting. I don't think I've ever run in to this type of bridge design even though it is very common. I'm betting most people design the substructures for a truck in each span.

I always make sure the bridge is continuous for live load and then there is not question regarding the substructure design. I think we all agree that AASHTO states that one truck is run across the bridge but the lane load can be discontinuous. For the superstructure design you obviously have to apply the truck to each span.

AASHTO 3.11.4 states that for continuous spans only one truck shall be considered on the structure but I'm not so sure it implies that this is different for simple spans but I don't know.

If I run one of my bridge girder design programs that allows multiple simple spans it is going to give reactions at the support based on a truck in each span. One way to handle it would be to average the reactions to the sub but make sure the eccentric moment from unbalanced spans is accounted for.

I asked the principle of our firm over structures if he had ever designed a multi-span bridge with simple spans and he had but considered the bridge continuous for live load even though it wasn't (for the substructure design only).

Interesting... I'd be interested in other opinions on this. Prestressed-precast bridges would be the obvious example along with railroad bridges. What's the standard of practice?

Everyone have a Merry Christmas,
MikeD

 

[rowe]

P.S.- You should also consider a truck load and a lane load on one span only (two separate load cases) - creating an eccentric bearing reaction (and therefore a moment) on the pier.

 

[dgidwani]

Aside of qshake ("For the maximum load on the pier you would load each span simultaneously to arrive at the max reaction. This isn't to much of a strech to visualize assuming, of course, the spans will accomodate the trucks length." and rowe ("I would place a truck load on each adjacent simple span to determine the max reaction at the pier due to "truck loads" as Qshake replied.&quot, its seems everyone else interprets one truck per lane. I wonder if there is a published reference that can be conclusive.

By the way, qshake, I do not design bridges for a living, but I think your personal remarks were unwarranted, and I have a message for you in return : the tax-paying public deserves their money to be efficiently utilized by engineers who understand and know what they're doing. Thats what we're doing here.

Happy holidays.

 

[Qshake]

dgidwani,

My apologies if I offended you. My point is, based on experience in this forum for a couple of years, that some users arrive at this site as a "cure-all" so-to-speak of whatever ails them or to obtain a "black and white" answer for what is likely to be a gray area at best. Obviously, not knowing what your background is and what you're likely to do with the information gleened from this site, I was strongly pointing out that, as engineers, we must have the public at heart and in our minds when designing infrastructure elements.

I agree with your statement as well. What concerns me is that there is no one else where you work that has the experience and knowledge (as the other forum members do) necessary to check your work and provide a valuable source of experience for you.

It was not meant as a personal attack on you.

Regards,
Qshake

 

[dgidwani]

Qshake,
I appreciate and accept your clarification, and that you meant well; and there may even be people here who need and have the time for your advice on ethics, but I only visit this website for serious technical discussion, and to learn from the technical knowledge others may have.
So shall we get back to technical?
Do you know of any reference that directly addresses this particular issue?

 

[rowe]

dgidwani,

The term "one truck per lane" refers to the to the longitudinal direction or "end to end" (NOT side by side).
AASHTO is specific in section 3.11.4 when stating that, FOR CONTINUOUS SPANS, one truck load per lane shall be considered ON THE STRUCTURE (therefore, KOODI's first post is correct because he qualifies it with "continuous" spans. From my point of view, the omitting of simple spans in the statement implies that as many truck loads (aka axle loads) shall be placed ALONG a 12 foot lane (in the longitudinal direction) to produce the maximum stress for whatever component you're designing.

AASHTO 3.6 addresses Traffic Lanes and basically specifies that you don't put more than one truck or lane load within a 12' lane WITH REGARD TO THE TRANSVERSE DIRECTION (i.e. if you have a 35'-11" roadway width, you can only place two 12' lanes on the roadway and each truck or lane load must be within their respective 12' wide lane.

KOODI's second post seems a little misleading (sorry KOODI). AASHTO states that LANE LOADS EQUAL TRUCK TRAINS (multiple trucks though not necessarily multiple HS20 loads)
however, the reduction for multiple lanes loaded is to account for the improbability of truck/lane loads side by side AT THE SAME LOCATION (longitudinally) THAT PRODUCES MAXIMUM STRESS IN THE COMPONENT - not just "on the bridge".

In conclusion, I would reiterate that you should place a truck load in each adjacent span, as close to the pier as possible, to determine the max reaction on the pier due to truck loads.

Also, with regard to ANDYZ's post, you would only use an impact coefficient based upon average span lengths of adjacent spans if the structure was continuous, otherwise, use individual impact coeff. based upon the span length of the individual simple span length.