How to determine the spacing between prestressed girders according to AASHTO LRFD ?

[Salim91]

Dear engineers,

For a given girder section, span length and span wIdth, how can we determine the spacing between girders according to the the AASHTO LRFD ?

 

[BridgeSmith]

The LRFD bridge design specs do not have prescriptive girder spacings. The girder spacing will be determined by analysis. Typically, the spacing is determined by the capacity of the deck that will be used. The girder section is then determined based on what is required for the loading. You need a qualified structural engineer to design the superstructure.

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

 

[MIKE_311]

PCI has beam tables. Identify your span lengths and then find a depth and spacing that works for the clearances you require.

 

[OSUCivlEng]

First, determine the total width of your bridge deck.
Second, subtract the width of the deck cantilever or overhang from the width of the bridge deck. Be sure to include both sides.
Third, divide that width by the number of girders less one. That will tell you your girder spacing.

No expensive AASHTO LRFD needed.

 

[STrctPono]

Salim, there are many factors that determine what girder spacing you should go with and this part of the design process is not constrained by spec requirements.

As Rod mentioned, your deck capacity typically governs the MAX O.C. girder spacing. This is an upper bound limit since likely your deck thickness is already set regardless of your girder spacing.

As Mike mentioned, your clearance is another important consideration. Many times in bridge design, shallow girder depths are desirable so as to meet clearances over roadways below or to meet hydraulic freeboard requirements.

You should also at this point know your out to out deck width as this is governed early on in the design process by roadway geometry requirements. Draw a typical section of your superstructure and lay out your girders and see what fits within the above mentioned constraints. Try and keep your deck overhangs to no more than half your girder to girder spacing. There are other things that can come into play that may affect this process. One example is deck drain locations. Figure out where these are going to be early on in the process. Do not wait until after your bridge has been designed. Deck drains can conflict with your girder locations especially if you are using wide flange prestressed girders.

Run a quick analysis to ensure the girder has enough capacity. Obviously, the wider your girder spacing, the greater your Live Load Distribution Factor and the greater your demand.