Advanced Creep Analysis for Two-Span Bridges Explained

Oct 18, 2023
1 minute read
BLOG BRIDGE INSIGHT

Creep Analysis 5 MIDAS Example 

 

 

In this article, the previous method explained in Creep Analysis 4 is applied to a real-world bridge and compares the output with MIDAS??

Consider a two-span bridge as shown below.

Picture1-4Two-span bridge
 
Picture2-1Stage 1
 

 

Picture3
Stage 2

 

The typical section is as shown.
Picture4Typical section
 

 

Section properties are:

 

 

Area

46.5119 ft2

Moment of Inertia

303.5361 ft4

Perimeter (outside)

74.0359 ft

Perimeter (inside)

35.1884 ft

Material properties are:

f'c

6 ksi

RH

70%

                 Concrete

Normal and rapid hardening

 

Graph1

Creep Coefficient

 

Graph2

Elastic Modulus (MPa)

 

From the graph,

φ(∞, 7) = 2.19                                   χ(∞, 7) = 0.74                                    φ(60, 7) = 0.975

φ(∞, 60) = 1.46                                 χ(∞, 60) = 0.82.

 

Ec(60)/Ec(7) = 1.040/0.882 = 1.179.

At time t1 = 60 days, the member end forces due to self-weight at span 2 can be calculated as below. Note that Ec(60)/Ec(7) = 1.179. (MIDAS file ex.2C)

 

Picture3-Jan-25-2022-02-32-29-52-AM

[Member End Force] = [Stiffness]{Displacement} + {Fixed End Force}

[Stiffness]{Displacement} = [Member End Force] - {Fixed End Force}

 

Picture6

Analyze two-span continuous beam for these member end loadings. Note that the section properties are as below and the loading directions may not be identical for each program. (MIDAS file ex.2D)

Picture7

Results (ft, kips)

Picture6-3

Finally, the member end forces are:


Span AB

Span BC

Check this result with MIDAS output (MIDAS file ex.2E).

Picture8

Creep secondary moment (ft-kips)

 

Construction analysis from MIDAS gives creep secondary moment at pier 2 as 6866 ft-kips. The error between the two methods is 6% and again we do not know where these differences come from. However, considering the uncertainty of creep itself, the author believes this error is acceptable in practical design.

 

Download File [Click]

 

 

About the Editor
Seungwoo Lee PhD, P.E., S.E.
Senior Supervising Engineer at WSP USAs (Tampa, FL)

He is a Senior Supervising Engineer with a high level of expertise in the design and analysis of suspension bridges. He has a solid fundamental understanding of structures and can apply his knowledge exceptionally well to the design and analysis of highly complex structures. He is also a strong communicator, able to identify and distill the key information that is relevant to the task at hand and present it in a clear and quickly understandable manner.

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