MSEW (3.0): Features
Here are some of the features of program MSEW:
MSEW(3.0) can follow AASHTO (ASD or LRFD) or NCMA design guidelines. See limitations related to the NCMA option at the end of this section.
MSEW can be applied to walls reinforced with geogrids, geotextiles, metal mats or metal strips. It allows for reduction factors (construction damage, aging and creep) associated with polymeric reinforcement. Corrosion of metallic reinforcement over the life span of the wall can be assessed.
MSEW (3.0) can generate DXF graphic files that are compatible with AutoCAD®. Tables can be exported to Excel or printed by a click of a button. Data can be exported for in-depth global stability analysis using ReSSA.
Repetitive reinforcement layers (same length, spacing) can be input quickly. Alternatively, each layer can be input individually using a spreadsheet-like table.
Up to five types of reinforcement can be specified for a single wall. Parameters such as strength, reduction factors (polymer), coverage ratio, and cross sectional area (metal) characterize each ‘type’ of reinforcement.
MSEW has two modes of operation: Design and Analysis. In the Design mode, the program computes the required layout (length and vertical spacing) corresponding to prescribed safety factors. In the Analysis mode, MSEW computes the factors of safety corresponding to prescribed layout. In either mode, the program produces the ‘ideal’ reinforcement values for strength or coverage ratio assisting the designer in optimizing utilization of reinforcement.
The user can run MSEW first in its Design mode to obtain a ‘computer-based’ initial layout. Upon clicking on Analysis, all results obtained in Design are preserved as input data. The user can then modify the layout or properties of the reinforcement layers to insure correspondence to local conditions. Upon clicking on run, MSEW produces the factors of safety corresponding to the data modified by the user.
Walls with a batter of up to 20 degrees and a backslope can be designed or analyzed. Zero-batter two-tiered walls, trapezoidal walls, bridge abutment walls, and back-to-back walls can also be designed or analyzed. The user can specify the backslope rise and angle in the lower tier wall in the two-tiered option. AASHTO does not deal explicitly with such an option, however, sound geotechnical engineering allows for simple extrapolation.
MSEW considers uniform load (dead and live), strip footing, isolated footing, point load, and horizontal load. One can specify 3 different strip footings and 3 isolated footings with the option for dead and live loads (live load may simulate traffic load away from the back of the face of the wall). The footings may be embedded. Two different horizontal loads can be input.
Seismicity can be invoked either in Design, with specified safety factors that are different than the static ones, or in Analysis, producing both the static and seismic safety factors.
In LRFD seismicity follows AASHTO 2007 or AASHTO 2008-2010. For external stability the user can utilize a displacement criterion to reduce the possible excessive conservatism existing in M-O approach.
M-O equation for Kae could reflect a problem that is not feasible in which the M-O critical wedge extend very deep into the retained soil (e.g., the retained soil in reality does not extend to infinite backslope or is not homogenous to great extent). The user can specify the maximum extent of M-O wedge and therefore determine a Kae that represent realistic conditions. Such an option may legitimately reduce conservatism associated with the M-O approach.
The interaction parameters (signifying the pullout resistance or interfacial resistance to sliding) can be ‘locked’ to the default values meaning that when soil properties are changed in another dialog, the interaction parameters are modified accordingly. If the values are left unlocked, the user can input alternative values.
The user can select different facing elements: blocks (i.e., connection is derived by friction), precast panels (similar to blocks but connection is mechanical), full height precast panels, or wrap-around. In case of frictional connection, the connection strength, which is a function of confining stress, is checked at the elevation of each reinforcement layer to assess the potential for reinforcement front-end pullout or break. The confining pressure between stacked blocks is estimated using the hinge height method. For mechanical connection, the strength and the corresponding factor o safety are evaluated for each layer.
Bearing capacity is calculated accounting for such factors as sloping soil at the toe or ground water table at the base. Ultimate bearing load, eccentricity and factor of safety, are part of these calculations. AASHTO procedure for bearing capacity for sloping toe, as applied to MSE walls, may be overly conservative. To overcome this potential difficulty, the designer may explore the use of presumptive ultimate bearing capacity pressure; however, the value of this pressure relates to the experience of the designer.
MSEW evaluates the factor of safety against the long-term strength and pullout of the reinforcement at each elevation. It also calculates the safety factors on the connection strength and the resistance to direct sliding at the elevation of each layer. Eccentricity is calculated at each layer thus, if needed, making it easier for the designer to use the ‘coherent mass’ approach (commonly used by some metallic wall proprietors) in assessing reinforcement stress. It should be noted, however, that MSEW does not calculate stresses based on the coherent mass method; using the eccentricity may make it easier for the designer to convert the calculated stresses by MSEW into those obtained from the coherent mass method (if so desired). The export of data to Excel may facilitate such further calculations.
The natural soil strata, composed of up to five different soils, as well as the phreatic surface, can be input for global/compound (slope) stability computations.
Slope stability analysis (Bishop’s method) can be invoked to assess the potential for compound and deepseated failures. The analysis automatically considers the reinforcement layout and its available long-term strength along its length (including connection strength), the soils strata (including retained and reinforced soils), the soil surface geometry, the phreatic surface, the external loading, and the seismicity. Re-runs of circles emerging at specific locations can be done at a click of a button. For judgment, the user can view each analyzed critical circle and the corresponding calculated driving and resisting moments, including the resistive moment contribution of the reinforcement. The user can also view the intersection of this circle with reinforcement layers, the force mobilized at each reinforcement layer for stability, and the inclination of that force.
Two methods of rotational stability analysis are available (clicking the ‘?’ provides a technical explanation and formulation associated with each method). One method is Demo 82; it is Bishop’s treating the contribution of the reinforcement as pure moment when calculating the factor of safety. The second method is ‘Comprehensive Bishop.’ This formulation considers force as well as moment effects of reinforcement.
The user can conduct straightforward deepseated stability analysis by bypassing the analysis for slip surfaces emerging at the face, a useful option when slip surfaces emerging at the face are not likely.
In case of unsatisfactory deepseated stability, the user can specify basal reinforcement of sufficient length and strength in the Analysis mode. MSEW will produce the resulted factor of safety considering this basal reinforcement, assuming circular slip surface.
Exportation of data to ReSSA makes in-depth stability analysis including safety maps convenient. The information transformed includes connection strength as well as all geometrical details.
MSEW checks for conflicting input data.
Units can be SI or Imperial (Imperial units same as English units).
The user can override default values to conduct instructive parametric studies or to allow for adaptation to unusual conditions. This combined with analysis makes the program useful in forensic studies.
One can specify interfacial friction angle between the reinforced soil and facia and then calculate Ka for internal stability based on Coulomb’s. Unlike the NCMA design, however, the vertical component of the inclined Coulomb resultant is not ignored. Users should use this option very carefully since it is beyond AASHTO.
In external stability, Direct sliding, Bearing Capacity and Eccentricity can be assessed using Ka based on Coulomb’s lateral earth pressure. The user needs to input the friction angle between the retained and reinforced soil. Once again, this option has to be selected carefully since it deviates from AASHTO. Both options using Coulomb’s are useful in the context of exploratory parametric studies as well as in forensic engineering.
Help includes all relevant chapters from Demo 82 and the entire FHWA NHI-00-043 and FHWA NHI-00-044 manuals.
MSEW contains extensive graphics for both input and output information. Some of the graphics display force resultants thus making evaluation of results easier. Other graphics provide instant visualization to verify whether prescribed safety factors have been attained (green) or not (red). Other graphics, such as critical slip surfaces or distribution of force along reinforcement, are instructive.
Most of the results are tabulated. In Design, results that are specific to each stability mode can be viewed (intermediate results). This feature is useful when assessing effects of all design criteria on the final synergistic layout. That is, the designer can identify the elements controlling the design and therefore, take specific actions to improve the design outcome.
When in Analysis mode and while viewing the main table of results, the user can access the reinforcement-input data via a shortcut. The user then is directed to the table where the type of reinforcement, its elevation and strength (or horizontal spacing) is specified. Upon modifying and exiting the table, clicking on run takes the user back to the table of results. This allows for quick iterations for examining ‘what if’ scenarios.
Results can be printed as a report. The user can select from a menu the material to be included in the report. The user’s logo can be embedded in the report.
The designated product name of the particular reinforcement specified can be used. This name will appear in the printout.
Results can be saved as a text file. Most word-processors can retrieve such a file for editing and integration into a custom-made report.
Layouts of reinforcement and slip circles can be captured as bitmap files. Most graphic programs and word-processors can retrieve these bitmap files.
MSEW allows the user to retrieve and display up to four sections (bitmap files). This feature is useful for visual comparison of layouts. The user can capture these four sections as a bitmap file.
MSEW calculates the quantities of reinforcement per unit length of wall. Upon entering unit cost of each reinforcement type, MSEW instantly produces the total cost. This information can be printed or saved as a text file.
The user can create databases for up to 100 different types of reinforcement, one database for each category of reinforcement. The database includes connection to facia information (particularly useful for block walls where input of data in consecutive runs can be cumbersome). One can easily retrieve and modify this database. Data retrieved from database can be overridden.
If reinforcement layers shorter than the distance from the face to the slip surface are input, these layers will be considered in internal stability as intermediate layers connecting the facia to the active wedge but not as layers capable of stabilizing the active wedge. That is, connection load carried by primary layers will be reduced (alleviated) since intermediate layers carry some of the load. However, Tmax at the slip plane defining the active wedge is calculated ignoring the intermediate layers since they cannot develop pullout resistance at the slip surface. When intermediate layers are detected, MSEW does the necessary calculations for connection and reinforcement strength automatically. The designer must insure that the intermediate layer can carry the connection load by being embedded deep enough into the active wedge. MSEW is limited to 10 intermediate layers between any two adjacent primary layers. Technical explanation is presented in MSEW. The user is referred to the article “Alleviating Connection Load,” Geotechnical Fabrics Report, October-November, 2000, Vol. 18, Number 8, 34-39, written by Dov Leshchinsky. Note that AASHTO does not deal explicitly with intermediate layers. If invoked, this layout is beyond AASHTO.
It is highly recommended to purchase and use the AASHTO manual (Bridge Section) when considering the LRFD option in MSEW. As LRFD uses different philosophy than ASD although the basic calculations are the same, the intuition one may have developed based on ASD may not be relevant for LRFD.
Switching from LRFD to ASD or vice versa in MSEW does not require to re-input data; just a click of a button. Hence, comparing the results of two (or three if NCMA is applicable) different approaches is easy and could be instructive.
When in the NCMA option, MSEW is limited as follows:
MSEW(3.0) follows NCMA design calculations for segmental block walls considering static and seismic conditions. It is highly recommended to have an access to the NCMA design guidelines; program MSEW does not include these details in its Help. Unlike the AASHTO design mode, overriding default values in NCMA is generally not permitted in MSEW.
Analysis in NCMA mode is limited to simple geometry following NCMA guidelines. MSEW(3.0) allows the user to invoke the NCMA method only when in Analysis Mode (i.e., results are produced for given reinforcement layout and strength).
Only geosynthetic reinforcement is used when invoking the NCMA option. Cases where reinforcement is not needed (i.e., gravity walls) are not considered by MSEW(3.0).
When in NCMA mode, report in format *.txt cannot be generated.
Surcharge for NCMA method is limited to a uniform surcharge.