Soil Supported Mat Foundation

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Design Guide No. PPSD-A-DG-024-SE-0129

SOIL-SUPPORTED MAT FOUNDATIONS USING STAAD.Pro

REVISION: 0 DATE: March 2007 PAGE 1 OF 5

OWNER(S): STAAD Foundation Committee

Approved: Rodney K. Simonetti

PPSD-A-DG-024-SE-0129

1.0 PURPOSE

To establish general recommendations for designing soil supported mat foundations usingSTAAD.Pro finite element analysis (FEA).

2.0 ABSTRACT

Over the past several years the design of soil-supported mat foundations have been typicallydesigned using STAAD.Pro finite element analysis. However, during this time, a standard

method of design has never been implemented to ensure consistency across projects. Thisdesign guide standardizes the methods of analysis for soil supported mat foundations andensures accurate results that are current with industry practices. Topics of discussion includemesh size, plate element length to width ratio, vertical and lateral spring supports based on soilsubgrade modulus, proper command usage; rigid links for load transfer, and foundation modelswith uplift. The recommendations in this Design Guide are based on the findings in PPSD-A-LI-024-0008, “Design of Soil-Supported Mat Foundation Using STAAD.Pro” (Reference 1).

3.0 DESIGN

3.1 ELEMENT MESH SIZE

Plate element meshes should be rectangular with local axes oriented in the direction ofthe primary reinforcing. The use of triangular (3 sided) meshes should be avoided.

Use STAAD “Structure Wizard” tools (Reference 2), quadrilateral mesh tool andtriangle mesh tool or department CBA STAAD Plate Element Generator Spreadsheetto create required mesh.

Maximum mesh size should not exceed plate thickness.

Minimum mesh size should not be less than 12 inches.

Element aspect ratio should not be excessive. The ratio should be on the order of 1:1and preferably less than 4:1.

When assigning nodes to an element using the STAAD “Structure Wizard” in the inputdata, it is essential that the nodes be specified clockwise, with the starting node being

in the upper left-hand cor ner of the foundation. This insures that the analytical “top”and “bottom” surfaces of the mat match the physical foundation.

When determining mesh size, be sure to consider factors that could impact the modellayout, such as applied loading, STAAD analysis time and post processing of data.


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3.2 SUPPORT CONDITIONS

Use the PLATE MAT command for generating soil springs (See Section 3.4 below).

Do not use the ELASTIC MAT command.

3.3 LATERAL SOIL SPRINGS

Use soil lateral spring values as part of the PLATE MAT command (Reference 2).

If no lateral soil sub-grade modulus is provided in the geotechnical recommendations,use 50% of the vertical soil sub-grade modulus for lateral spring generation.(Reference 3)

Note that foundation lateral displacement results may not be realistic due to soil lateralspring support assumptions. If lateral displacement is a concern the engineer shouldconsider the effects of embedment or other boundary conditions.

The Engineer should decide if in-plane membrane stresses should be evaluated andconsidered for reinforcement design. In most cases this is not an issue and can beignored since values are usually insignificant.

3.4 “PLATE MAT” COMMAND (Reference 2)

Use STAAD.Pro Command:

Plate list PLATE MAT DIR AL L SUBGRADE f3 f4 f5 (PRINT) (COMP)

YR -0.1 0.1 PLATE MAT DIR AL L SUBGRADE f3 f4 f5 (PRINT) (COMP)

to generate soil spring support constants. Refer to the STAAD Technical ReferenceManual for additional information.

Consider special support conditions such as spring tension/compression, nonlinearsprings, dynamic load cases and inclined supports.

Do not use the SET Z UP command. (Global Y must be the vertical axis).

Use the PRINT command to include the node influence areas for each support in theoutput file.

3.5 RIGID LINKS FOR LOAD INPUT

Rigid links may be used to simplify the application of loads to the mat and to accountfor the eccentricity of the load relative to the centroid of the mat.

Use releases at the bottom of the links. Release MZ and MY.

The height of the angled portion of the link should equal ½ the thickness of theconcrete slab. Do not provide a vertical link to the slab directly under the load (ifapplicable).

The angle of the link will be determined by the plate mesh size and the thickness of theslab. Varying angles will not affect the results.


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Try to use as many points of attachment to cover an area at least equal to the area ofthe pier, column, etc (within reason) to allow for greater distribution of the load.

Adjust the member properties (AX, IX, IY, & IZ) of the rigid link to equal 10,000 (unitsof feet). In general, this approach will work; however, care should be taken whenusing these property values with short rigid links. The engineer should review on acase-by-case basis that the results are reasonable. Rigid links should have a densityequal to zero.

Use the same modulus of elasticity for the rigid link as concrete.

Example of a typical rigid link used for application of a load on a mat:

The use of solid elements instead of rigid links is not recommended.

In general, it is not recommended to apply point or pressure loads directly to the plateelements. It may be acceptable to apply relatively small loads as direct point loads ordirect pressures.

3.6 FOUNDATION MODELS WITH UPLIFT (See Figure 1)

If no uplift exists in the base pressures under the service level loading combinations, itis acceptable to use linear tension/compression spring supports and generate factored

load combinations using superposition (STAAD Load Combination commands,Reference 2).

o Ignore displacements, reactions, and bearing pressures results for factored loadcombinations.


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If uplift exists in the base pressures under the service level loading combinations, anon-linear analysis with compression-only spring supports is required (Reference 2).

o Add SPRING COMPRESSION command to supporting nodes.

o Use REPEAT LOAD command instead of LOAD COMB command.

o Use CHANGE command for service loading combination cases to reset springsfor each loading.

o Complete serviceability design, checking displacements and bearing pressures.

o Use one conservative load factor (such as 1.6) based on the applicable codeand design criteria, applied outside the program to obtain the plate elementforces in strength level for concrete mat strength design. Instead of using oneconservative load factor for all combinations, it may be prudent to groupcombinations according to an appropriate average load factor.

If only minimal uplift occurs in the bearing pressures under the service level loadingcombinations use good engineering judgment to determine if a minimum amount ofuplift is acceptable and a non-linear analysis is not required.

4.0 REFERENCES

1. Design of Soil-Supported Mat Foundation Using STAAD.Pro, PPSD-A-LI-024-0008.doc

2. STAAD.Pro Technical Reference Manual

3. F.E.Richart, Jr., R.D.Woods and J.R.Hall, Jr., Vibration of Soils and Foundations p. 340-343, 1969


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5.0 APPENDICES

Figure 1 - FLOW CHART FOR MAINTAINING STAAD MODEL STABILITY

Prepare Foundation Design Data

Prepare Loading Information

No

Yes

Yes

No

Input Primary Load Cases

Such as

"LOAD 1 SELF WEIGHT"

"LOAD 2 DEAD LOADS"

. . . .

Service Level

"LOAD COMBINATION"

"PERFORM ANALYSIS"

Use good engineering

udgment to determine

if a minimum amount

of uplift is

acceptable…hence,

will not affect mat

reinforcement design

Evaluate base

pressures or

support reactions

to determine if

there is any uplift

at the supports

Continue with serviceability

design, checking

displacements and base

pressures

Add strength level load

combinations using “LOAD

COMB” commands

Complete STAAD post

processing using “Plate Centre

Stress” results for


Add “SPRING

COMPRESSION” & "node_list

KFY" commands to supporting

nodes

Use “REPEAT LOAD”

command ( not “LOAD

COMB” command)

Use service level load

combinations (unfactored) to

run "PERFORM ANALYSIS"

Use “CHANGE” command

after each service load

combination to reset support

springs for each load

combination

Complete serviceability design

Complete strength level design

using serviceability

combinations with manually

applied load factors 1.2~1.6

according to applicable code for


It may be prudent to use more than one

conservative factor for all combinations and to

group combinations according to an average

factor for a particular combination

Documents

Soil Supported Mat Foundation