Load Bearing Masonry Software

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Monday, December 19 2011 17:51 Jay Desai, PE, SE, SECB, CBIE, Marc Steinhobel, PE, SECB

KNOWLEDGE EDGE | Loadbearing Masonry Software

Putting It to the Test

Leap Forward in Technology

RAM Advanse Masonry Module is a leap forward in technology. It gives engineers the ability to model the entire building taking

into account the effects of both global and local forces on each wall segment. It saves the engineer time from having to

generate separate analytical models that only determine member forces and separate design calculations using results from

the analytical model or other hand calculations.

As with RAM Structural System, RAM Advanse Masonry Module reduces the time spent by engineers crunching numbers. It

allows engineers the flexibility to model, analyze and evaluate various options quickly and effectively. Alternative framing

options, wall locations, shear wall layout, pier size and requirements are quickly and accurately analyzed and their effect on

the entire structure considered.

Does the masonry module reduce the time required to design a multi-level loadbearing masonry building? Yes. How much?

We estimate the time savings in the range of 40% to 50% for analysis and design. More importantly, it provides a tool

engineers can use to optimize their design, resulting in a better, more economical design thereby reducing construction costs,

allowing masonry to compete on a level playing field against other structural loadbearing wall systems.

Putting It To the Test

The new 77,500 sf residence hall at Michigan Technological University is in the design phase. Located in Houghton, Michigan

Tech was established in 1885 as a mining college with only 23 students. Now, the school has expanded with more than 400

faculty and more than 6500 students. The community of Houghton has expanded along with the Universitys needs and the

need for new student housing. The new student housing building is a six-story concrete masonry loadbearing structure with

precast plank floors. The building has repetitive punch-out windows between larger bay windows along the exterior. Along the

interior, corridor walls are loadbearing CMU, designed to resist both gravity and lateral loads. Several partition walls,

perpendicular to the corridor walls, are used as shear walls to resist lateral wind and seismic forces. Special consideration was

taken for snow loading due to the Universitys location in Michigans Upper Peninsula where ground snow loads are 100 psf.

The RAM System

Desai/Nasr Consulting Engineers used Bentleys RAM Structural System V8i

(release 13.03) and RAM Advanse V8i (release 9.6) published in December 2008

to model and analyze the loadbearing masonry structure. RAM Advanse V8.0 was

released in June 2006 and introduced its first masonry design capabilities. V9.5,

released in August 2008 introduced hybrid masonry design capabilities. Editors

Note: RAM Elements V8i (release 10) was published in July 2009 and introduces

the products name change, as well as enhancements to the masonry capabilities.

RAM Structural System V8i Modeling Module was used to quickly and effectively

build the structural frame including all structural members. Wall openings were

modeled to include the effect these openings had on the global stiffness of the

structure.

Gravity surface, line and point loads as well as seismic masses were input into the RAM Modeler Module. RAM Frame

Module was used to input the parameters for the lateral wind and seismic loads for the main lateral load resisting system.

Frame generated the required lateral loads as point loads imposed on the rigid floor diaphragms.

Since RAM Structural System does not have the capability to design masonry walls, in order to complete the design, the

model was exported to RAM Advanse for further analysis. Loads input in RAM Structural System are converted to nodal point

loads on shell elements and point and line loads on beam elements when imported into RAM Advanse. After the full gravity

and lateral model was imported, some adjustments were required to change the material properties and member sizes to

conform to the different naming convention used by RAM Advanse. Gravity column end releases were adjusted and corrected

to conform to the pure finite element structure of the masonry module.

Figure 1 RAM Structural System

Model

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Finite Element Analysis

RAM Advanse is a full blown finite element analysis program that completes full iterative

convergence of the model during the P-Delta analysis. Careful model consideration was

given to limit the size of the stiffness matrix and number of load cases considered to reduce

analysis time. Finite element analysis programs generally provide results that are a good

representation of the forces, stresses and displacements based on the input parameters. A

good understanding of finite element analysis, program assumptions and limitations isessential to ensure good results. For example, in this masonry module, nodes between wall

panels (located at end of wall panels modeled in RAM Modeler) are considered attached to

each panel, effectively making the panels continuous across joints. If interior non-

loadbearing walls are rigidly attached to the loadbearing exterior walls, then the results

generated may not reflect the design engineers intent and assumptions when it comes to the foundations.

To effectively isolate walls at these locations, or at any control joints desired, the walls are stopped

short at intersection points and dummy beam members are added to provide continuous support for

floor elements.

After adjusting the model to comply with RAM Advanse parameter requirements, a full P-Delta

analysis produced good results which were used to check the main lateral force resisting shear walls

for in-plane loading and story drift. Vertical compression and in-plane shear were easily reviewed and

checked for each shear wall using the masonry module.

The model imported from RAM Structural System was not suitable

to carry out detailed design of individual wall panels. Effects of load

eccentricity were not included due to precast panel bearing on half

the wall and out-of-plane loading due to component wind loading. RAM Frame cannot

generate pressures on walls, however, these out-of-plane loads are easily added in the

masonry module.

To analyze exterior walls, a section of wall was

isolated. Dead and live load moments were

added to include the out-of-plane effects of load

eccentricity on the wall. It is important to

understand that RAM Advanse imports loadsfrom RAM Structural System as point loads on the nodes at the corners of the wall

panels. The size of wall panels and location of wall joints in RAM Structural System

directly impact the loading of the shell elements and accuracy of the results.

Results were improved by limiting openings to one per panel, by splitting the walls

between window openings and maintaining a vertical to horizontal aspect ratio of

not more than 1:1.5. Limiting the aspect ratio ensured that the concentrated nodal

loads at the ends of the panels were evenly distributed and piers between window openings carried the correct gravity loads.

The effect of out-of-plane wind loads on the exterior walls was included by adding

localized wind pressure on the face of the wall shell elements.

The refined model was analyzed in the general

finite element module of RAM Advanse before

the selected wall panel was imported into the

masonry module.

The masonry module imports the analyzed

forces, stresses and strains from the general

module. Any change to the loads or load

combinations in the module deletes all analysis

results. The analysis is then re-run in the general

module. Wall reinforcing and other parameters

are set in the masonry module. Parameters such as CMU material properties have to be set

Figure 2 Imported Model in

RAM Advanse

Figure 3 Dummy

Beam Members

Figure 4 Exaggerated Drift

Figure 5 Panel Selection for Design

Figure 6 Adding the Effects of Out-of

-Plane Wind Loads

Figure 7 Maintaining an

Aspect Ratio 1:1.5




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in the general module. The masonry module cannot be analyzed if there is not at least one service and one design load

combination set up.

After analyzing various configurations of the wall panel with different wall joint locations in RAM Structural System, the analysis

results were comparable to our hand calculations. Multiple load cases, with in-plane and out-of-plane forces, were quickly

analyzed and designed. Good graphical display options allow the engineer to review the forces and displacements calculated

at sections through the wall panel. The detailing module did a good job of detailing the reinforcing steel in the wall and around

the openings.

The lintel design module could not design the lintels as tension stresses were generated in the top of the shell above the

windows due to the arching effect and distribution of forces around the window openings. The lintel design module is

programmed to provide tension reinforcing at the bottom of the lintel beam and not at the top of the beam, thereby generating

a No Good design message in the output.

Editors Note: RAM Elements V8i Release 10.0 corrects the lintel issue, along with several

other updates. IMI has also placed on its website information to assist designers with how to

handle the negative moment at the lintel.

Similarly, using large panels, the narrow piers between the windows were under designed as

the point loads applied to the corners of the panels and the effects of arching reduced the

amount of axial load in the pier.

Carefully locating the wall joints to transfer the loads correctly into the wall and limiting wallpanel sizes eliminated the lintel design issues and properly accounted for axial load in the

piers. When large wall panels were modeled with multiple window openings, the program did

not correctly design the lintels over the openings or the piers between the openings.

The detailed analysis printout gave capacity curves for each design segment for in-plane and out-of-plane forces. The output,

however, does not give a detailed breakdown of the individual calculations, making it difficult to cross check program results

with hand calculations.

Comparative Results

At Desai/Nasr, it has always been our practice to use new software products on smaller projects.

This is done mainly to compare results of hand calculations versus those obtained with the use of

the software. This practice allows the engineers to become familiar with the program and

develop a higher level of comfort with the use of a particular software product. Since the release

of RAM Advanse Masonry Module, Desai/Nasr has used it to analyze and design small-scale one

- and two-story buildings and individual wall panels with openings.

The Michigan Tech student housing project was the first large

multi-level loadbearing masonry building Desai/Nasr analyzed

and designed using this masonry module. After thorough

understanding of how RAM Advanse imports the RAM

Structural System models and loads the masonry shell

elements with few modifications to the model, a workable result

was achieved. The RAM Structural System modeler quickly and

effectively generates a full building model with the required

gravity and lateral loads with minimal input. Careful consideration is recommended when

setting up wall joint locations to ensure that loads are correctly transferred into the wall shellelements. Dummy members are an effective way of isolating the wall continuity at wall intersections and control joints,

improving analytical results. Analysis time can be reduced by limiting load combinations and carrying out detailed analyses on

typical wall panels only.

Before the release of RAM Advanse Masonry Module, masonry design software focused on component design with

independent tools to design separate components such as shear walls, lintels, bearing walls, piers and columns.

The independent tools often are constrained to specific geometric and load

parameters. Engineers will surely find time-saving benefits from the introduction of

this masonry module to the suite of structural software available.

Figure 8 RAM Advanse

Masonry Module

Figure 9 Analysis

Results

Figure 10 Detailing Module




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Last Updated on Tuesday, December 20 2011 12:36

Editors Note: Steel-framed and concrete-framed buildings have been designed for

years using whole building design software. This is not the case for masonry

buildings. Most have been designed using hand calculations, spreadsheets written

by individual engineers or commercial packages that design elements or

components. The suite of Bentley software makes whole building design a reality

for loadbearing and hybrid masonry.

Engineers who design even modest-sized loadbearing buildings recognize that

hand calculations can be tedious and repetitious. For a simple four-story building, it

is not uncommon for an engineer to spend six to eight weeks developing design

calculations. To put the effect of using this software into perspective, that time can be reduced to approximately half. In

addition, it is possible to evaluate alternate designs or modifications to a design without starting over.

RESIDENCE HALL | MICHIGAN TECHNOLOGICAL UNIVERSITY | HOUGHTON

Architect Neumann/Smith Architecture Southfield

Engineer Desai/Nasr Consulting Engineers West Bloomfield

General Contractor Gundlach Champion Houghton

Budget $14.2 millionExpected date of completion July 2010

Figure 11 Interaction Diagrams


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