Modeling of wall elements for stiffness consideration

[Waqas Haider]

Assalam o alaikum. 

Why dont we model masonary wall in etabs while modeling frame? Walls are there with their stiffness participation by reducimg displacements in an earthquack. Is it because we consider walls will fall when seismic force will act on walls perpendicular to plan of walls?

More over one of my friend told me that now a days practice of construction in islamabad, quetta and kashmir , houses are built with masorany walls along with columns. The frame provided is designed for lower seismic zone than original and for lower ductility. My question is does it act as dual system? Means walls are considered to resist lateral forces and the frame ( in which beam is not designed to take slab load) is provided just to have some extra stiffness.... what exact concept is used behind such design, if any one is familiar with? 

Edited December 27, 2016 by Engr Waqas

[EngrJunaid]

W.salaam Waqas,

We have various Seismic-force resisting systems defined in different codes/standards with different nomenclature.

You can study these Systems in detail in these tables;

      BCP SP-2007, Table 5.13-Structure Systems

      ASCE 41-13, Seismic Evaluation and Retrofit of Existing Buildings,Table 3-1,Common Building Types

      ASCE/SEI 7-10,  Minimum Design Loads for buildings & other structures,Table 12.2-1 Design Coefficients and Factors for Seismic Force-Resisting Systems

Modelling Structure either as BARE FRAME or FRAME with MASONRY INFILL in ETABS/SAP/STAAD PRO/ROBOT Structures/Seismostruct is the designer design approach that how he want to resist the gravity/lateral Loads.

By Modelling wall in model it becomes a DUAL System (Concrete Frame with masonry shear walls) BUT the idealization of masonry walls is not as simple as other elements like Beams/columns/slabs/RC Shear walls etc. It requires more experties/understanding (Study Section 11.4-Masonry infills of ASCE 41-13,Seismic Evaluation and Retrofit of Existing Buildings for detail). Proper anchorage should be provided b/w masonry and RC frame. 

We mostly/commonly design our Concrete structures as BARE FRAME (Moment resisting frame without Infill walls) and assign wall load on the beams. By doing this we are neglecting the masonry walls stiffnesses which results more ductile frame (Ductility is more desirable in structures).Proper steel reinf. detailing is required and gap filled with thermopore or any other insulating material is provided b/w the RC frame and masonry wall (inorder to prevent the hammering action of masonry on RC Frame during EQ).

Most of the 2,3 story houses are constructed as CONFINED MASONRY STRUCTURES and these structures perform very well in EQs.

 

 

Edited December 28, 2016 by EngrJunaid

[Ayesha]

For the case of frame structure, the key is to have free standing masonry, i.e. provide gaps b/w masonry and frame on sides (between masonry and frame column) and top (between masonry and frame beam) and have it anchored so that it doesn't fall incase of a seismic event (this is very important or the wall will fall down in case of a seismic event). You can also find the attached example useful in understanding how free standing masonry should be anchored to structural system. 

Masonry Veneer Shelf Angle Design.pdf

[UmarMakhzumi]

FYI- Video is more related to confined mansonry but shows the out of plane behaviour of wall. Might be of interest to few.

Thanks.