Infill Brick Frames

Infill Brick Frames

Introduction

Brick infill frame is a composite structural system in which the concrete or steel frame is bounded by the brick masonry. The term “bounded” is of prime importance since it dictates the interaction of frame and brick wall. The interaction changes the behavior from moment frame to an infill frame.?

Concrete frame members (slab, beam, column & foundation) are designed to act as load bearing elements. On the other hand, the brick wall is “intended” to be used as a non-load bearing (non-structural) element. The brick walls are used either for partition or as a fa?ade to act as an envelope for the building. At times, concrete masonry units (CMU) or block walls are used as a substitute for brick.

The concrete frame with brick as an infill is the most used structural system for multi-story buildings in Pakistan. The paper aims to briefly discuss the limitations of the conventional analysis approach for infill frames, its structural performance, positive contribution of infill walls by acting as a fuse & presents an alternate analysis procedure. A sample example is also given towards the end of the paper.

Infill Wall Systems

Brick infill frames are quite popular in South Asia, i.e., Pakistan, India, Nepal, Sri Lanka & Bhutan. Some nations have however transitioned from the use of brick walls to light gauge steel (metal studs). Table 1 is a brief comparison between the two systems.

*Light gauge walls are approx. 80% lighter compared to brick walls. This can significantly reduce the dead weight of the building.

**Higher material cost of light gauge steel will be offset by savings from the concrete frame design. This is due to the reduction of dead weight.

Building Codes And Infill Frame Structures

American Building Codes i.e., Uniform Building Code (UBC) or International Building Code (IBC), both do not recognize infill frames as a valid structural system.

Building Code Pakistan (BCP) 2021 & 2007, both refer to ASCE 7 for structural systems & hence as a result do not cover the infill frame as a structural system. This is unfortunate since infill frame is the most used structural system for multi-story buildings in Pakistan.

Most Building Codes do not recognize infill frames as a structural system due to its brittle & undesired failure patterns in past seismic events.

However, Indian Building Code (IS 1893: 2016) & Eurocode 8 (EC 8) do recognize infill frames as a valid structural system. IS 1893 & EC 8 provide guidelines for modeling the infill walls & determining its effects on the overall building response. EC 8 codes take the approach of designing the infill frame as a dual system, concrete frame & infill wall are checked combinedly. Additional checks in the Euro 8 code intend to take the advantage of infill walls & at the same time ensure that they do not cause a brittle failure in the concrete frame. These additional checks are covered under section 4.3.6 of EC 8. However, the Indian Code IS 1893 under section 7.9 intends to model the infill walls as struts to check its impact on irregularity. The concrete frame is still designed for the full seismic force. In my opinion the approach adopted by the Indian code is more practical & relevant to the construction industry of Pakistan.?

Standard Practice Followed

Below is a brief description of the design practice typically followed to design concrete frame buildings in Pakistan.

Modeling

Such structures have historically been analyzed by modeling only elements which are part of the concrete frame i.e., slab, beam, column & foundation. Typically, two analysis models are developed, one for the super structure & another for the foundation. Brick walls are not modeled in the analysis models irrespective of their nature, brick walls bounding the frame or interior partitions.

This technique assumes brick walls only add to the dead weight and do not contribute to the stiffness of the structure. The reality is however different. Brick infill walls have high lateral stiffness till they crack significantly. Seismic performance of a structure depends on seismic mass & stiffness. Hence, this modeling practice wrongly omits the contribution of the brick walls (unless the brick walls are detailed with a gap to avoid interaction with the concrete frame).

Design

Concrete frames are typically designed as intermediate (IMF) or special moment frames (SMF). Adaptation to IBC is still in the transitional phase in Pakistan & hence most designers still tend to use UBC-97. In case of IBC, the seismic design category & structural height dictates the frame type. In case of UBC-97, seismic zone & structural height dictates the frame type.

In case of linear analysis, seismic response modification factor (R) for IMF or SMF are used to reduce the forces to design level. Nonlinear analysis works in the inelastic region & hence does not require the demands to be reduced by the “R” factor. However, designers opting for any of the above methods tend to typically ignore the infill wall stiffness & assume that the concrete frame will act as a “moment frame”. Contribution of infill walls to structural irregularities (horizontal or vertically) is also ignored. These are incorrect assumptions as explained in later sections.

Another issue is the use of the correct “R” value. Developing a generic “R” value for the infill frame is highly complicated if not impossible. BCP, UBC and ASCE 7 do not have any “R” value for the infill brick frames (not recognized as a valid system). Hence, designers typically tend to use the “R” value for moment frames.

Detailing

Beams, columns & joints are detailed corresponding to the selected frame type (IMF or SMF). The connection between infill wall and concrete frame is usually not detailed. However, some designers do provide rebar dowels which connect columns to brick infill walls.

Structural Performance Of Brick Infill Frames

This section describes the structural performance of brick infill frames in case of a seismic event. Infill brick walls can change the global structural response by either contributing favorably or unfavorably. The second aspect of infill frames is the local adverse effects of infill walls on concrete frame structure. The third aspect is the cracking of the infill walls & its potential for out of plane failure.

A.??Global Structural Response Of Frame Structure

1.???Lateral Stiffening Effect

The brick infill walls provide additional stiffness to the structure. The bare frame is composed of the concrete framing only (that's what we typically model). Stiffness of the infill frame is in fact significantly higher than that of the bare frame. The increased stiffness is due to the ability of infill walls to act as diagonal strut/ brace (compression only). This holds true till the infill walls crack significantly and transfer the complete load to the bare frame. The typical practice is to ignore this stiffening effect in analysis.

Effect On Static Produce:

The increased structural stiffness translates into a lower time period. Hence, if the designer opts to use the static procedure (ELF) from UBC-97 and ignores the stiffening effect of the infill walls the base shear will mostly be underestimated. This is because the time period is in the denominator of the base shear equation (Eq 30-4). Static procedure in ASCE 7-22 (Section 12.8.1) however computes seismic response coefficient (Cs) based on four equations 12.8-2, 12.8-3, 12.8-4 & 12.8-5. All except 12.8-2 are dependent on the time period. Hence by ignoring the stiffening effect, the base shear based on equation 12.8-3 to 12.8-5 will be underestimated.?

Effect On Dynamic Produces:

If the designer opts to use response spectrum analysis (RSA) the effect of ignoring the infill wall on the base shear can be one of the following:

●????If the time period of the bare frame is on the rising part of the curve (between 0 & To) then it means that the base shear is overestimated.

●????If the time period of the bare frame is on the flat part of the curve (between To & Ts) then it means that the seismic forces will remain the same or are overestimated.

●????If the time period of the bare frame is on the receding part of the curve (beyond Ts) then it means the base shear is underestimated. This can be an applicable case for high rises or structures built on soft soils.

The effects of ignoring the stiffening effect of infill walls in case of response history analysis (RHA) will be close to that stated for the response spectrum analysis (RSA).

2.???Change Of Structural Behavior From Frame To Truss Action

The infill walls change the structural behavior of the concrete frame from a moment frame to a truss dominated behavior. As stated above, this is due to the diagonal compression braces formed by the infill frames.

As a result, the concrete frame elements (beams & columns) will experience less flexure moments till infill walls significantly crack. However, these elements might experience higher axial forces due to truss action. Refer figure 3, showing difference in structural behavior.

3.???Irregular Stiffness Distribution

The most detrimental effect of infill frames is the irregular stiffness distribution. Irregularity in stiffness is one of the most common causes of catastrophic failures in the infill buildings. Indian Building Code (IS 1893:2016) & Eurocode 8 (EC 8) while allowing the use of infill frames impose strict irregularity checks to avoid such failures. Irregularity effects of infill frames cannot be understood unless they are modeled as equivalent diagonal struts.

●????Irregularity In Plan (Horizontal)

The irregularity in plan due to infill walls can be observed in most of the frame structures in Pakistan. This is because the front side of the building is largely open while the back side is blocked with infill brick walls. Hence, the front side is much softer compared to the back side. This feature shifts the center of rigidity closer to the back side & hence can cause extreme torsion irregularity. As a result, the flexure demand on columns on the front side of the building will be much higher than assumed.

●????Irregularity In Elevation (Vertical)

The irregularity in elevation is most observed in commercial plazas & apartment buildings where the front side of the ground floor is largely open.

In the case of commercial buildings this is due to the presence of shops on the ground floor. In case of apartment buildings, the ground or lower ground floor is at times kept open for parking. The true nature of this soft story mechanism cannot be understood without modeling infill walls as struts.

4.???Energy Dissipation

Energy dissipation of infill walls is in fact a positive effect. Brick walls are typically the first element to show signs of failure in case of a seismic event, sliding or diagonal cracking. Hence, infill walls act as a “fuse” or in other words act as a source of energy dissipation. This is the key reason why poorly engineered or non-engineered concrete frames have survived big earthquakes in the past.

B. Local Adverse Effects

1.???Extreme Pressure On Columns

Infill walls exert extreme pressure at top and bottom of the concrete columns. This pressure is due to the diagonal strut action of the infill walls. This is most likely to occur in panels with no opening in the brick wall. Chances of such failure can be minimized by diligently checking the columns for shear force from the diagonal struts.

2.???Captive or Short Columns

Captive or short column effect is an adverse phenomenon which causes very high shear demand on the frame columns. This is most likely to occur in?panels with window openings in the brick wall. The portion of the column next to the opening is free to move while the column next to the wall is restricted to move. This causes the behavior of columns to change from being moment dominated (desired) to be shear dominated (undesired).

C. Infill Brick Wall Failure

Infill brick walls due to their higher stiffness but poor ductility are usually the first element to show signs of failure. The two common types of failures can be seen in images below: diagonal cracking & out of plane failure of brick walls. The out of plane failure can become a life safety hazard.

Methods To Address Infill Frames:

Extensive research & documentation by building codes have shown that there are two commonly used solutions to address the challenge of infill frames.

The first is to isolate the infill walls from the concrete frame so the frame can truly act as a moment frame. The second is to consider the stiffness of the infill walls in the analysis model. This approach is adopted by IS 1893 & EC 8 codes.

Another method that is quick and conservative is to design the concrete frame based on the governing load cases from the two models. However, the designer still needs to be cautious of the out of plane failure of infill brick walls. Using this method, the concrete frame elements will be designed for the governing case from:

Model 1: Bare concrete frame

Model 2: Concrete frame with infill walls modeled as diagonal struts.

The model 2 will help in the following:

●????Identify torsional irregularity due to infill walls

●????Identify columns experiencing high moments due to torsional irregularity

●????Identify high axial loads in beams or columns due to frame action

●????Identify short or captive columns

●????Give more realistic seismic drifts/ displacements

Method 1: Isolate Concrete Frame From Infill Brick Walls (Drift Joints):

Drift joints ensure the frame can truly act as a moment frame & not as an infill frame. By providing the drift joints our current prevalent analysis practice (ignoring the infill wall in the analysis model) will hold true to its spirit. Also, this will help to avoid brittle failure patterns due to brick walls. Drift joints ensure the infill wall and concrete frame do not interact by providing a low compressive strength or flexible material in between. Drift joints should at least be equal to the drift determined through analysis for bare frames. However, drift joints require additional cost. Also, this increases the potential for out of plane falling of brick wall which needs careful consideration in the detailing.?

Method 2:?Equivalent Diagonal Strut:

Another solution as adopted by IS 1893, Eurocode 8 (EC 8) & FEMA-356 is to model the infill walls as diagonal struts. This approach saves the cost of drift joints. Refer figure 8, 9 & 10 for modeling diagonal struts in case of different wall conditions. For details on modeling different conditions refer to FEMA-356.?

There are several equations developed for the modeling of infill walls. The equations convert the infill walls to an equivalent strut. Some of the commonly used equations are stated in table 2. The equation developed by Mainstone has been adopted by Indian Code 1893 & FEMA-356.

*Equation is adopted by FEMA-356 & Indian Code IS 1893: 2016

?Note:

w = Width of the strut in model (to be computed)

d = Diagonal length of brick wall (known)

For Mainstone equation, λh is found by following equation:

Where:

tm=Thickness of brick wall

hm=Height of brick wall

Theta=Angle of diagonal strut

Em=Modulus of elasticity of masonry wall

Ec=Modulus of elasticity of concrete column

Ic=Moment of inertia of concrete column

hc=Height of column to beam centerline

Sample calculation using Mainstone (1974) Equation

Model tm x w = 9” x 39” strut in analysis software, ETABS, STAAD or others. Refer results on following pages.

The following example is intended to show the drastic difference in structural performance of a bare frame & infill frame. Static forces of the same magnitude are applied to both the models.

Refer example frame in figure 12. The diagonal struts are modeled (using the calculations provided above). For ease the struts are modeled only where walls have no opening.

Table 3 summarizes the results of the two models.

Model 1: Governs in axial forces. The displacements are closer to reality.?

Model 2: Governs in bending moment & shear forces.